david c lindberg- the genesis of kepler's theory of light light metaphysics from plotinus to...

The Genesis of Kepler's Theory of Light: Light Metaphysics from Plotinus to KeplerAuthor(s): David C. LindbergSource: Osiris, 2nd Series, Vol. 2 (1986), pp. 4-42Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/301829 .

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Engraving of Kepler (first half of nineteenth centulr 'I. Courtesy of David C. Lindberg.



The Genesis of Kepler's Theory of Light: Light Metaphysics from

Plotinus to Kepler

By David C. Lindberg*

ISTORIANS OF JOHANNES KEPLER's astronomy and cosmology have ceased to be embarrassed by his metaphysical speculations. They ac-

knowledge that Kepler undertook his defense of heliocentrism because its "love- liness" filled him "with unbelievable rapture"; that he regarded the cosmos as the image of the Creator and searched for its trinities and underlying geometrical reasons, placed there by a God who "is forever doing geometry."' They have made their peace with the once incredible notion that the man who discovered new laws of planetary motion and set the astronomical enterprise on modern foundations was also a Neoplatonic enthusiast, astrologer, and panpsychist.

Historians of Kepler's optics paint quite a different portrait. No aura of Neo- platonic mystery surrounds this Kepler. Rather, we observe a skilled mathematical physicist absorbed in technical detail. We are shown a Kepler who broke with the past, clarified the foundations of optics, and solved a series of problems that had confounded his predecessors-vision, the camera obscura, and the geometrical theory of the telescope. His biographer Max Caspar has written that "Kepler, with an abundance of new thoughts and insights, with his clear grasp of the problems and proofs, prepared the ground for a new treatment of optics and solved, if not all, at least a good portion of, the tasks which he set for this science, in such exemplary fashion that today we still build upon the foundations laid by him." Vasco Ronchi has portrayed a Kepler "free from concepts, which although two thousand years old now appeared insufficient and absurd," able to "proceed rationally and logically towards a new structure of the intricate problem of light-image-vision." Alistair Crombie has discussed Kepler's mechanization of the problem of vision through the use of artificial models-the

* Department of the History of Science, University of Wisconsin, Madison, Wisconsin 53706. I am indebted to the National Science Foundation and the Graduate School of the University of

Wisconsin for support of the research on which this paper is based. I am also indebted to Catherine Chevalley, Stephen Straker, Daniel Siegel, and Ronald Numbers for critical commentary.

I See Max Caspar, Kepler, trans. C. Doris Hellman (London: Abelard-Schuman, 1959) (orig. Jo- hannes Kepler, Stuttgart: Kohlhammer, 1948), p. 397; quoting Johannes Kepler, Mysterium cosmographicum, ed. Max Caspar, in Gesammelte Werke, ed. Walther Von Dyck and Max Caspar, 18 vols. (Munich: Beck, 1937- ), Vol. I, p. 26. For an excellent review of Keplerian scholarship, see E. J. Aiton, "Johannes Kepler in the Light of Recent Research," Histoiy of Science, 1976, 14:77-100. On the metaphysical background of Kepler's astronomy and cosmology, see esp. Alexandre Koyrd, The Astronomical Revolution, trans. R. E. W. Maddison (London: Methuen, 1973) (orig. La reAolution astronomique, Paris: Hermann, 1969). See also W. Pauli, "The Influence of Archetypal Ideas on the Scientific Theories of Kepler," in C. G. Jung and W. Pauli, The Interpretation of Nature and the Psyche (New York: Pantheon, 1955).

OSIRIS, 2nd series, 1986, 2: 5-42 5



6 DAVID C. LINDBERG

camera obscura and the glass lens. According to Crombie, when Kepler "applied the mechanistic hypothesis to this case he did so as an explicitly strategic decision enabling him . .. to restrict the analysis of vision simply to discovering how the eye operates as an optical instrument like any other." The theme of mechanization has also been addressed by Stephen Straker, who argues that Kepler gave up Robert Grosseteste's metaphysics of light and the notion of light as the "pro- creative paradigm of natural activity," replacing it with a totally passive light that would provide a basis for mechanization of the science of optics.2

Occasionally the faint scent of Neoplatonism slips past the gatekeepers of Kepler's optical reputation, but in general students of Kepler's optics have over- looked possible Neoplatonic influence in order to concentrate on his correct solutions to important problems.3 Can we accept the resulting portrait of a di- chotomized Kepler, who, in turning from astronomy to optics, thrust aside Neo- platonic aims and presuppositions in order to direct his formidable intellect to an array of technical problems, despite the fact that Neoplatonic philosophy had a great deal to say about light? The astronomical Kepler intimately mingled Neo- platonic metaphysics and meticulous concern for technical detail; can the optical Kepler have done less?

To learn the answer we must undertake a two-part investigation. First, we must examine the history of Neoplatonic speculations about light. Neoplatonic light metaphysics has attracted considerable attention from historians, who have produced some useful studies." However, these studies have tended to investigate lofty issues in theology and metaphysics while overlooking the mundane detail that must concern us: for example, they have not generally treated the nature of light or dealt adequately with light as a physical or cosmological agent. Moreover, they have not always done justice to the complexity of the Neopla- tonic tradition; many tributaries emptied into the Neoplatonic stream, giving rise to a variety of opinions and serious doctrinal debates that require careful analysis. From a chronological standpoint, coverage has sometimes been spotty. In

2 Caspar, Kepler, pp. 149-150; Vasco Ronchi, The Nature of Light: An Historical Survey, trans. V. Barocas (London: Heinemann, 1970), p. 87; Alistair Crombie, "The Mechanistic Hypothesis and the Scientific Study of Vision: Some Optical Ideas as a Background to the Invention of the Microscope," in Historical Aspects of Microscopy, ed. S. Bradbury and G. L'E. Turner (Cambridge: Heffer, 1967), p. 54; and Stephen Straker, "Kepler's Optics: A Study in the Development of Seventeenth-Century Natural Philosophy" (Ph.D. diss., Indiana Univ., 1970), p. 423 et passim.

3 For the Neoplatonism, see, e.g., Ernst Mach, The Principles of Physical Optics: An Historical and Philosophical Treatment, trans. John S. Anderson and A. F. A. Young (London: Methuen, 1926), pp. 12-13; on Kepler's solutions to specific theoretical problems, see, in addition to the authors cited in n. 1, Edmund Hoppe, Geschichte der Optik (Leipzig: Weber, 1926), pp. 26-30; Svetlana Alpers, The Art of Describing: Dutch Art in the Seventeenth Century (Chicago: Univ. Chicago Press, 1983), pp. 33-38; and David C. Lindberg, Theories of Vision from al-Kindi to Kepler (Chicago: Univ. Chicago Press, 1976), pp. 185-208. Since no paper or book can deal with all important aspects of its subject, omission of Kepler's metaphysics is not sure evidence of intellectual turpitude but may reflect a strategic decision made in the interests of economy and clarity. However, now that we have scrupulously examined the "enduring" core of Kepler's optics, it is urgent that we investigate the context.

4 For general coverage, see Clemens Baeumker, Witelo: Ein Philosoph und Naturforscher des XIII. Jahrhunderts (Beitrage zur Geschichte der Philosophie des Mittelalters, 3.2) (Munster: Aschen- dorff, 1908), pp. 284-523; Klaus Hedwig, Sphaera lucis: Studien zur Intelligibilitat des Seienden im Kontext der mittelalterlichen Lichtspekulation (Beitrage zur Geschichte der Philosophie und Theo- logie des Mittelalters, N.S., 18) (Mtinster: Aschendorff, 1980); and James McEvoy, "The Meta- physics of Light in the Middle Ages," Philosophical Studies, 1979, 26:126-143. Studies of specific figures are innumerable.



KEPLER'S THEORY OF LIGHT 7

this paper, therefore, we will probe in detail the origins of Neoplatonic thought concerning light, concentrating on those aspects germane to an assessment of Kepler's optical achievement. We will, moreover, trace the development and transmission of these notions through generations of optical practitioners and Neoplatonic philosophers until the eve of Kepler's own work, hoping thus to depict the alternatives as they presented themselves to him.

Having portrayed the tradition, we must attempt to place Kepler within it. We must give Kepler's optics the broadest possible reading, going beyond the geometrical accomplishments on which so much attention has been lavished to consider his opinion on the nature of light and the place occupied by light within his physics, cosmology, and metaphysics-for it is here, if anywhere, that Neopla- tonic influence will lurk. In the course of this investigation, it will become evident that Kepler's optics, like his astronomy, was profoundly inspired and shaped by Neoplatonic thought.

Some may argue that I have actually written two papers-one on Neopla- tonism before Kepler, the other on Kepler's response to the Neoplatonic tradition. Even if this is so, I prefer the papers side by side, under the same title, because both are illuminated by this juxtaposition. It captures the symmetry in Kepler's relationship to the Neoplatonic tradition. Kepler's optical achievement, as I hope to demonstrate, cannot be fully appreciated without a consideration of the Neoplatonic themes that pervade it; equally, our understanding of the tradition of Neoplatonic light metaphysics will remain incomplete unless we explore its continuation and fulfillment in Kepler's optics. This paper is an attempt to balance the two sides of the equation.

I. ARISTOTLE

Despite our interest in Neoplatonism, we must begin with a brief look at Aristotle (384-322 B.C.), who presented an enormously influential theory of the nature of light and established important parts of the conceptual framework within which light would be discussed for two thousand years. Fundamental to Aristotelian metaphysics is the division of substance into matter and form. The tangible indi- viduals that make up the sensible universe are substances. Substance, however, is not simple, but a composite of matter and form. Matter is the indeterminate substrate, itself devoid of properties, which is endowed with form to produce a definite substance. Form, the bearer of properties, must have a substrate, matter, in which to inhere. Note that "matter" and "substance," synonymous today, were not so within the peripatetic tradition.5

This much is very standard Aristotelian metaphysics, upon which we need not dwell. We must, however, inquire briefly where to place corporeality in relation to form, matter, and substance. To be corporeal is to be a corpus or body-that is, to occupy space. Thus matter, devoid of all properties, including quantity, is necessarily incorporeal.6 So also is form, which may bring to matter the property

5For a good recent discussion of the Aristotelian notions of substance, matter, and form, see Abraham Edel, Aristotle and His Philosophy (Chapel Hill: Univ. North Carolina Press, 1982), Chs. 4-8.

6 That matter is devoid of quantity is specifically stated by Aristotle in Metaphysics 8.3, 1029a20- 25.



8 DAVID C. LINDBERG

of quantity but which is not of itself a dimensional thing. It is substance, then, that is corporeal, for substances are the three-dimensional beings that populate our universe. Are substances necessarily corporeal, or are there also incorporeal substances? Here we must proceed cautiously. Aristotle does grant the existence in the superlunary world of nonspatial beings, such as the prime mover and the intelligences that move the planetary spheres.7 Since these are independently existing, nonspatial entities, they are by definition incorporeal substances. Note, however, that they are immaterial as well as incorporeal. Although substances, they are devoid of matter and consist, therefore, of bare form. It may come as something of a surprise to learn that somewhere in the universe are substances that are not composites of matter and form, in apparent violation of Aristotle's own metaphysical principles. Aristotle's successors, looking for a better solution and building on Aristotle's own cryptic remarks about the intelligible matter of mathematical objects, would develop the notion of a spiritual matter that can receive spiritual forms to produce spiritual (and therefore incorporeal) substances.8

With this framework in mind, we are prepared to examine Aristotle's own theory of the nature of light. Aristotle expresses indignation toward the atomistic theory of corporeal emanations, arguing that light "is neither fire, nor in general any body, nor an emanation from any body (for in that case too it would be a body of some kind)."9 Light, for Aristotle, is not a self-existent but a state of something else, not a substance but the actuality of a transparent medium. Now transparent media are substances that partake of the same nature as the upper- most heavenly body, such as air and water. Light is the actualization of this transparency, the achievement of that state in which transparency is no longer potential, but actual, so that bodies separated from the observer by the medium become visible. Such actualization occurs in the presence of fire or some other luminous body. As a state of a medium rather than a local motion, light requires no time for passage, since the entire medium may have its state transformed in an instant.10 The true object of vision, however, is not light but color. Color is a characteristic of the surfaces of bodies, which has the capacity to produce further qualitative change in the actually transparent. Thus the transparent must first be brought to actuality by the presence of a luminous body. This actually transparent is then moved by the colored body to further qualitative change (a "second actuality"), and the latter is transmitted to the observer, who perceives color.

Several conclusions follow from this analysis. First, it seems clear that light is immaterial. To be sure, light is affiliated with matter, for there is a material

W. D. Ross, Aristotle: A Complete Exposition of His Works and Thought (New York: Meridian, 1959), pp. 175-180; and Edel, Aristotle (cit. n. 5), pp. 129-133.

8 Aristotle, Metaphysics 7.10, 1036a2-12; 7.11, 1036b32-1037a5. For a discussion of Aristotle's intent in these passages, see Ross's note in Aristotle, Metaphysics, ed. David Ross, 2 vols. (Oxford: Clarendon, 1924), Vol. II, pp. 199-200; and Thomas Heath, Mathematics in Aristotle (Oxford: Clar- endon, 1949), pp. 213-214. On spiritual matter, see Augustine, De genesi ad litteram 7.5-6; Theodore Crowley, Roger Bacon: The Problem of the Soul in His Philosophical Commentaries (Louvain: Editions de l'Institut Superieur de Philosophie; Dublin: James Duffy, 1950), pp. 81-91; and D. E. Sharp, Franciscan Philosophy at Oxford in the Thirteenth Century (Oxford: Clarendon, 1930), pp. 92-96, 103-105, 247, 262, 390-391.

9 Aristotle, De anima 2.7.418bl4-16, trans. W. S. Hett (London: Heinemann, 1936), p. 105. '0 Aristotle, De sensu 6.446b28-447a3.




substrate of the transparent substance of which light is a state. But surely light is not itself that substrate. Second, it is equally clear that light is not a substance. As a state of the transparent medium, light has no independent existence; without a transparent medium to be actualized, there can be no actualization and consequently no light. Can we infer, further, that light is incorporeal? Here we must proceed very carefully, for this question came to dominate the subsequent analysis of light, and its answer depends on precisely what we mean by "incorporeal." Light is surely not a body and of itself possesses no dimensionality; strictly speaking, therefore, it is incorporeal. Nonetheless, light participates in corporeality by virtue of being a state of a corporeal substance, the transparent. Similarly, it has a secondary or derived dimensionality through the dimensions of the actualized medium. In short, light is corporeal secondarily and by participation, even though not of itself a corpus.

II. PLOTINUS AND THE NEOPLATONIC TRADITION

Twentieth-century physicists may think of light merely as a physical agent representing a narrow band of the electromagnetic spectrum and the object of a narrow scientific specialty. But many of their predecessors regarded light as a central feature of the world-at once a transcendental reality and a physical agent, one of the fundamental principles of cosmogony and epistemology, the source of life and movement, and a powerful theological symbol. This tradition goes back to antiquity, particularly to Plato, who made heavy use of light symbolism in his theory of knowledge and other aspects of his philosophy. Light metaphors also pervade the Bible and patristic literature, largely through Platonic influence.

However, it is within Neoplatonism, as developed by Plotinus (d. 270), that light did fullest philosophical duty. Plotinus developed a metaphysical system based on the principle of emanation or radiation. The source of all being, according to Plotinus, is the transcendent, self-sufficient One, from whom proceeds all lesser being by an overflowing or emanation of its essence, just as rays of light emanate from the sun. Emanation from the One gives rise to nous or mind; further emanation carries one down the scale of being to soul and ultimately to the world of sense experience.1' The image of the One becomes progressively weaker through successive emanations; when we reach matter, we have "an image which has escaped from being and truth," total privation or negation, absolute nonbeing.'2 Matter, then, is both the antithesis of the One and its product through a process of radiation.

If the One is truly self-sufficient, how can it "stir" itself to the production of any secondary being? "What are we to conceive," Plotinus inquires, "as rising in the neighbourhood of that immobility [the One]?" He replies that "it must be a circumradiation-produced from the Supreme [the One] but from the Supreme

11 On the Stoic sources of Plotinus's doctrine, see A. H. Armstrong, " 'Emanation' in Plotinus," Mind, N.S., 1937, 46:62-64; Armstrong, The Architecture of the Intelligible Universe in the Philos- ophy of Plotinus: An Analytical and Historical Study (Cambridge: Cambridge Univ. Press, 1940), pp. 53-54; on nous, see ibid., Chs. 4-5.

12 Plotinus, Enneads 2.4.15, trans. A. H. Armstrong, 3 vols. (London: Heinemann, 1966-67), Vol. II, p. 147; on matter, see also Enneads 2.4 and 3.6 and Armstrong's synopses at the beginning of each chapter.



10 DAVID C. LINDBERG

unaltering-and may be compared to the brilliant light encircling the sun and ceaselessly generated from that unchanging substance." Plotinus has thus far been discoursing on a metaphysical plane, about the source of being. But at this point, in order to elucidate the process by which being emanates from the One, he appeals to an analogy from the physical realm:

All existences, as long as they retain their character, produce-about themselves, from their essence, in virtue of the power which must be in them-some necessary outward-facing hypostasis continuously attached to them and representing in image the engendering archetypes: thus fire gives out its heat; snow is cold not merely to itself; fragrant substances are a notable instance, for, as long as they last, something is diffused from them and perceived wherever they are present. Again, all that is fully achieved engenders; therefore the eternally achieved engenders eternally an eternal being. 1'

This passage conveys two important truths. First, it states a principle of universal activity: everything that exists produces an image or likeness of itself, which it directs into its surroundings. This idea would become one of the hall- marks of Neoplatonic emanationism. Second, Plotinus proclaims a unity between the physical and metaphysical realms. The visible is the image of the invisible and the invisible is equally the archetype of the visible; both obey the law of emanation. Visible light thus acquires a place of special importance in Plotinian philosophy; as the instance of emanation most accessible to the senses, light inevitably becomes the paradigm case, the door to an understanding of the universal principle of emanation. Let us, then, take a close look at Plotinus's theory of the nature of visible light.

Plotinus begins with an analogy. Discussing beauty in the first Ennead, he asks how an architect is able to judge a house to be beautiful. The answer is that the corporeal house is the incorporeal idea of a house stamped upon external matter. "When sense-perception, then, sees the form in bodies binding and mastering the nature opposed to it, which is shapeless, . . . it gathers into one that which appears dispersed and brings it back and takes it in, now without parts, to the soul's interior and presents it to that which is within as something in tune with it and fitting it and dear to it." Just as the house represents the mastery of matter by the incorporeal form in the architect's mind, so color results from the mastery of matter by the incorporeal form that we call "light": "And the simple beauty of colour comes about by shape and the mastery of darkness in matter by the presence of light which is incorporeal and formative power and form."1'4 This state-

13 Plotinus, Enneads 5.1.6, trans. Stephen MacKenna, rev. B. S. Page (2nd ed., London: Faber & Faber, 1956), p. 374, with altered paragraphing and punctuation. I have been forced to rely on a myriad of different editions and translations of Plotinus's Enneads because Armstrong's excellent translation currently extends only through the third Ennead, MacKenna's translation is always florid and occasionally incomprehensible, and both this and Guthrie's translation (see n. 14) are frequently awkward. I have therefore chosen from among the various translations and, when necessary, introduced corrections and revisions.

14 Plotinus, Enneads 1.6.3, trans. Armstrong, Vol. I, p. 241. Plotinus proceeds to identify fire too as form in relation to the other elements and notes that "it has colour primarily and all other things take the form of colour from it." Armstrong has argued that this association of light with fire, the least substantial of bodies-both given the status of color-producing form-gives "light a very special status on the frontier of spirit and matter." This, Armstrong thinks, was an early and naive tumble into the doctrinal borderland between Stoicism and Neoplatonism that was not to be repeated in Plotinus's philosophical works; Architecture of the Intelligible Universe (cit. n. 11), p. 55. However,




ment was to prove enormously influential, giving rise to the idea of light as the form of all corporeal substance, which became a major theme in the work of later writers.

Plotinus returns to light in the fourth Ennead, now employing the Aristotelian notion of activity or actualization (energeia). Light, he argues, is the actualization of a luminous body-an actualization that has the capacity to radiate itself outward into its surroundings:

On the one hand, there is in the luminous body an actualization, a kind of super- abundant life, a principle and source of activity; on the other hand, beyond the limits of the luminous body exists a second actualization characteristic of this body, and which never separates itself from the body. Every being has an actualization which is its image; so that, as soon as the being exists, its actualization exists also; and so long as the being subsists, its actualization radiates nearer or further.... Thus the light that emanates from bodies is the actualization of the luminous body which is active exteriorly.

Plotinus then adds, returning to the idea of form, that "the light in the bodies whose original nature is such, is the formal being of the originally luminous body. When such a body has been mingled with matter, it produces color."'15 Thus in every luminous body there is a form (eidos) or actualization (energeia) that is the source of the body's activity, sending forth its power or image into the surroundings.

Several points made in the last two paragraphs require clarification: first, the claim that light is incorporeal; second, the idea of two actualizations or lights, the one internal to the luminous body, the other radiating outside it. Both claims are clarified in a passage from the second Ennead on the fiery nature of the sun. After quoting Plato on the origin of the sun, Plotinus continues: "By fire he [Plato] does not mean either of the other kinds of fire but the light which he says is other than flame, and only gently warm. This light is a body, but another light shines from it which has the same name, which we teach is incorporeal. This is given from that first light, shining out from it as its flower and splendour."16 The first light is light within the luminous body, representing its form or actualization; this light is held to be corporeal. The other light, offspring and image of the first, is light radiating spherically outward from the luminous body; Plotinus affirms this second light to be incorporeal.

How are we to understand the corporeality of Plotinus's first light? How can the form of a luminous body-how can the form of anything-be corporeal? Indeed, when Plotinus referred earlier to light as "incorporeal and formative power and form," did he not have this first light in mind? The answer is found in the distinction, which we have already seen in Aristotle, between intrinsic and derived corporeality. Forms of themselves are nondimensional and are therefore, strictly speaking, incorporeal. But the form of a corporeal body has secondary or derived corporeality. Just as Aristotelian light has corporeal being by virtue of its

Plotinus expresses the same idea in Enneads 4.5.7, written later in his career (quoted in the next paragraph). On the dating of various portions of the Enneads, see Armstrong's note, Enneads, Vol. I, p. 231, and the ordering in Plotinus, Complete Works, trans. Kenneth S. Guthrie, 4 vols. (Alpine, N.J.: Platonist Press, 1918).

15 Plotinus, Enneads 4.5.7, trans. Guthrie, Vol. II, pp. 526-527, with a change in punctuation. 16 Plotinus, Enneads, 2.1.7, trans. Armstrong, Vol. II, pp. 29-31.




presence in the transparent medium, so Plotinian first light is corporeal by virtue of its bodily existence in the sun or other luminous body; it participates in corporeality through its host.

What, then, are we to make of the incorporeality of Plotinus's second or radiating light? This is, first of all, a denial of the atomistic theory that light represents the flow of corpuscles from the luminous body.'7 More important for our purposes, it is also an emphatic rejection of the Aristotelian view that radiating light has corporeal existence as a state or property of a corporeal medium. Plo- tinus does not believe that a corporeal medium is necessary for the radiation of light: "If [light] is a quality, some quality of some substance, then light, equally with other qualities, will need a body in which to lodge: if, on the contrary, it is an activity rising from something else [Plotinus's view], we can surely conceive it existing, though there be no neighbouring body but (should it be possible) a blank void which it will overleap and so appear on the farther side." Radiating light employs no medium because "no other object can possess it. . . . Light therefore is not a modification of the air, but a self-existent in whose path air happens to be present." The actualization that is light goes forth from the luminous body, leaps over the intervening space, and actualizes a suitable recipient, bringing forth color or a mirror image according to the nature of the recipient; the actualization "must be thought of as being lodged, both in the active and powerful source and in the point at which it settles."' 8 It is never lodged in the medium.

III. THE NEOPLATONIC TRADITION OF THE MIDDLE AGES

The influence of Plotinus was powerful and widespread. A succession of followers developed and systematized his teaching, disseminating it in both Chris- tian and non-Christian circles. This Plotinian legacy can be seen in Saint Augus- tine (354-430), who made extensive use of light metaphors, though on the nature of light he adopted the Stoic position, treating sensible light as the most subtle of the elements-a corporeal substance and yet the corporeal substance most nearly akin to the incorporeal. 19 Neoplatonic books, including an abridgement of a portion of Plotinus's Enneads (which circulated as The Theology of Aristotle, thereby introducing enormous confusion into many questions, including the nature of light), were translated into Arabic and gave rise to a Neoplatonic tradition in Islam. One of the early members of this tradition was al-Kindi (d. ca. 873), known as "the first philosopher of the Arabs." Neoplatonic metaphysics per- vades al-Kindi's philosophy: being is arranged into a hierarchy of perfection, the One is portrayed as the totally self-contained and unmoved first cause, and soul is explained as an emanation from the divine being analogous to the radiation of light from the sun.20

17 Against the atomistic theory, Plotinus argues that a visible object gives rise to its image in a mirror "without letting any of its substance escape by wastage" (Enneads 4.5.7, trans. Guthrie, Vol. II, p. 528).

18 Plotinus, Enneads 4.5.6, trans. MacKenna, pp. 334-335 (with some slight revision). 19 For Augustine, see Frangois-Joseph Thonnard, "La notion de lumiere en philosophie augusti- nienne," Recherches augustiniennes, 1962, 2:125-175; and Roger Bacon's Philosophy of Nature: A Critical Edition, with English Translation, Introduction, and Notes, of De multiplicatione specierum and De speculis comburentibus, by David C. Lindberg (Oxford: Clarendon Press, 1983), pp. xxxix- xli. For Augustine's theory of the nature of light, see his De genesi ad litteram 7.15, 7.19, 12.16; and De libero arbitrio 3.5.16.

20 This discussion of al-Kind! parallels that in Bacon's Philosophy of Nature, ed. Lindberg, pp.




But the portion of al-Kindi's philosophy that we must trace, because of its influence on subsequent emanationist doctrines, is his astrology, which was embodied in a work later translated into Latin as De radiis or Theorica artium magicarum. Al-Kind! believed that each star sends radiation into the terrestrial region, radiation that varies according to the nature and position of the star, the precise mode of radiation, and the combined effect of radiation from different stars. The astrologer must also take into account the motions of the stars and planets and the condition of the recipient matter.

This much is standard astrological doctrine. But al-Kindi presses further, probably under the influence of Plotinus, to argue that not only do stars radiate force, but "everything in this world, whether substance or accident, produces rays in the manner of stars." Such radiation proceeds in all directions, "so that every place in the world contains rays from everything that has actual existence," and thereby each thing in the sublunary world acts on all other things.21 Al-Kind! devotes the remainder of his treatise to examples and applications of this doctrine. He argues that rays of fire transmit heat, while the earth's rays transmit cold; medicines, taken either internally or externally, diffuse their rays through the patient's body; magnets attract iron through their radiation; bodies in colli- sion send forth rays that convey sound; even images in the mind radiate. Al- Kind! also deals at length with the rays produced by words, thus explaining the efficacy of incantations and prayers. Finally, he discusses the rays issuing from images and figures and explains the operation of ritual sacrifice. Certain features of De radiis (its determinism and its analysis of prayer) were obviously heretical from a Christian standpoint and had little or no long-term influence in the West. Its more lasting contribution was to teach certain medieval scholars, including Robert Grosseteste and Roger Bacon, that every creature in the universe is a source of radiation and the universe a vast network of forces.

Al-Kindi also wrote another treatise, De aspectibus, in which he investigated the mathematics and physics of radiation. Here he argues that rays are three- dimensional bodies, rather than the one-dimensional lines of Euclidean optics, and indeed that radiation issuing from a source forms a single continuous radiant body. As for the nature of this radiant body, al-Kindi's conclusion harks back to Aristotle (through Galen). He repeatedly refers to light as an "impression" on the medium; a ray of light, he argues, is "the impression of luminous bodies in dark bodies, denoted by the name 'light' because of the alteration of accidents produced in the bodies receiving the impression."22 Al-Kindi thus endows radiating light with a corporeality that it did not have for Plotinus: whereas the second light of Plotinus "leaps over" the medium to actualize a recipient beyond,

xliv-xlvi. On al-Kindi, see also cAbdurra'man Badawi, Histoire de la philosophie en Islam, 2 vols. (Paris: Vrin, 1972), Vol. II, pp. 385-477; and Majid Fakhry, A History of Islamic Philosophy (New York: Columbia Univ. Press, 1970), pp. 82-112.

21 "Al-Kindi, De radiis," ed. M.-Th. d'Alverny and F. Hudry, Archives d'histoire doctrinale et litteraire du moyen age, 1974, 41:139-260, on 224, 228. For summaries of De radiis, see Lynn Thorn- dike, A History of Magic and Experimental Science, 8 vols. (New York: Columbia Univ. Press, 1923-1956), Vol. I, pp. 642-646; and Graziella Federici Vescovini, Studi sulla prospettiva medievale (Turin: Giappichelli, 1965), pp. 44-47.

22 Al-Kindi, De aspectibus, Prop. 11, in "Alkindi, Tideus and Pseudo-Euklid: Drei optische Werke," ed. Axel Anthon Bjornbo and Sebastian Vogl, Abhandlungen zur Geschichte der mathema- tischen Wissenschaften, 1912, 26(3):1-176, on p. 13; see also Lindberg, Theories of Vision, pp. 24-26, 30-31, and Ch. 2 generally.




al-Kindi's (like Aristotle's) is a quality embodied in the medium. If for Plotinus the medium is a hindrance, for al-Kindi it is a necessity.23

Emanationist doctrine continued to flourish in the Islamic world. One recipient of this doctrine who had a wide influence in medieval Latin Christendom was the Spanish Jew Avicebron (d. ca. 1058). In his Fons vitae, written in Arabic and later translated into Latin, Avicebron discusses the metaphysics of emanation:

The First Maker, sublime and holy, liberally bestows His inner being, for all that is flows from Him. And since the First Maker bestows the form that is within Him, consequently there is nothing to prevent it from flowing forth; and thus He is the source who preserves all that is, embracing and comprehending it. Therefore, it is necessary for all substances to obey His action and imitate him in conferring their forms and bestowing their forces, as long as they encounter suitable recipient matter.... In sum, the first emanation, which embraces all substances, compels all other substances to emanate into one another. And as an example of this, consider the sun, which does not emanate of itself nor bestow its rays except by reason of the fact that it falls under the first emanation and obeys it.24

All substances emanate their forms in obedience to a universal law of emanation. Neoplatonism reached the Latin West by many routes. Augustine's writings,

composed in Latin, were influential throughout the Middle Ages. Other sources were translated into Latin: the works of Pseudo-Dionysius from the Greek; the Liber de causis of Proclus, al-Kindi's De radiis and De aspectibus, Avicebron's Fons vitae, and Avicenna's Shifa from the Arabic. But there were also rival philosophical traditions in the West. Platonism in its original form was present in Plato's Timaeus, translated early in the Middle Ages; the physical and metaphysical works of Aristotle, newly available by the end of the twelfth century, exerted a powerful influence; and Christian theology was, of course, omnipresent. Fi- nally, there were technical treatises on a wide variety of subjects, including geometrical optics. The attempt to reconcile these diverse materials gave rise to a variety of philosophical products, including a Neoplatonized Aristotelianism within which the doctrine of emanation and other aspects of Plotinian light metaphysics remained potent elements.25

One of the founders of the Western tradition of Neoplatonized Aristotelianism and a central figure in the intellectual life of the thirteenth century was Robert Grosseteste (ca. 1168-1253). Grosseteste adopted as much of the Neoplatonic emanationist metaphysics as he could without falling into the heresy of pan- theism, and light therefore permeates his philosophy as symbol and analogue. In De ordine emanandi causatorum a Deo he points out that "the Father is the cause of the Son, and the Son emanates from the Father." An emanationist metaphysics is again evident in De luce, where it is argued that "every higher body, in virtue of the light which proceeds from it, is the form and perfection of the body that comes after it. And just as unity is potentially every number that comes after it, so the first body through the multiplication of light is every body

23 Note that al-Kindi's radiating light has the same kind of derived corporeality in the medium as Plotinus's first light has in the luminous body.

24 Avencebrolis fons vitae, 3.13, ed. Clemens Baeumker (Beitrage zur Geschichte der Philosophie des Mittelalters, 1.2) (Munster: Aschendorff, 1892), pp. 107-108.

25 There was no purely (or even predominantly) Platonic or Neoplatonic tradition in medieval Latin Christendom from the thirteenth century to the end of the fifteenth. When one speaks of Platonism or Neoplatonism during this period, Neoplatonized Aristotelianism is always meant.




that comes after it."26 Yet Grosseteste holds firmly that God does not create through intermediaries, by a successive overflowing of essences; rather, all things emanate directly from the divine being. James McEvoy, who has bril- liantly analyzed Grosseteste's light metaphysics, summarizes as follows:

God is the very substance and essence of spiritual light, and no light of the spirit can shine save by participation in the uncreated light. Light streaming from a source tends to form a hierarchy of diminishing power, and the created lights in their varying degrees of participation imitate the nature of the source of light, each shining or reflecting upon the next the light which it has itself received from above. The guiding principle of the whole conception is that of unity. There is a single infinite fountain of light, whose unity remains perfect even when its ray is participated in by many. The generation and extension of light downwards from this spiritual sun leaves it in itself unchanged, undivided, and still transcendent to the hierarchy formed by its effusion. Although totally present to all it shines upon, it loses nothing of its interiority in its outgoing. Since all light comes from one single source, downwards from the Father of Lights, the work of the assimilation and transmission of light within the grades of the hierarchy, down even to the lowest, is as much the activity of the lux suprema as of the lower beings themselves.27

Light is prominent not only in Grosseteste's theology and metaphysics but also in his cosmogony and physics, where he addresses the visible world. In De luce Grosseteste develops an elaborate cosmogonical scheme that aims to reconcile Neoplatonic emanationism with the Biblical account of ex nihilo creation-all, of course, within a broadly Aristotelian framework. Grosseteste's cosmogony has been frequently and eloquently expounded; let us look briefly at the main lines of the argument.28 In the beginning, God created first matter, an altogether dimensionless entity, and first form, a dimensionless point of light. In Grosseteste's identification of light as first form, Plotinian influence is already apparent. The point of light instantaneously diffused itself into a sphere, drawing matter with it (since matter and form are inseparable except in thought), thus bringing dimension to matter and giving rise to body. Light is thus the first form of corporeity or the first corporeal form:

The first corporeal form, which some call corporeity, is in my opinion light [lux]. For light of its very nature diffuses itself in every direction in such a way that a point of light will produce instantaneously a sphere of light of any size whatsoever, unless some opaque body stands in the way. Now the extension of matter in three dimensions is a necessary concomitant of corporeity, and this despite the fact that both corporeity and matter are in themselves simple substances lacking all dimension. But a form that is in itself simple and without dimension could not introduce dimension in every direction into matter, which is likewise simple and without dimension, except by multiplying itself and diffusing itself instantaneously in every direction and thus extending matter in its own diffusion. For the form cannot desert matter, because it is inseparable from it, and matter itself cannot be deprived of form.

26 Robert Grosseteste, De ordine emanandi causatorium a Deo, in Die philosophischen Werke des Robert Grosseteste, ed. Ludwig Baur (Beitrage zur Geschichte der Philosophie des Mittelalters, 9) (Munster: Aschendorff, 1912), p. 147; and Grosseteste, De luce, ibid., p. 6, quoting from Grosseteste, On Light, trans. Clare C. Riedl (Milwaukee: Marquette Univ. Press, 1942), p. 15.

27 James McEvoy, The Philosophy of Robert Grosseteste (Oxford: Clarendon, 1982), pp. 65-67, 136.

28 The best account is that of McEvoy, ibid., pp. 151-158; see also Lindberg, Bacon's Philosophy of Nature (cit. n. 19), pp. xlix-lii.




The result of this multiplication was a great sphere of simple body, the basis of the corporeal universe:

Thus light, which is the first form created in first matter, multiplied itself by its very nature an infinite number of times on all sides and spread itself out uniformly in every direction. In this way it proceeded in the beginning to extend matter, which it could not leave behind, by drawing it out along with itself into a mass the size of the material universe.29

The spherical diffusion of light came to an end when the outermost parts reached the limit of rarefaction, thus fully actualizing the potentiality of matter and giving rise to the firmament or outermost sphere (a perfect body "because it has nothing in its composition but first matter and first form"). The firmament then radiated its light (lumen) inward, since "light is the perfection of the first body and naturally multiplies itself from the first body."30 In the return of light toward the center of the universe, additional rarefaction was produced, for light always carries matter with it. Successive celestial spheres were thus produced, each rarefied to the maximum degree and thus perfect in that it lacked potentiality for any additional change, while the lower region acquired greater density through the influx of additional matter. This region of great density was differen- tiated into the four elements of the terrestrial region, which were subject to further condensation and rarefaction, and thus to change, because of their incomplete actualization.

One of the most striking features of this cosmogonical scheme is its firm state- ment of the unity of the cosmos. To be sure, the celestial-terrestrial dichotomy of Aristotle is still apparent in the claim of celestial perfection (total actualization through maximum rarefaction) and terrestrial imperfection (the potentiality of incomplete rarefaction). But underlying the differences is a fundamental unity, based on light as first corporeal form. All of the bodies of the universe, celestial and terrestrial alike, are the products of radiating light: "the form and perfection of all bodies is light, but in the higher bodies it is more spiritual and simple, whereas in the lower bodies it is more corporeal and multiplied."'31 Moreover, through the unity based on light, motion is engendered in all bodies of the universe: "But since lower bodies participate in the form [first light] of higher bodies, the lower body by participation in the same form as the higher body receives motion from the same incorporeal motive power as does the higher body."32 One feels almost that the difference between heaven and earth is quan- titative rather than qualitative, for terrestrial elements are denied the diurnal motion of the heavens only because the "light in them is impure, weak, and far removed from the purity which it has in the first body."33

Finally, in the mundane world light acts as the universal agent of causation. Grosseteste adopts the essentials of al-Kindi's claim that everything acts on

29 Grosseteste, Philosophischen Werke, ed. Baur, pp. 51-52; quoting from On Light, trans. Riedl, pp. 10-11, with altered punctuation (both cit. n. 26).

30 Grosseteste, On Light, trans. Riedl, p. 13; Philosophischen Werke, ed. Baur, p. 54. 31 Grosseteste, On Light, trans. Riedl, p. 15; Philosophischen Werke, ed. Baur, pp. 56-57. 32 Grosseteste, Philosophischen Werke, ed. Baur, p. 57; cf. On Light, trans. Riedl, p. 16, from

which I have borrowed some wording. 33 Grosseteste, On Light, trans. Riedl, p. 16; Philosophischen Werke, ed. Baur, p. 57. On light as

the principle of unity in the cosmos, see McEvoy, Philosophy of Grosseteste, pp. 156-158.




everything else through the radiation of force or light: "A natural agent multiplies its power from itself to the recipient, whether it acts on sense or on matter. This power is sometimes called species, sometimes a likeness, and it is the same thing whatever it may be called, and the agent sends the same power into sense and into matter, or into its contrary, as heat sends the same thing into the sense of touch and into a cold body." That such radiations are responsible for all causation can be inferred from another passage in the same work, where Grosseteste states that "all causes of natural effects must be expressed by means of lines, angles, and figures, for otherwise it is impossible to grasp their explanation." And in De natura locorum he writes of the geometrical rules governing the propagation of light: "These rules, foundations, and fundamentals having been given by the power of geometry, the diligent investigator of natural things can in this manner specify the causes of all natural effects. And he can do this in no other way."34 If all natural effects are expressible in terms of lines, angles, and figures, it is because all natural effects are reducible to the radiation of light.

Many features of Grosseteste's philosophy of light reappeared in the works of the influential Franciscan (eventually minister general of the Franciscan Order) Saint Bonaventure (ca. 1217-1274). Light was one of the central realities of Bon- aventure's world. As for Grosseteste, it was the first corporeal form, the source of all corporeal existence and activity. It served cosmogonical functions, as God's principal instrument in the creation; indeed, Bonaventure associated six different illuminations with the six days of creation. All knowledge, Bonaventure held, comes by divine illumination. Visual sensation, of course, occurs through the radiation of light; all other forms of sensation occur through an analogous multiplication of similitudes. Light is also responsible for the linkage between body and soul.35 Even the Son descended from the Father by a process of emanation resembling that by which the object of sense sends its similitude into the sense organ:

A sense object can stimulate a cognitive faculty only through the medium of a similitude which proceeds from the object as an offspring from its parent.... In like manner, know that from the mind of the Most High, . . . from all eternity there emanated a Similitude, an Image, and an Offspring; and afterwards ... He was united to a mind and a body and assumed the form of a man.36

-In the beginning, according to Bonaventure, God created the empyreum (the outermost celestial sphere), from which light radiated instantaneously in all directions. Insofar as it was spiritual in nature, this light brought angelic being into existence. Insofar as it was corporeal, it conferred extension and gave rise to body. Light is thus the first form of every corporeal substance; as pure activity, it endows corporeal substances with their active powers; according to their

34 Robert Grosseteste, De lineis, angulis, et figuris, quoted from my translation in A Source Book in Medieval Science, ed. Edward Grant (Cambridge, Mass.: Harvard Univ. Press, 1974), pp. 385-386, 385; and Grosseteste, De natura locorum, in Philosophischen Werke, ed. Baur, pp. 65-66.

35 Saint Bonaventure's De reductione artium ad theologiam: A Commentary with an Introduction and Translation, ed. and trans. Sister Emma Therese Healy, Vol. I of Works of Saint Bonaventure, ed. Philotheus Boehner and Sister M. Frances Laughlin (Saint Bonaventure, N.Y.: Franciscan Insti- tute, 1955), pp. 46-53, 61-68; and Joseph Anthony Mazzeo, Medieval Cultural Tradition in Dante's Comedy (Ithaca, N.Y.: Cornell Univ. Press, 1960), pp. 73-76. On Bonaventure's philosophy of light, see also Hedwig, Sphaera lucis (cit. n. 4), pp. 161-173.

36 Bonaventure, De reductione artium, trans. Healy, pp. 29-31.




degree of participation in the form of light, all bodies in the world can be hierar- chically arranged. The affinities between the cosmogonies of Bonaventure and Grosseteste are apparent, though it should be noted that whereas creation for Grosseteste began with a central point of light instantly emanating outward to form a sphere, for Bonaventure it began with the spherical empyreum sending its light inward.37

Bonaventure draws a distinction between spiritual light and corporeal light, admitting an analogical relationship but no more.38 He discourses at some length on the corporeality of cosmic light, presenting arguments on both sides of the question. He concludes from a variety of considerations that the cosmic light of the first day of creation, the first corporeal form by which the material universe came into being, is itself corporeal. As the argument unfolds, however, it becomes clear that Bonaventure has only secondary or derived corporeality in mind. Light is not body, but the form of body. Aristotle teaches that form is the principle of activity; since light "is the most active of all corporeal things," it must be form. Moreover, every body is a composite of form and matter; therefore, if light is form, it cannot also be body. Finally, if light were body, since it has the capacity of multiplying itself, then "some body could multiply itself outside itself without the presence of other matter, which is impossible for any created thing."39

But is light substantial or accidental form? Bonaventure reports arguments for both sides of the question. He then ingeniously resolves the issue by accepting both conclusions. What made this possible was a distinction between two kinds of light that had been formulated by Avicenna. In his De anima (a portion of his Shifa) Avicenna had distinguished between the luminous quality of the sun or fire (lux in the Latin translation of the work) and the splendor or effect of lux on the surrounding medium (translated into Latin as lumen). In the West, Grosseteste carried the distinction between lux and lumen in a somewhat different direction, identifying lux with first corporeal form, emanating from its punctiform origin into a great cosmic sphere, and lumen with the light emanating inwardly from the outermost sphere-though Grosseteste did manage to retain the notion that lux is the primary light and lumen its offspring.40 Bonaventure grasps the distinction in its original Avicennan form, arguing that "Light [lux] can be understood in two ways: in one sense, it is the very form that gives being to a lucid body and by which, principally, a luminous body is active ... ; in the other sense light is the fulgor surrounding the luminous body and resulting from the existence of light in such matter, and this light is the object of sense and the instrument of activity." Light in the former sense is substantial form; in the latter sense it is accidental form. Here Bonaventure employs the term lux equivocally to cover both senses;

37 Bonaventure, Super quatuor libros sententiarum 2, Dist. 13, Art. 2, Quest. 2, Opera omnia, ed. A. C. Peltier, 15 vols. (Paris: Vives, 1864-1871), Vol. II, pp. 553, 538-539; and Bonaventure, De reductione artium, trans. Healy, pp. 45-47.

38 Mazzeo, Medieval Cultural Tradition (cit. n. 35), pp. 76-77; and Gertrude Kelly Hamilton, "Three Worlds of Light: The Philosophy of Light in Marsilio Ficino, Thomas Vaughan, and Henry Vaughan" (Ph.D. diss., Univ. Rochester, 1974), p. 65.

39 Bonaventure, Super quatuor libros sententiarum 2, Dist. 13, Art. 1, Quest. 1, Opera omnia, ed. Peltier, Vol. II, pp. 545-548; and ibid., Art. 2, Quest. 1, p. 552.

40 Grosseteste, Philosophischen Werke, ed. Baur, pp. 55-56, quoting from On Light, trans. Riedl, pp. 13-14. On this distinction, see David C. Lindberg, "The Science of Optics," in Science in the Middle Ages, ed. Lindberg (Chicago: Univ. Chicago Press, 1978), pp. 356-357.




subsequently he substitutes lumen for light as accidental form. Lumen, he proceeds to argue, is not body, but a corporeal form or accidental quality that passes through a medium instantaneously by a process of "generation and diffusion." "Since lumen is a corporeal form," he notes, "it requires a medium to convey it."'41

We have seen that Grosseteste and Bonaventure, while adopting large portions of Neoplatonic light metaphysics, including the notion of light as the form of a luminous body, firmly rejected Plotinus's interpretation of radiating light as an incorporeal entity needing no medium. They preferred the opinion of Aristotle and al-Kindi, which made radiating light a qualification of the medium-in their view, corporeal form. Roger Bacon (ca. 1220-ca. 1292), Grosseteste's follower and Bonaventure's contemporary, adopted a similar position.42 Bacon never developed a full metaphysics of light; rather, he took the physics of light as expressed by al-Kindi and Grosseteste-the claim that radiation is the universal instrument of natural causation-and developed it into a systematic doctrine. In De multiplications specierum, Bacon painstakingly explored the physical details of this emanation-which he called the multiplication of species. In the course of writing De multiplications specierum, Bacon gave the nature of radiating light a more extended analysis than it had ever before received.

The term species goes back to classical Latin, with the meaning "aspect" or "appearance." Augustine employed it to denote the likeness in the senses and intellect of a perceived object, and this usage remained current throughout the Middle Ages. With Grosseteste and Bacon, however, the meaning of the term was broadened to denote the likeness emanating from an object, whether or not a percipient being is present to receive it. It became the force or power by which any object acts on its surroundings, a synonym for al-Kindi's universal force. Bacon points out that it has different names in different contexts: form, image, similitude, species, idol, phantasm, simulacrum, intention, virtue, passion, and impression. Whatever its name, it

is the first effect of an agent; for all judge that through species [all] other effects are produced. Thus the wise and the foolish disagree about many things in their knowledge of species, but they agree in this, that the agent sends forth a species into the matter of the recipient, so that, through the species first produced, it can bring forth out of the potentiality of the matter [of the recipient] the complete effect that it intends.41

We need not follow Bacon's exploration of the nature of species and their multiplication in all of its detail." A few claims, however, require our attention. Bacon argues that a species resembles the agent from which it issued-in nature, definition, specific essence, and operation-and that by its action it converts the

41 Bonaventure, Super quatuor libros sententiarum 2, Dist. 13, Art. 2, Quest. 2-Art. 3, Quest. 1, Opera omnia, ed. Peltier (cit. n. 37), Vol. II, 555-559, quoting from pp. 555, 558, 559.

42 On Bacon's sources, see Bacon's Philosophy of Nature, ed. Lindberg (cit. n. 19), p. liv; and Lindberg, Theories of Vision (cit. n. 3), pp. 107-109. To admit Grosseteste's influence on Bacon is not to grant that Bacon actually studied with Grosseteste.

43 Bacon, De multiplications specierum 1.1.42-69, 75-80, trans. Lindberg (cit. n. 19), pp. 4-7. On the term species see Pierre Michaud-Quantin, Etudes sur le vocabulaire philosophique du moyen aige, with Michael Lemoine (Rome: Edizioni dell'Ateneo, 1970), pp. 113-150, on p. 113; and Bacon's Philosophy of Nature, ed. Lindberg, pp. liv-lv.

44 For a detailed account, see Bacon's Philosophy of Nature, ed. Lindberg, pp. liii-lxxi.




recipient into the likeness of the agent. Lumen, for example, is the species of lux, and its effect is to convert the recipient medium into the likeness of the parent lux. Bacon insists not only that celestial things influence the terrestrial realm through their species, but that terrestrial things act reciprocally on the heavens. Bacon thus joins Grosseteste in affirming the physical unity of the cosmos.

The question that must principally concern us is the nature of light or species and the mode of its generation and multiplication. Bacon considers five possible modes of generation and dismisses all but one. The most significant of those dismissed is that which maintains that something (either substance or accident) actually departs from the agent; this, he claims, is impossible because "neither accident nor a piece of substance can depart from a subject without corruption of the whole substance," and the most active producers of species are spiritual and celestial substances, which are incorruptible. After further discussion, Bacon concludes that species are generated by a "bringing forth out of the active potentiality of the recipient matter."45 Moreover, this occurs only by direct contact between agent and recipient. Thus an agent produces its species in the part of the medium immediately adjacent to itself; that first part of the medium then acts as agent, replicating itself in the immediately adjacent second part of the medium, and so forth until resistance of the medium or loss due to secondary radiation brings the process to an end. Bacon summarizes in his Opus maius:

But a species is not body, nor is it moved as a whole from one place to another; but that which is produced [by an agent] in the first part of the air [or other medium] is not separated from that part, since form cannot be separated from the matter in which it inheres unless it should be mind; rather, it produces a likeness to itself in the second part of the air, and so on. Therefore, there is no change of place, but a generation multiplied through the different parts of the medium; nor is it body which is generated there, but a corporeal form that does not have dimensions of itself but is produced according to the dimensions of the air; and it is not produced by a flow from the luminous body, but by a drawing forth out of the potentiality of the matter of the air.46

What is the significance of Bacon's position? It is important to see that he allies himself with Aristotle and al-Kindi and Grosseteste, against Plotinus, on the relationship of light to the medium. All, of course, reject the atomistic view that light is body issuing from a luminous source. But whereas Plotinus also rejected any connection between light and material media, Bacon followed Aris- totle in making light an actualization of potentialities present in the medium; without a medium, there is no propagation of light. While maintaining that species are not themselves bodies (we know this because species entering a medium, which for Bacon is a plenum, do not cause its sides to bulge outward), Bacon insists that species are embodied in the medium and acquire dimensionality through it. They play corporeal form to the matter of the medium. "There- fore," Bacon writes, "since the medium is the material cause, in which and from the potentiality of which a species is generated by the agent and generator, this species cannot have a corporeal nature distinct from the medium." This is evident because an effect, whether complete or incomplete, "does not have a new corporeal dimension, but that which belongs to the medium or body in which [it]

45 Bacon, De multiplicatione specierum 1.3.6-8, 52, trans. Lindberg, pp. 44-45, 46-47. 46 The Opus Majus of Roger Bacon, ed. John Henry Bridges, 3 vols. (London: Williams & Nor-

gate, 1900), Vol. II, pp. 71-72.




is generated." Bacon also rejects the ancient distinction between corporeality and materiality. He can see no difference between the terms and informs us that he will employ them interchangeably.47

Does it follow that the species of corporeal things are corporeal (that is, material)? Unequivocally yes! The species of a corporeal agent must resemble the agent that produced it in nature, definition, and specific essence. Moreover, such a species is "neither soul nor intelligence nor the first cause; and everything else is truly corporeal." Therefore, since the opinion that species are incorporeal "cannot be saved by any rational judgment, nor does it have any probability, as is evident to any man who wishes to dismiss the foolishness of the vulgar and to follow reason, I therefore state unconditionally that the species of a corporeal thing is truly corporeal and has truly corporeal being."48

If the species of a corporeal agent is necessarily corporeal, can it nonetheless have a spiritual mode of existence in the medium? Absolutely not, since the mode of existence must agree with the essence having that mode. Furthermore, the mode of existence in a recipient must be consistent with the mode of the recipient: "since the corporeal medium receiving the species . . . has entirely corporeal being, this species can have only corporeal being."49 But several ob- jections must be dealt with. People have been disposed to attribute immaterial or "spiritual" being to species in a medium because of the need to explain why radiating species can intersect (as when two or more rays of light pass through a single aperture) without interference; for if the species were truly material, it would seem that intersection should result in mixing and loss of individual identity. Bacon's reply, an ingenious dodge, is that although the species truly mix, nonetheless from the point of mixture to the observer's eye come one species that continues its original line of propagation and others that do not; the former conceals the latter because of its greater strength, thus giving the same result as though no mixing had occurred.50 How, then, are we to interpret the statements of Aristotle, Avicenna, and Averroes that have been taken to imply a spiritual mode of existence for species in the medium, the senses, or the brain? Bacon believes that these authors have been mistranslated and therefore misunder- stood. If Averroes seems to argue that the species of a corporeal agent has immaterial or spiritual existence in the medium, what he really means is "insensible being, to which some vulgar [scholar] or the translator applied the name 'spiritual' because of the similarity between spiritual things and insensibles." Like- wise, when Aristotle argues that the senses receive the species of things without their matter, he does not mean to endow these species with spiritual being, but merely to indicate their insensibility. "For spiritual things are insensible, and therefore in common usage we interchange the terms, converting the name 'insensible' to 'spiritual,' so that everything that lacks being sensible to us is said to have intelligible and spiritual being; but this is to use 'spiritual' equivocally, and

47 Bacon, De multiplications specierum 3.1.37-42, 3.2.56-60, trans. Lindberg, pp. 180-181; 190-191. On the merging of the ideas of materiality and corporeality, see Section VII below.

48 Ibid., 3.2.6-7, 76-80, pp. 186-187, 190-191. 49 Bacon, De multiplications specierum 3.2.33-36, trans. Lindberg, pp. 188-189. Bacon pins the

blame for the notion that the species of a corporeal thing might have a spiritual mode of existence chiefly on Averroes; see ibid., 3.2.17-36, pp. 188-189, and accompanying notes; and Lindberg, "Science of Optics," pp. 357-358.

50 Bacon, De multiplications specierum 3.3.1-55, in Bacon's Philosophy of Nature, ed. and trans. Lindberg, pp. 195-205. On the unsatisfactory character of this solution, see ibid., pp. lxvii-lxviii.




[the thing lacking sensible being] remains in truth [an object] of corporeal nature and corporeal being."'51

Bacon's resolute defense of the corporeality of light or species, supported by the authority of Aristotle, Grosseteste, Bonaventure, and others, determined the majority view of generations of European scholars.52 Two immediate followers were his younger contemporary Witelo, a Polish scholar attached to the Papal Curia, and John Pecham, Bacon's Franciscan brother-though, in truth, neither man wasted a great deal of ink on what must have seemed a thoroughly settled issue. Witelo (d. after 1281) stated his position in the preface to his weighty Perspectiva:

Light is the diffusion of the supreme corporeal forms, applying itself through the nature of corporeal form to the matter of inferior bodies and impressing the descended forms of the divine and indivisible artificers along with itself on perishable bodies in a divisible manner, and ever producing by its incorporation in them new specific or individual forms, in which there results through the actuality of light the divine formation of both the moved orbs and the moving powers. Therefore, because light has the actuality of corporeal form it makes itself equal to the corporeal dimensions of the bodies into which it flows and extends itself to the limits of capacious bodies.53

Light is the diffusion of corporeal form, adapted to the dimensions of a corporeal medium that receives it. The corporeal nature of light is reinforced in Witelo's theory of refraction, where the resistance of the medium to the passage of light is a central axiom.54 Pecham (d. 1292) says even less on the subject. In his Trac- tatus de perspectiva he argues that, properly speaking, light (lux) is accidental form-and he clearly has in mind the accidental form of a corporeal substance. And in both that treatise and his better-known Perspectiva communis, he drops hints broad enough to indicate that light or species proceeds through the medium by a process of self-replication.55

In the fourteenth century, the focus of the problem shifted to the epistemolog- ical realm. The cause of this shift was the attempt by a number of scholars, of whom William of Ockham (d. ca. 1349) ultimately became the most prominent, to dispense with species as the agents of perception. Ockham opposed the existence of species on a number of grounds, including our inability to perceive them. He also argued that the notion of an intermediary between the object of perception and the percipient being was an unnecessary multiplication of entities; he thought it quite sufficient to suppose that an object acts at a distance on the sense organ or the intellect without the mediation of species. Although Ockham may have

51 Ibid., 3.2.91-98, pp. 192-193. 52 Aristotelians like Albertus Magnus, uncontaminated by Baconian influence, of course expressed

a similar view. On Albertus, see Lindberg, Theories of Vision (cit. n. 4), pp. 105-106. 53 Witelo, Perspectiva, in Lindberg, Theories of Vision, pp. 118-119. 54 See David C. Lindberg, "The Cause of Refraction in Medieval Optics," British Journalfor the

History of Science, 1968, 4:30-34. Witelo also echoes Baconian themes in treating light as a visible instance of all natural action; see A. C. Crombie, Robert Grosseteste and the Origins of Experimental Science, 1100-1700 (Oxford: Clarendon Press, 1953), p. 215; Aleksander Birjenmajer, Etudes d'histoire des sciences en Pologne (Studia Copernicana, 4) (Wroclaw: Ossolineum, 1972), pp. 275-302; and Lindberg, Theories of Vision, p. 119.

55 John Pecham, Tractatus de perspective, ed. David C. Lindberg (Franciscan Institute Publica- tions, Text Series, 16) (St. Bonaventure, N.Y.: Franciscan Institute, 1972), pp. 29-30; John Pecham and the Science of Optics: Perspectiva communis, edited with an Introduction, English Translation and Critical Notes by David C. Lindberg (Madison: Univ. Wisconsin Press, 1970), pp. 39-40.




had a few followers, the principal response to his theory seems to have been dismay and protest. Among the younger members of his own university (Oxford) and his own order (the Franciscan) in the 1320s and 1330s (the decades immediately following his Oxford career), we find universal condemnation of his teaching and a return to the Baconian theory of species. Walter Chatton, Robert Holcot, Adam Wodeham, and others insisted that perception and cognition occur through species or similitudes multiplied in a corporeal medium from the sense object to the sense organ. It might be debated whether these similitudes are of the same essential nature as the objects from which they issue (Bacon's own position), but it was undisputed that species are forms or qualities in a corporeal medium.56

IV. FICINO AND THE RESTORATION OF PLOTINIAN NEOPLATONISM

In the second half of the fifteenth century, the Neoplatonized Aristotelianism of Grosseteste, Bonaventure, and Bacon came to be rivaled-in some quarters, eclipsed-by a purer version of Neoplatonic philosophy. Chief agent in this development was Marsilio Ficino (1433-1499), founder of the Platonic Academy in Florence, whose translations of Plato and Plotinus provided direct access to the primary sources and inspired a Neoplatonic revival. Along with his translations, Ficino produced original compositions of considerable power and influence, in which he endeavored (as had Augustine, Grosseteste, and Bonaventure before him) to develop a synthesis of Neoplatonic philosophy and Christian theology.57

For Ficino, as for Plotinus, light was the connecting tissue of the universe. God was conceived as the invisible and infinite light, from which descends visible and finite light-mere shadow in comparison to the infinite light. All creatures participate more or less in the true divine light, thus arranging themselves hierar- chically. Light is the animating force of the universe and the active power in bodies, a manifestation of the "inner fecundity" of things. Light has the power of instantaneous expansion, whereby it liberally imparts its goodness to all of creation. These are themes that pervade Ficino's works. In De lumine he writes:

What is light [lux] in God? Immense abundance of his goodness and truth. What is light in angels? Certitude of understanding flowing from God and profuse delight of will. What is light in the heavens? Abundance of life from the angels, unfolding of power from the heaven, and laughter of the sky. What is light in fire? A certain vital energy planted by the heavenly bodies, and efficacious propagation. In things that lack sense, light is grace overflowing from heaven. In sentient beings it is exhilaration of spirit and vigor of the senses. In all things, then, light is the emanation of inner fecundity; everywhere, it is the image of divine truth and goodness.

Light in this "worldly temple" is

to be revered above all else as the image of God. It is also a certain instantaneous expansion of utmost breadth, which, without harm to itself and owing to its

56 Katherine H. Tachau, "The Problem of the species in medio at Oxford in the Generation after Ockham," Mediaeval Studies, 1982, 44:394-443; Tachau, "Vision and Certitude in the Age of Ockham" (Ph.D. diss., Univ. Wisconsin-Madison, 1981); and Lindberg, Theories of Vision, pp. 140-142.

57 On Ficino, see Paul Oskar Kristeller, The Philosophy of Marsilio Ficino (New York: Columbia Univ. Press, 1943); see also Kristeller, Eight Philosophers of the Italian Renaissance (Stanford: Stanford Univ. Press, 1964).




abundance, goodness, and liberality, imparts itself most freely and gladly to all things. It is the cause, preservation, and animation of all things that are born; and other things it elevates to the life, truth, and joy from which it descends. Without it, all things appear to die; in its presence they revive.... Light in all things made by God is a certain radiance of divine brightness and, so to speak, God making Himself as though finite, adapting Himself to the capacity of His works.58

De lumine is a veritable hymn to the metaphysics of light. In none of his works did Ficino present a well-developed cosmogony or cos-

mology of light. Gertrude Kelly Hamilton is quite right, however, to see the suggestion of a cosmogony in Ficino's Praedicationes.59 There we find a scheme left undeveloped, but in outline reminiscent of Grosseteste and Bonaventure. Commenting on the first day of creation, Ficino points out that God's first creative act was to produce matter that had no form or magnitude and therefore was not yet body. In the second moment of creation-Ficino can only have in mind the creation of light in Genesis 1:3-matter was extended in length, breadth, and depth, thus producing corporeal substance. Light is therefore the first corporeal form, or "the first form of the first body."60

Cosmological themes are also touched upon in Ficino's frequent encomia to the power and majesty of the sun. The sun, he points out, "regulates and guides all things celestial, like a veritable lord of the sky."'6' It imparts vital heat to all things, generating and moving them; it furnishes light to all the stars and gives life to the signs of the zodiac; and it determines planetary movements. Ficino was, of course, a believer in astrology. He held that the sun determines the qualities of the seasons and sheds light and warmth; moreover, "the return of the Sun to the degree and the minute of the birthdate of each person renews his fortune for one year."62 The relationship of the moon and sun at the time of birth determines the individual horoscope. If context does not make it clear that Ficino supposed some or all astrological influence to be exerted through the radiation of light and analogous powers, the point is explicitly stated in his commentary on Plotinus: "Our spirit is consonant with the heavenly rays which, occult or manifest, penetrate everything. "63

However important Ficino's cosmology of light may be, his return to a purer version of Neoplatonic philosophy is most evident in his discussion of the nature of light. To begin with, although he appears to accept the general view that lux and lumen are related as parent and offspring, Ficino does not in practice sharply distinguish the two lights.64 This reflects the heritage of Neoplatonism, which

58 Marsilio Ficino, De lumine 5, 16, in Opera omnia, 2 vols. (Basel, 1576; rpt., Turin: Bottega d'Erasmo, 1959), Vol. I, pp. 978, 984-985. Cf. Marsile Ficin, Thgologie platonicienne de l'immorta- lite des dimes, ed. and trans. Raymond Marcel, 3 vols. (Paris: Soci&t6 d'6dition "Les Belles Lettres," 1964), Vol. II, p. 239.

59 Marsilio Ficino, Praedicationes, Opera omnia, Vol. I, pp. 492-493; and Hamilton, "Three Worlds of Light" (cit. n. 38), p. 58.

60 Ficino, De lumine 9, Opera omnia, Vol. I, p. 979. Note Ficino's careful observation of the traditional distinction between body and matter.

61 Marsilio Ficino, De sole 3, in The Italian Philosophers: Selected Readings from Petrarch to Bruno, ed. and trans. Arturo B. Fallico and Herman Shapiro, Vol. I of Renaissance Philosophy (New York: Modern Library, 1967), p. 121; cf. ibid. 6, p. 127.

62 Ibid. 2-4, pp. 120-122, quoting 3, p. 122. 63 On Ficino's astrology, see D. P. Walker, Spiritual and Demonic Magic from Ficino to Campan-

ella (Studies of the Warburg Institute, 22) (London: Warburg Institute, 1958); for the quoted passage, see p. 23.

64 For the lux-lumen distinction, see Ficino, De lumine 11, Opera omnia, Vol. I, pp. 980-981.




emphasizes continuity, rather than discontinuity, in the world. If I read him cor- rectly, Ficino holds, ultimately, that there is a single infinite light, variously participated in; and although he adheres more or less to the conventional terminology of lux and lumen, he understands that in fact there is no abrupt discontinuity in the line of descent of the various lights where a linguistic break can be unequivocally made.

What is the nature of these lights? Once again the dominant theme is continuity. Corresponding to the hierarchy of lights is a hierarchy of natures-the infinite, divine light is altogether incorporeal, while lights descending from it partake increasingly of corporeality. Nevertheless, they do not reach very far into corporeality, and Ficino asserts repeatedly that the visible light in which we are principally interested is incorporeal-though he also admits occasionally that it falls short of perfect incorporeality. For example, in De lumine he argues that the

light [lux] apparent to our eyes cannot be the first light, both because it is propor- tioned to . . . corporeal eyes and because it is mobile and present in one thing while dependent on another. Therefore, we must ascend to some higher light, unfettered by such conditions, already existing in and through itself, incorporeal through and through. Since even this light visible to us is almost incorporeal, and likewise the cause of incorporeal images, . . . it is clear that it cannot have its origin in corporeal mass [mole] or form or power; rather, it originates from a light higher than bodies, clearer, and immeasurably greater.65

Ficino defends the incorporeality- (or quasi-incorporeality) of light on several grounds. Its apparent instantaneous propagation across vast cosmic spaces is perhaps the most striking evidence. Moreover, light penetrates hard and soft bodies, large and small spaces, with equal ease and in the same time. Light can issue continuously from a lucid body without any corruption or diminution of the body. And then there is the authority of Plotinus, who denies the occurrence of corporeal change (such as approach, recession, contraction, and expansion) in light. All these things prove "that light [lumen] is spiritual rather than corporeal."66 "Lumen is a certain spiritual, instantaneous, and extremely diffuse emanation from bodies by their nature, without detriment to themselves." And yet Ficino does not fail to remind us on occasion that light, in truth, is only "quasi-spiritual. "67

As a spiritual or quasi-spiritual entity, how does light interact with material things? Into what sort of relationship can it enter with body? Light, Ficino as- sures us, is not a quality of the body or medium that receives illumination, but the "proper and natural activity of the illuminating body." Thus light is never confined to a specific place; it is not "so much in the illuminated body as present to it." It is not dependent for its existence on a corporeal medium, but only on the activity of a luminous source. A corporeal medium, far from being a necessity, is a hindrance: "Light does not mingle with the illuminated transparent substance, . . . for mixing of this sort does not pertain to celestial activity and powers (that is, to rays), but to elemental qualities. No elemental qualities are necessary for the presence of light; it is sufficient that light encounter no

65 Ibid. 6, p. 978. 56 Ibid. 9, 13, pp. 979, 982; cf. Theologie platonicienne (cit. n. 58), Vol. I, p. 236. 67 Ficino, De lumine 2, 13, Opera omnia, Vol. I, pp. 977, 982.




terrestrial opacity unsuited for receiving celestial things."68 The Plotinian character of Ficino's remarks is striking: light as the activity of a luminous body and the superfluity of a medium had been basic themes of Plotinus's theory of light.

It follows from the foregoing argument, Ficino believes, that light (lumen) is "between substance, which is (as it were) self-subsistent, and quality, which depends on both an agent and a recipient."69 Light is totally dependent on and inseparable from the lucidity of its source, while serving as the intermediary through which the source communicates illumination and heat to bodies in its vicinity. This can be clarified by an analogy: Ficino maintains that light is to the illuminated medium as soul is to the body that it animates. In the scale of being, God and the angels are, of course, located at the summit, while body and quality are at the base. Soul, Ficino argues, is the intermediary between these superior and inferior beings, participating simultaneously in the immortality of angels and the changes and motions of quality, thus introducing unity into the universe. Soul "embraces the superior without abandoning the inferior, and so through it the two are united." In the same way, the visible light of the sun unites the celestial and terrestrial regions:

Soul does the same thing as the light [lumen] of the sun. For light descends from the sun into fire and fills it without abandoning the sun. It always adheres to the sun and always fills the fire. It perfects the air without being corrupted by the corruption of the air. In the same way, the third essence [soul] must adhere to the divine and fill that which is mortal. While adhering to the divine, it knows the divine because it is spiritually united to the divine, and spiritual union begets knowledge. While it fills bodies, moving them from within, it animates them.70

Soul is not united to its body quantitatively, point for point, but is wholly present in each individual part of the body.71 This distinguishes it from quality, which can be divided along with the body in which it inheres. In a passage dealing with color, Ficino again stresses the analogy between light and soul:

On the earth's surface light [lumen], infused into various mixtures of the four elements, especially the earthly, assumes the forms of various colors, like corpuscles whose little souls [animulae] are scintillae of light infused into them. If you could separate these [scintillae] from those mixtures and preserve them, you would perhaps see what rational souls are like when separated from body.72

If Ficino sometimes seems ambivalent about the nature of light, this is (in part) the legacy of Neoplatonism, which forever confronts the dilemma of maintaining unity while acknowledging diversity. In every feature of the universe there is both continuity and polarity. Distinctions must be made, but there are no places on the continuum where lines of demarcation can be drawn. It is vacillation between these two truths that leads Ficino into seeming contradictions: light is spiritual, yet not quite spiritual; light proceeds without a medium, yet "perfects the air" and "fills that which is mortal." But vacillation does not entirely explain

68 Ibid. 11, p. 980; cf. The'ologie platonicienne, Vol. III, p. 35. 69 Ficino, De lumine 11, Opera omnia, Vol. I, p. 980. 70 Ficino, The'ologie platonicienne, Vol. I, pp. 137, 139; cf. De lumine 16, Opera omnia, Vol. I, p.

985. 71 Ficino, The'ologie platonicienne, Vol. I, p. 140. 72 Ficino, De lumine 13, Opera omnia, Vol. I, p. 982.




the ambiguities of Ficino's account of light. In the final analysis, light is of itself mysterious and ineffable. In Ficino's judgment, there is no possibility of defining absolutely what lumen is, and we are often reduced to declaring what it is not. Ficino thus associates the investigation of light with the via negativa of the pseudo-Dionysian tradition. Light "is not color, not transparency, not an image of the heavenly bodies. . . . Therefore, we know light, as we know God, only through negations and comparisons."73

Ficino's attempt to revive Plotinian light metaphysics, and with it Plotinian teaching on the nature of light, was not without impact in the sixteenth century. Nonetheless, majority opinion continued to favor traditional Aristotelian- Baconian doctrine. The best optician of the century, Francesco Maurolico (1494- 1575), addressed himself exclusively to the mathematics of light, avoiding any discussion of its nature or metaphysical basis; at the very most, one might infer from his use of the word "species" that he was prepared to accept the customary connotations of that term.74 Girolamo Cardano (1501-1576) included a chapter entitled "De luce et lumine" in his De subtilitate; there he noted that lux is a quality of bodies, which gives rise to its likeness, lumen, in the adjacent transparent medium or to color in opaque bodies. A few years later Julius Caesar Scaliger (1484-1558) issued a hot attack on some of Cardano's doctrines, particularly his claims that the moon has its own light and that heat and light are inseparable and therefore ultimately the same thing. In the course of his diatribe, Sca- liger examined the nature of lux, lumen, radius, and calor and their relationships. He argued that lux, the quality of a luminous body, does not exit from that body; what issues from the body is lumen (the species or likeness of lux), also meta- phorically called radius. Lumen is the actuality of the adjacent transparent medium; heat and lumen are companions but are not inseparable and certainly are not the same thing.75 For Cardano and Scaliger, and probably for Maurolico, radiating light is still the qualification of a corporeal medium.

The same may be true of the greatest English mathematician of the sixteenth century, John Dee (1527-1608).76 Unfortunately, Dee remained almost entirely silent on the nature and mode of propagation of light, and we can infer his opinion on the subject only from his general allegiance to the Baconian doctrine of the multiplication of species. What makes Dee's work worthy of our attention is his firm defense of al-Kindi's and Bacon's physics of light. Dee maintains that everything in the universe-substance and accident, spiritual substance as well as corporeal substance-sends its species in all directions, so that "every place in

73 Ibid. 12, p. 981. Cf. Ficino, De sole 2, in Italian Philosophers (cit. n. 61), p. 120. 74 Francesco Maurolico, Photismi de lumine et umbra ad perspectivam et radiorum incidentiam

facientes. Diaphanorum partes, seu libri tres.... Problemata ad perspectivam et iridem pertinentia (Naples, 1611). For an analysis of Maurolico's optical contributions, see Lindberg, Theories of Vision (cit. n. 3), pp. 178-182; and Lindberg, "Optics in Sixteenth-Century Italy," in Novita! celesti e crisi del sapere: Atti del Convegno Internazionale di Studi Galileiani, ed. P. Galluzzi (Supplement to the Annali dell'Istituto e Museo di Storia della Scienza, 2) (Florence: Istituto e Museo di Storia della Scienza, 1983), pp. 131-148.

75 Hieronymi Cardani De subtilitate libri XXI (Nuremberg: Johannes Petreius, 1550), p. 93; and Julius Caesar Scaliger, Exotericarum exercitationum liber XV: De subtilitate ad Hieronymum Car- danum (Paris: Michael Vascosan, 1557), pp. 207-208, 213-216. Kepler cites the optical writings of Cardano and Scaliger.

76 This is the view of John Heilbron in his introduction to John Dee on Astronomy: Propaedeumata aphoristica (1558 and 1568), Latin and English, ed. and trans. Wayne Shumaker (Berkeley: Univ. California Press, 1978), pp. 61-63.




the universe contains rays of all things that have actual existence."77 All species obey the laws of optics and must therefore submit to optical analysis. Radiation may be sensible or insensible; and it is responsible for magnetic action, solar heat, sensation, and astral influence. Of the latter, Dee states: "I affirm it to be most certain that about every single thing in the universe the rays of all seven planets-the rays of a more secret influence, or sensible rays, either principal or accidental-converge and mingle at all times, and that there remains a perpetual conjunction of all these in everything in the universe."78

The sixteenth-century optical writer most heavily influenced by Plotinus and Ficino was the professor of Platonic philosophy at the University of Ferrara, Francesco Patrizi (1529-1597). In his Nova de universis philosophia, Patrizi developed his own version of Neoplatonic light metaphysics. God the Father is the first light (lux), source of all others; from Him descend, as lesser lights, the Son and the Holy Spirit, images of the Father and His goodness.79 Further emanation gives rise to supercelestial lux, celestial lux, and corporeal lumen. Patrizi discourses at length on the nature of lux and lumen and their relationship. Lux is neither corporeal nor incorporeal but both simultaneously; it is an "intermediary between incorporeal divinity and corporeal nature." Lux in the corporeal realm "is in one way infinite, the truest image of that truly infinite and purely incorporeal light. As it happens, it falls short of that incorporeal light in no way except that it issues forth as a body (owing to its three-dimensionality), simultaneously finite and infinite." Lux is corporeal simply insofar as it occupies space. Lux is the purest and simplest of all things; it is neither matter nor form, but both matter and form in perfect unity.80

Lumen is derived from lux through the mediation of rays; rays can thus be considered second light and lumen third light. Lumen "is nothing other than a diffused ray, while a ray, by the same token, is nothing but unified lumen gath- ered into itself." Patrizi, who has no fear of paradox, maintains that lumen is "an incorporeal, immaterial, three-dimensional body," a corpus incorporeum. It is not a quality of the transparent medium, but a self-subsistent: "It is not so much contained within the transparent as present to it." We know this because the transparent can be moved without affecting lumen passing through it. Patrizi concludes that lumen functions

as life extending through all things; by its most alluring sweetness it gives life and nourishes, both within and without.... Along with celestial lumen comes gentle and vital heat, in which is the power of moving and forming all things. And in all things lumen is the pouring forth of innermost fecundity, the surest and most evident image

77 Dee, Propaedeumata aphoristica, trans. Shumaker, p. 123, with one correction. Dee here quotes al-Kindi verbatim; see above, n. 21. The resemblance between Dee's Propaedeumata aphoristica and Bacon's De multiplications specierum is striking. On Dee's use of Bacon, see Nicholas H. Clulee, "Astrology, Magic, and Optics: Facets of John Dee's Early Natural Philosophy," Renaissance Quar- terly, 1977, 30:632-680. On Dee's other sources (including al-Kindi), see Heilbron's introduction to John Dee on Astronomy, pp. 53-55.

78 Dee, Propaedeumata aphoristica, pp. 127-129, 133, 179-181, 189, quoting from p. 191. 79 Francesco Patrizi, Nova de universis philosophia (Venice: Robertus Meiettus, 1593), fols. lv,

23r; on Patrizi's philosophy, see Kristeller, Eight Philosophers (cit. n. 57), Ch. 7. 80 Patrizi, Nova de universis philosophia, fols. 2v, 3r; on lux as occupying space, see Elizabeth E.

Maechling, "The Doctrine of Light in the Philosophy of Francesco Patrizi (M.Phil. diss., Univ. London, 1977), pp. 23-24.




of divine beauty and goodness. And so through lumen the entire world becomes luminous and visible and aesthetically pleasing.81

Lumen can also serve as the basis for the return of the creature to the Creator. If, in the order of being, all lights descend ultimately from God, then in the order of knowing we can ascend, by the study of light, to God, the "Father of lights"- just as Aristotle ascended from the study of motion to knowledge of its cause, the prime mover.82

V. KEPLER'S METAPHYSICS AND COSMOLOGY OF LIGHT

We have examined the course of Neoplatonic emanationism and the major alternatives available to Kepler on the nature of light. Although it is generally impossible to identify the specific sources of Kepler's thought on light, since (with a few exceptions) he does not cite them, we can learn a great deal about Kepler's position from similarities of doctrine. Viewed against this background, Kepler's works unmistakably reveal their philosophical origins.

Kepler left no explicitly theological or metaphysical treatise, but theological and metaphysical remarks pervade his scientific works. Specifically, Kepler expresses a deep and abiding commitment to Neoplatonic emanationism. He associates light with the underlying mathematical archetypes and presents it as one of the principal agents employed by the Creator in His governance of the universe. In Kepler's view, it is light that links spiritual and corporeal realities, that de- lights the eye and animates all things. With Plotinus, Ficino, and Patrizi, he believes that the student of light is rewarded with an understanding of the ultimate realities.

In his earliest work on optics, Ad Vitellionem paralipomena (1604), Kepler introduces light as the offspring of sphericity and both light and sphericity as images of the Trinity. All things, he insists, are meant as images of their Creator insofar as their essences permit. In seeking to create the best possible world, the Creator

found nothing more beautiful or more excellent than Himself. That is why, when he conceived the corporeal world, he gave it a form as like Himself as possible. Thus originated the whole genus of quantities, and in it the distinction between the curved and the straight and the most excellent of all figures, the spherical surface. For in forming the sphere, the all-wise Creator produced for his pleasure the image of his holy Trinity. Therefore, the central point may be regarded as the origin of the sphere, the surface as the image of the innermost point, and each path coming to the surface as an infinite movement of the point outside itself, so as to produce a certain equality of all movements.... Hence, everywhere between the point and the surface there is absolute equality, closest unity, loveliest harmony, connection, relation, proportion, and symmetry. And although there are plainly these three things-center, surface, and space between-nevertheless they possess such unity that none of them could be removed, even in thought, without destroying the whole.83

81 Patrizi, Nova de universis philosophia, fols. 9v, lOr, lv. 82 See ibid., title page; see also Patrizi to Baccio Valori, quoted in Maechling, "Doctrine of Light"

(cit. n. 80), p. 8. 83 Johannes Kepler, Ad Vitellionem paralipomena, quibus astronomiae pars optica traditur

(Frankfurt, 1604), ed. Franz Hammer, in Gesammelte Werke (cit. n. 1), Vol. II, p. 19 (hereafter Paralipomena). In treating this work I have benefited from two translations: Johann Kepler, Les




The center of the sphere is the counterpart of God the Father, origin of all else; the circumference, image of the central point, represents Christ the Son; and the intervening space symbolizes the Holy Spirit.84

Whatever aspires to perfection must acknowledge and imitate this most perfect shape. "Therefore bodies, although of themselves confined within their own surfaces and unable to multiply themselves into a sphere, are endowed with various virtues; these virtues, nesting within the bodies but a little freer [to leave] than the bodies themselves, deprived of corporeal matter but consisting of a certain matter of their own that can assume geometrical dimensions, issue forth and strive for sphericity." Magnetic virtue is one such emanation; virtus motrix, which drives the planets, is another. Preeminent in beauty and importance, however, is light:

How marvelous, then, if that source of everything beautiful in the world, which the divine Moses introduces into newly created matter on the first day [of creation] for the shaping and animating of all things-how marvelous, I say, if this source and most excellent thing in the whole corporeal world [i.e., light], origin of animal faculties and link between the corporeal and spiritual worlds, should proceed according to the same laws by which the world was to be ordered. Thus the sun is a certain body in which resides that faculty of communicating itself to all things that we call light. On this account, it requires the middle place in the whole world, the center, so that it can uniformly and perpetually diffuse itself into the whole sphere. All other things that participate in light imitate the sun.

Thus light is not only the image of the triune God; it is also the origin of the soul's faculties, the bond that connects the spiritual world with the world of corporeal matter, and a mirror of the laws of nature.85

The association between light and soul reappears in the Harmonice mundi (1619), where Kepler develops a panpsychic philosophy and, in particular, explores the nature of the terrestrial soul. Kepler presents a variety of arguments to prove the animation of the earth; for example, he finds terrestrial analogues of animal functions (nourishment, respiration, sense perception, and memory) and products (excrement, tears, mucus, pus, and ear wax). In the course of this discussion, he inquires into the nature of the anima telluris and concludes that it is a kind of light:

Since already [we have determined] as a certainty that the earth possesses a soul, we come to a consideration of its essence. Not only is it a light of the same sort as that of fires and fireflies-light that depends only on itself and not on illumination from the

fondements de l'optique moderne: Paralipomenes a Vitellion (1604), ed. and trans. Catherine Che- valley (Paris: Vrin, 1980); and (for Ch. 1) Jole Shackelford, "Kepler on the Nature of Light, or Eine kleine Lichtmetaphysik, including an English translation of Chapter One of Kepler's Ad Vitellionem paralipomena" (1983).

84 This symbolism, drawn from Nicholas of Cusa, is spelled out in Kepler's Mysterium cosmographicum, trans. A. M. Duncan, ed. E. J. Aiton (New York: Abaris, 1981), p. 95; and Kepler, Epitome of Copernican Astronomy 4, trans. Charles Glenn Wallis, in Nicolaus Copernicus, On the Revolu- tions of the Heavenly Spheres (Great Books of the Western World, 16) (Chicago: Encyclopaedia Britannica, 1952), pp. 839-1004, on pp. 853-854. On Kepler's spherical symbolism, see also Gerard Simon, Kepler: Astronome, astrologue (Paris: Gallimard, 1979), pp. 133-143.

85 Kepler, Paralipomena, pp. 19-20. It is certain that light is intended in this passage because of the reference to its introduction on the first day of creation. Similar views are expressed in Johannes Kepler, Harmonice mundi, 4.1, ed. Max Caspar, Gesammelte Werke (cit. n. 1), Vol. VI, p. 224; quoted by Pauli, "Archetypal Ideas" (cit. n. 1), pp. 160-162.




sun . . . but it also plainly appears to be a certain flame.. .,as we demonstrate from the perpetual and sensible existence of subterranean heat. For without soul, no [such heat] can persist in actuality in bare matter; indeed, it cannot exist potentially in substances issuing from animals and plants unless produced by a soul and by forms derived from fire.

A little later in the same work the luminous nature of the soul is.again defended. The soul, Kepler argues, assumes in actuality the form of a point, but potentially the figure of a circle:

Since it is energy (energia), the soul spreads itself from its punctiform dwelling into a circle. Whether it must perceive external things that surround it spherically or rule the body, which also surrounds it, the soul lies hidden within, rooted in its fixed point, from which it issues to the remainder of the body through its species. But how should it issue forth if not in straight lines, for that is what it means to issue forth? Since it is light and flame, how else could it proceed than in straight lines, as other lights proceed from their sources?86

Light is associated with life not only through soul, the animating principle of every living thing, but also through vital heat. In the Paralipomena, Kepler speaks of the flame in the heart, "the hidden lamp of the heart," nourished by blood from the vena cava, ruler over the vital functions of the body. "Thus animal heat depends on light, to say nothing of the fact that the soul, of itself invisible, is fittingly held to have an essence cognate to light; and by virtue of this designation it would come with light into association with heat, inasmuch as light is the offspring of the soul." Heat in the earth is also related to light, the "guardian of heat," as one can see from the volcanic flames of Mount Aetna. In roots and seeds there is a little fire, which gives forth its light in glowworms and rotting wood. The thrust of the argument is that light is the universal source of heat: "heat in all other things is adventitious, being dependent on the heat of light. "87

VI. KEPLER'S ASTROLOGY

If light is active in the terrestrial realm, it is equally prominent in the linkage between heaven and earth. Despite his scorn for many features of astrology as traditionally practiced (he accuses its practitioners of "infantile credulity"), Kepler was a firm believer in the reality of astrological influence. For a philosopher of Neoplatonic persuasion, committed to the unity of the cosmos, it could hardly be otherwise. Kepler developed his position on the true nature of astrological influence in a small treatise entitled On the More Certain Fundamentals of Astrology, published in 1601. 88

Two kinds of astrological cause are at work, he argues, one physical and the other mathematical. The physical cause proves to be totally optical, dependent

86 Kepler, Harmonice mundi, Gesammelte Werke, Vol. VI, pp. 268-269, 271, 275. I was led to this last passage by Pauli, "Archetypal Ideas," pp. 178-179, and have borrowed several phrases from his translation.

87 Kepler, Paralipomena, pp. 35-36. 88 "Johannes Kepler's On the More Certain Fundamentals of Astrology, Prague, 1601," ed. J.

Bruce Brackenridge, trans. Mary Ann Rossi, in Proceedings of the American Philosophical Society, 1979, 123(2):85-116, quoting from p. 99.




on the radiation of light from the heavens to earth. Through its light, the sun supplies heat to the earth, imparting more or less according to the season. As the sun withdraws in the winter, the earth returns to its natural cold state. The moon is influential through reflected solar light, which has a humidifying function. Kepler advances empirical evidence for this claim, noting that "all things invari- ably moist swell with the waxing moon and subside with the waning moon." Thus ocean tides are produced, and the humors of the body are altered in quantity. Finally, the planets send both an innate and a reflected light to the earth. Insofar as the light is innate and therefore direct, it heats; insofar as it is reflected, it humidifies: "Both of these powers they [the planets] possess and exer- cise through benefit of light, which they have received and send down to us continuously. For the proper quality of light inasmuch as it is light is heating; but the proper quality of light insofar as it is reflected, is humidifying."89 In the absence of light, the earth is cold and dry; light brings warmth and humidity.

But how can different planets have different effects? Despite his contempt for the astrological tradition, Kepler retained the traditional planetary qualities (Mars, for exam-le, is hot and dry, Saturn cold and moist); he simply furnished a new explanation. The sun, which supplies only direct light, gives us mean heat; the moon, which has no light of its own and merely reflects solar light, imparts mean humidity. But each of the planets has two lights: its innate light, supplying heat, and reflected solar light, supplying humidity. The reflected light is altered by the color of the planetary body; thus, for example, Mars has a black surface that weakens the solar light reflected from it, giving the planet a ruddy appearance. Innate light is also altered, acquiring color and heat-producing properties from the internal disposition of the planetary substance. The weakening of both the reflected and innate light will, of course, affect the heat-producing and moisture-producing power of the rays incident on the earth. If we examine all of the possible combinations of heating and humidifying-each of these two qualities being present in excess, deficiency, or mean degree-and assign them appro- priately to the planets, we can explain planetary influence. Saturn's rays convey deficiency of heat and excess of humidity; Jupiter's rays convey the mean of both qualities. Mercury, by contrast, is represented by two possibilities: mean heat and excess humidity or deficiency of heat and mean humidity; but the two cases offer the same ratio between the heating and the humidifying qualities and therefore have the same effect.90

This physical cause of astrological influence is accompanied by a geometrical cause, far nobler because it is formal rather than material. Pairs of planets send their rays of light to the earth, and the angle between these rays is perceived by the terrestrial soul, which is "appreciative of geometry." If the angles are har- monic, that is, correspond to one of the five traditional aspects or the three others added by Kepler, the soul demonstrates its appreciation by increasing its vegetative activity. "Every animate faculty," Kepler argues, "is the image of God the Geometer in creation, and He is inspired to His task by his celestial geometry of aspects or harmony." Therefore, the soul of the earth must respond

89 Ibid., pp. 91-93; quoting from pp. 92 (with several changes), 93. For similar ideas in Ficino, see De sole, in Italian Philosophers (cit. n. 61), p. 128.

90 "Kepler's On the More Certain Fundamentals," ed. Brackenridge, trans. Rossi, pp. 93-95 and n. 18, pp. 109-1 10.




in the same manner: as "the ear is stimulated by harmony to listen carefully and thus to hear much more, . . . so the earth is stimulated through the geometric concourse of vegetating rays (which we said warmed and humidified) so that it applies itself so much more diligently to its function of vegetation and exudes a copious supply of vapors. "91 Once again the agent of celestial action is light, although here it is not the nature of light that is efficacious but the response of the terrestrial soul to its geometrical configuration.

Light (or a force akin thereto) makes its final cosmological appearance as the motivating force of Kepler's planetary system, the virtus motrix, which we will consider after we have investigated the nature of visible light.

VII. KEPLER ON THE NATURE OF LIGHT

If Neoplatonic influence is apparent in Kepler's metaphysics and cosmology of light, its hold on Kepler is demonstrated beyond doubt by an examination of his views on the nature of visible light. Kepler touched upon light in almost every book he wrote, but he dealt with its nature most fully in the first chapter ("On the Nature of Light") of Ad Vitellionem paralipomena.

Proposition 1 of this chapter maintains that it is the nature of light to emanate from its source and communicate itself to distant bodies. Proposition 2 informs us that every point of the luminous body is a source of emanation, and from each of these. point sources proceeds an infinity of lines in spherical fashion. In the third proposition, Kepler maintains that light is suited for propagation to infinity, arguing that light is weightless and thus offers no resistance to the power of the luminous body that propels it: "therefore, the proportion of the power to the weight is infinite." The same line of argument demonstrates that light is propagated with "infinite swiftness" and that it is attenuated latitudinally, as rays from a given point spread out, but not longitudinally.92

In Proposition 4, Kepler affirms the rectilinear propagation of light-surely the most traditional and basic of all optical principles. However, he defends this principle with novel arguments. Whereas his predecessors simply assumed its truth or defended it on observational grounds or by appeal to the principle of economy, Kepler gives the rectilinear propagation of light metaphysical foundations. Light is propagated in straight lines because all things, insofar as their essences permit, imitate their Creator. The most perfect geometrical expression of the divine Trinity is the sphere, and light must therefore "strive for sphericity." The equality and symmetry of rectilinear propagation produce the sphere: "If light were to utilize a curved path, there would be no equality in its diffusion and consequently no resemblance to a sphere." Moreover, nature seeks either greatest unity or most complete separation. Both are best achieved through rectilinear motion-the former through rectilinear motion toward a center, as in the descent of heavy bodies, the latter through rectilinear motion away from a center, as in the emanation of light.93 The straight line along which light is propagated is called the ray. A ray is not light and is certainly not corporeal, but simply

91 Ibid., pp. 96-98 and n. 28, p. 112, quoting from pp. 97, 98. 92 Kepler, Paralipomena, Props. 1-3, p. 20; Props. 5-7, p. 21. 93 Ibid., pp. 19-21, quoting from p. 20; cf. Les fondements de loptique moderne, ed. and trans.

Chevalley [cit n. 83], p. 109). On Kepler's predecessors, see David C. Lindberg, "Laying the Foun- dations of Geometrical Optics: Maurolico, Kepler, and the Medieval Tradition," in Lindberg and




a geometrical line representing the motion of light; it represents the motion rather than the mobile. Kepler ignores the distinction between lux and lumen, employing the terms interchangeably to denote luminosity, whether in a luminous body or emanating from it.94

We have thus far learned how light is named, certain rules governing its propagation, and something of the metaphysics underlying those rules. But what is the nature of light? Kepler is better at telling us what it is not. He attaches an ap- pendix to the first chapter of his Paralipomena in which he assembles a long series of Aristotelian theses about light and vision and then proceeds systemati- cally to refute them. It is his purpose, as he puts it, to take the Aristotelians to the "school of the Optici."95 Kepler attacks Aristotle on a broad front but concentrates his forces on the Aristotelian notion that light is a state (habitus) of the potentially transparent medium whereby its transparency becomes actual-a state induced by the presence of a luminous body-and the further notion that color moves or alters the actually transparent, thus sending a qualitative change through the transparent medium to the eye of an observer. To define transparency in terms of light and light in terms of transparency, as Kepler believes Aristotle does, is to beg the question. Rather, transparency is solely a function of the "internal disposition of the body," and this is totally independent of the presence or absence of light.96 Light, for its part, is an emanation from a luminous or illuminated body and does not depend for its existence on a transparent medium. It is a "species"-we will return in a moment to this term-which not only can be propagated without a medium but which, in the absence of all media, would yield perfectly precise vision. As for color, Kepler maintains that its visi- bility depends not on illumination of the medium (in order to bring it to a state of actual transparency) but on illumination of the colored object itself. Experience teaches that you can illuminate the medium all you wish, but colored objects will not be seen unless illumination falls on them.97

We must return to the term species, sometimes employed by Kepler to denote emanating light. I disagree vehemently with those who have argued that Kepler continued to employ the archaic terminology of species while divesting it of its traditional content.98 In the absence of arguments to the contrary, we must suppose that Kepler understood and accepted the customary connotations of the terms he employed. As a rule Kepler was extraordinarily precise in his choice of terminology (for example, he coined terms when no existing one would suffice), and there is no reason, unless one is determined at all costs to portray him as a modern, to suppose that the term species is an exception. Kepler's usual prefer- ence is for the terms lux (in optical contexts) and virtus (in astronomical or cosmological contexts); radius and radiatio appear when the geometry of radiation is at issue, and lumen is employed as a synonym of lux. Species is introduced as yet another synonym-or, on occasion, as part of an explanation of the nature of lux or virtus. For example, in the Astronomia nova (1609) Kepler argues that "just as

Geoffrey Cantor, The Discourse of Light from the Middle Ages to the Enlightenment (Los Angeles: Clark Memorial Library, 1985), pp. 1-41.

94 See, e.g., Kepler, Paralipomena, p. 40; see also Chevalley's introduction, Fondements, p. 71. 95 Kepler, Paralipomena, p. 38. 96 Ibid., pp. 22, 45-46. 97 Ibid., reply to Thesis 16, p. 42; cf. reply to Thesis 10, p. 40. 98 Ronchi, Nature of Light, p. 92; and Straker, "Kepler's Optics," p. 504 (both cit. n. 2).



KEPLERWS THEORY OF LIGHT 35

light [lux], illuminating all earthly things, is the immaterial species of the fire in the body of the sun, so this power [virtus motrix], embracing and drawing the planetary bodies, is the immaterial species of the virtue residing in the sun."99 In Chapter 1 of the Paralipomena he maintains that what moves the eye in vision is "neither the body of the sun or of color, nor the medium, but species (whether lights [lumina] or rays of the sun and colors)." Another passage in the Paralipo- mena is particularly telling because in it Kepler foists the term species onto Witelo, who had never employed it. Arguing against Witelo's belief that forms (translated by Kepler as "species") are fixed in the humors of the eye, thus explaining the persistence of vision, Kepler points out that "a species is always present along with the body of which it is the species, and if the body is hidden by an opaque obstruction, the species is destroyed by the shadow."100

It is clear that Kepler drew the term species from the medieval philosophical tradition, where (as we have seen) it was employed to denote the likeness or power that all things emanate in order to influence their surroundings-the "first effect," as Roger Bacon put it, "of any naturally acting thing." It was an expression of Neoplatonic emanationism-of the notion that all existing things are centers of activity, radiating similitudes and influences in all directions. I see no reason to doubt that Kepler understood this and intended it in his own use of the term. Indeed, this very idea appears in his claim that bodies, though unable to escape their own boundaries, contain virtues "nesting within" them, which "issue forth and strive for sphericity." The idea equally appears in the claim that "the sun is a certain body in which resides the faculty of communicating itself to all things which we call light."'0' It thus seems undeniable that in using the term species Kepler accepted the conceptual framework that lay behind it.

However, there were unresolved issues within Neoplatonic emanationism. Both the corporeality and the incorporeality of species had been defended, and Kepler would be compelled to choose between these alternatives. On one side was the considerable weight of the Baconian tradition, allied on this issue with the pronouncements of Aristotle and ratified by centuries of scholastic commentary, proclaiming the corporeality of light or species. On the other side were Plotinus, Ficino, Patrizi, and a few others, urging the incorporeality-or quasi incorporeality-of light. Kepler, as we shall see, repudiated the Baconian tradition and cast his lot with Plotinus and the incorporealists.

Before we examine Kepler's argument on the subject, we must take note of certain conceptual and linguistic changes that had occurred since antiquity- changes that induced Kepler to identify matter with corporeal substance.'02 In defining matter as the indeterminate substrate, devoid of properties, Aristotle had in effect declared it beyond visualization. Yet generations of philosophers

9 Johannes Kepler, Astronomia novta 3.33, ed. Max Caspar, Gesammelte Werke (cit. n. 1). Vol. III, p. 240. On Kepler's terminology and its meaning, see Chevalley's introduction, Fondements (cit. n. 83), pp. 69-71; and Catherine de Buzon [Chevalley], "La propagation de la lumiere dans l'optique de Kepler," in Roemer et la vitesse da la lumiere (Paris: Vrin, 1978), pp. 75-78.

'?? Kepler, Paralipomena, pp. 45, 37. For a similar claim, see Plotinus, Enneads 4.5.7. On Witelo, see Perspectiva 3.6, in Opticae thesaurus Alhazeni Arabis libri septem .... Item Vitellonis Thurin- gopoloni libri X, ed. Friedrich Risner (Basel, 1572), rpt. ed. David C. Lindberg (New York: Johnson Reprint, 1972), pp. 87-88.

101 Bacon, De multiplicatione specierum, 1.1.27-29, trans. Lindberg (cit. n. 19), p. 3; and Kepler, Paralipomena, p. 19.

102 See Ivor Leclerc, The Nature of Physical Existence (London: Allen Unwin, 1972), Chs. 9-10.




could not resist the temptation to visualize it; and to visualize it was, of course, to invest it with properties and to conceive it (however unwittingly) as corporeal substance. They were encouraged in this deed by the terminology at their dis- posal, for the primary meaning of hyle (Greek) and materia (Latin) had always been that of timber, the material of construction-very substantial stuff. Further support could be found in the relativity intrinsic to Aristotle's idea of matter: the matter of a building is brick and mortar, while the form is the shape given them as the building is erected; the matter of brick, in turn, is clay; and the matter of clay, finally, is a certain "prime matter" beyond which it is impossible to go. Only the last of these matters lacks substantial existence of its own; bricks have independent existence before anything is built with them, and clay exists independently before it is made into bricks. Among scholastic commentators we thus see the development of the idea of "second matter," carrying with it the notions of substantiality and corporeality.103 A final step in the shift of meaning resulted from the tendency of Nicholas of Cusa, Giordano Bruno, and other Renaissance scholars to elevate the status of matter by regarding it not merely as a passive recipient of forms but as the active container or mother of forms. Bruno argues that "matter sends the forms out from itself, and does not receive them from without"; that matter is "deprived of forms and without them, not in the manner in which ice lacks warmth, . . . but in the manner in which the pregnant woman is without progeny, which she obtains and sends forth from herself."' 04 Kepler clearly accepts this redefinition, so that when he speaks of matter or material he has in mind corporeal substance; that is, he employs the terms with essentially their modern meanings. No other interpretation makes sense of his equation of matter with weight, density, and three-dimensionality or his association of matter and resistance.105

We are now prepared to appreciate Kepler's defense of the immateriality or incorporeality of light and species. Kepler informs us of the immateriality of light in the first chapter of his Paralipomena, where he defines it as a two-dimensional geometrical surface and maintains that it has "neither matter nor weight nor resistance." 106 But on what arguments does this conclusion rest? One is that only an immaterial or incorporeal entity could be propagated instantaneously:

For, as is demonstrated in Aristotle's books on motion, there is a certain relationship between the time and the ratio of motive force to weight or quantity of the mobile.... But here the ratio of the motive power to the light that it moves is infinite, since light has no matter and therefore no weight. Therefore, the medium does not

103 James A. Weisheipl, O.P., "The Concept of Matter in Fourteenth Century Science," in The Concept of Matter in Greek and Medieval Philosophy, ed. Ernan McMullin (Notre Dame, Ind.: Univ. Notre Dame Press, 1963), pp. 150-156. We have seen Bacon's equation of matter and substance (see in Section III above, near n. 47).

104 Giordano Bruno, De la causa, trans. Sidney Greenberg, in The Infinite in Giordano Bruno, With a Translation of His Dialogue: Concerning the Cause, Principle, and One (New York: King's Crown Press, 1950), p. 153. On matter, see also pp. 134-157; and Leclerc, Nature of Physical Existence (cit. n. 102), p. 133.

105 Kepler, Paralipomena, props. 5-14, pp. 21-23. However, for an exception, ibid., p. 20, quoted in Section V above (at n. 85), where Kepler refers to an incorporeal or mathematical matter possessed by the virtues issuing from bodies.

106 Kepler, Paralipomena, Props. 3, 8, pp. 20-21. In Prop. 1, p. 20, he also refers to light as a "geometrical body"-a characterization he later rejects in the Astronomia nova (see Section VIII below, atn. 113).




resist light, since light lacks matter by which resistance could occur. Therefore the swiftness of light is infinite.107

Another argument for the immateriality of light is drawn from the phenomena of refraction. If light were corporeal substance, it would encounter continuing resistance within a transparent substance; consequently, it would be continuously retarded and continuously bent. But no such thing is observed. On the contrary, refraction is exclusively a surface phenomenon; light is bent as it crosses the boundary of a transparent medium and thereafter continues on a straight course. If, as Kepler believes, things are able to act on each other only if they belong to the same genus, then it follows from the surface character of reflection and refraction that light is itself a surface.108 But how do surfaces interact, and why should the course of light (itself an immaterial surface) be altered by an encounter with the surface of a dense body? Kepler argues that both the surface that is light and the surface of the dense body participate in density. Light undergoes lateral attenuation as it spreads from a point source into a spherical form, and this introduces rarity and density (immaterial in kind, of course). The surface of the reflecting or refracting body participates in the density of the body to which it belongs: "Density is an effect of matter, which occupies three dimensions, two of which belong to the surface. Therefore, the surface participates in the density of bodies in its own measure." Thus a dense medium resists light "not insofar as it has solidity . . ., but insofar as it is bounded by a dense surface."109

On the mechanism of resistance and encounter, Kepler argues:

It is inquired .. . by what faculty a translucent surface could act on light. I answer that motion is applicable to light by the first [proposition of this chapter] and thus also the species and the remaining accidents of motion, [including,] certainly, impact on a denser surface, the overcoming of it, and a certain resistance from the thing that is overcome. It is evident that the same thing necessarily occurs in the motion of physical objects, [as] when a sphere is propelled into water and passes beneath the surface. "0

Kepler proceeds to additional mechanical analogies, but none of them satisfacto- rily answers the basic question, and we are left to wonder at the notion of two surfaces, one material and the other immaterial, interacting in mechanical or quasi-mechanical fashion. In Kepler's defense, we must acknowledge just how intractable the problem was. Both the claim that light is immaterial and the assertion that it interacts with material surfaces seemed to rest on solid observational ground-the former on the infinite swiftness of light and the fact that light is not further retarded once it has penetrated the surface of a refracting medium,

107 Kepler, Paralipomena, Prop. 5, p. 21. Note that Kepler appears to derive the infinite swiftness of light from the premise of immateriality. I do not believe, however, that we can infer the mode of discovery from the mode of presentation; I think it more likely that Kepler began with a commitment to the infinite speed of light (an ancient and medieval commonplace) and took this as evidence of immateriality.

108 "Whatever is impeded is impeded or expelled by that which is of the same genus, as body by body" (ibid., Prop. 10, p. 22). The best analysis of Kepler's theory of refraction is that of Straker, "Kepler's Optics" (cit. n. 2), Ch. 7.

109 Kepler, Paralipomena, Prop. 6, p. 21, Props. 13-14, p. 23. 110 Ibid., Prop. 20, p. 27.




the latter on the manifest phenomena of reflection and refraction. If reconcilia- tion of the apparently conflicting claims was difficult, surely an attempt to discard one of them in order to embrace the other would have been fraught with equal difficulty. We can hardly blame Kepler, then, for endeavoring to reconcile rather than discard.

VIII. KEPLER'S VIRTUS MOTRIX

We turn, finally, to Kepler's search for the motivating force of the planetary system, the virtus motrix-at once an expression of Kepler's cosmology of light and a measure of his views on the nature of light. It is well known that Kepler applied himself not only to the problem of planetary kinematics (ultimately dis- carding the Ptolemaic and Copernican apparatus of deferents and epicycles in favor of elliptical orbits) but also to the dynamics of planetary motion. It was not enough simply to devise techniques for predicting planetary positions; it was necessary also to discover the nature of the force that moves them. How does one explain this concern for the dynamics of motions that had always been treated, with fairly satisfactory results, kinematically? There were undoubtedly many reasons. To explore nature was to learn about the Creator. Moreover, the incompatibility-or, at least, serious tension-between the geometrical models of astronomical kinematics, in either its Ptolemaic or its Copernican form, and Ar- istotelian physics was acknowledged by everybody; Kepler was far from the first to look for ways of alleviating the problem. Furthermore, the traditional kine- matic approach was only fairly satisfactory; it did not give perfect results, particularly as measured against the new astronomical data of Tycho Brahe, and Kepler saw in planetary dynamics the making of an alternative method. He reports that he attempted to perfect astronomy by following the methods of the ancients but that "none succeeded except that which concentrates on the physical causes." And what must have been most encouraging is that this success led to another-to a proof of the Copernican heliocentric cosmology, of which Kepler was already an adherent: "I inquired into celestial physics and the natural causes of motion, and from this investigation at last were born clear arguments by which the Copernican opinion . .. is proved true and the other two [Ptolemaic and Tychonic] false.""'

Kepler knew from Ptolemy that the speed of a given planet varies with its distance from the center of the universe and, indeed, that at perihelion and aphe- lion the speeds are inversely proportional to the distance. Kepler generalized this inverse proportionality to the entire orbit and argued that it results from a force, a virtus motrix, emanating from the sun situated at the central point. The sun rotates on its axis, and this rotational motion is transmitted by the virtus motrix to the planetary body. The speed of any planet at any particular point of its orbit is determined by the strength of the virtus motrix at that point and the inherent inertia, or resistance to motion, of the planet.'12

Is the virtus motrix to be identified with light? Almost, but not quite. Kepler maintains that light and the motive virtue are "akin." He even claims, in a burst of enthusiasm, that "light [lux] and the motive virtue of the sun agree in all of

I Kepler, Astronomia nova (cit. n. 99), p. 20. For an illuminating analysis of Kepler's celestial physics, see Koyrd, Astronomical Revolution, trans. Maddison (cit. n. 1), pp. 167-264.

112 Ibid., pp. 185, 202-204, 206. On virtus motrix, see also pp. 185-224; and Kepler, Astronomia nova, 3.33-34, pp. 236-246.




their attributes." But as Kepler explores the nature of the virtus motrix, it becomes apparent that in fact it shares only certain attributes with light. Both are immaterial species of a quality of the sun. Both emanate instantaneously from their source to a surface or body capable of receiving them, and neither loses any of its power in passage. Both are geometrical surfaces:

This [motive] virtue (or species) will not be a geometrical body, but a certain surface, exactly as light. Thus in general the species emanating immaterially from things do not proceed according to corporeal dimensions, even though they issue from a body (in this case the body of the sun). This follows, indeed, from the law of emanation, for emanation is terminated not in and of itself, but by the surfaces of illuminated bodies; and therefore, as light is considered a certain surface because its emanation is received and terminated by surfaces, so the virtus motrix is considered as though it were a certain geometrical body because it is terminated or received by the entire bodies of the things it moves.1"3

Finally, neither of them can be said to exist in the intervening space: "The virtus motrix cannot be or subsist anywhere in the entire world except in the moved bodies; consequently, it can never be in the medium between the source and the mobile, although, so to speak, it was there, exactly as light." This bears a re- markable resemblance to Plotinus's claim that light is a "self-existent in whose path air happens to be present" but which is never "possessed" by the air, and to Ficino's assertion that light is not "so much in the illuminated body as present to it."11I4 The medium is not a requirement for transmission but merely intervening substance, an obstacle to be leapt over.

But there are cold facts that make it impossible to maintain a true identity between light and virtus motrix or to view light as the vehicle of virtus motrix. One planet can be eclipsed by another in such a way as to receive no visible light from the sun; yet it must continue to receive virtus motrix, since it does not immediately cease to move. Moreover, light emanates spherically from its source, while the virtus motrix emanates circularly from the sun. Kepler's defense of the circularity of virtus motrix is confusing and perhaps, as Alexandre Koyre has argued, weak."' Nevertheless, what required this solution was the fact, not open to dispute, that a planet's speed varies inversely with its distance from the sun. If this is so, then there is no good alternative but to substitute circular for spherical emanation; it is instructive to see the details of Kepler's theory of the virtus motrix being dictated ultimately by observational data. A final difference between light and motive virtue is the mode of their reception. Light is terminated by the surfaces of bodies, motive virtue by their "whole corporeity. "n116

We must acknowledge, then, that light and virtus motrix are similar rather than

113 Ibid., 3.33, p. 240. This last passage is quoted by Koyr6, Astronomical Revolution, pp. 200-201. On the relationship between light and virtus motrix, see also Simon, Kepler (cit. n. 84), pp. 334-336.

114 Kepler, Astronomia nova, 3.35, p. 240 (for similar claims, see ibid., p. 241; and Kepler, Parali- pomena, p. 40); cf. Plotinus, Enneads 4.5.6 (quoted at n. 18 above); and Ficino, De lumine 11 (quoted at n. 68 above). The notion of virtus motrix as an immaterial emanation appears also in Kepler's notes to the 1621 edition of his Mysterium cosmographicum, trans. A. M. Duncan, ed. E. J. Aiton (New York: Abaris Books, 1981), pp. 169 (n. 4), 171 (n. 7), and 219 (n. 5). Peter Barker and Bernard R. Goldstein, "Is Seventeenth Century Physics Indebted to the Stoics?" Centaurus, 1984, 27:155, have called attention to probable Stoic residues in these passages from the Mysterium.

115 Kepler, Astronomia nova, 3.35, 36, pp. 247-248, 250-252; and Koyr6, Astronomical Revolution, pp. 212-213.

116 Kepler, Astronomia nova, 3.33, p. 240; and Koyrd, Astronomical Revolution, pp. 200-201.




identical. "It remains, therefore, that just as light, illuminating all earthly things, is the immaterial species of the fire that is in the body of the sun, so this [motive] power embracing and drawing the planetary bodies, is the immaterial species of the virtue residing in the sun, of inestimable strength and thus the first source [actus primus] of every motion in the world." As far as I can see, there is nothing in Kepler's metaphysics that called for a distinction between light and virtus motrix. On the contrary, Kepler's Neoplatonic instincts cried out for their identification, and to these instincts he very nearly yielded. But in the end, his sensi- tivity to the phenomena prevailed over his metaphysics and forced a distinction between the two solar emanations. Nevertheless, Kepler yielded the minimum; if light and motive virtue could not be identical, then they would be different species of the same genus. "Companions," Kepler called them, and investigated them together, drawing from the one such understanding as he could not gather from the other.'"7

IX. CONCLUSIONS

It must now be apparent that Kepler's thought on light was deeply rooted in Neoplatonic philosophy. When Kepler argued that light is the "most excellent thing in the whole corporeal world, origin of animal faculties and link between the corporeal and spiritual worlds," he was echoing themes that we have en- countered in the writings of Plotinus and Ficino. His conclusion that light is the universal principle of animation is standard Neoplatonic doctrine. The unity of Kepler's cosmos-in which center, periphery, and intervening space are as closely linked as the three members of the holy Trinity, and solar force binds all celestial bodies into a single physical system-is a natural development of Neo- platonic emanationism and of the cosmogonical speculations of Grosseteste, Bonaventure, and Ficino.

But Kepler was no uncritical consumer of Neoplatonic lore. He was profoundly influenced by Neoplatonic doctrine, but this had to be harmonized at every point with the teachings of sense, intellect, and biblical revelation. Plo- tinus's emanationism, it turned out, could not be accepted without modification. In Plotinian philosophy, it is the very being of the One that overflows into lower things through a process of emanation; light is the image of the One insofar as it partakes of the same essence. The same notion is found in Avicebron, Ficino, and Patrizi. Kepler, however, insists on a much deeper cleavage between creature and Creator. Light is a divine image, but not the divine offspring. Light is the product, not of emanation from the Godhead, but of a divine decision to produce, ex nihilo, an image of Himself: "The Creator found nothing more beautiful or more excellent than Himself. That is why, when He conceived the corporeal world, He gave it a form as like Himself as possible." Light is still made in God's image, but Kepler has expressed the point in such a way as to separate himself from the pantheistic notion that the two partake of the same essence.

Kepler clearly accepts the notion, rooted in Neoplatonic emanationism, of all creatures as centers of activity. We have seen this principle in Plotinus, who argued that all things send forth a virtue representing the archetype within, and in al-Kindi, who developed the point into a full astrological system, maintaining

117 Kepler, Astronomia nova, 3.33, pp. 240, 241.




that everything sends radiation into everything else, "so that every place in the world contains rays from everything that has actual existence." Grosseteste and Bacon took over this theory and developed it into a comprehensive physics or etiology of light that saw all bodies as centers of energy, emanating species in all directions; Grosseteste and Bacon went so far as to deny all other forms of natural causation. And John Dee, of course, adopted and disseminated the Baconian gospel. Now Kepler expressed precisely the same point, arguing that corporeal things, "although of themselves confined within their own surfaces . . ., are endowed with various virtues." The latter, "nesting within the bodies but a little freer [to leave] than the bodies themselves, . . . issue forth and strive for sphericity." Under pressure of the phenomena, Kepler found it necessary to differentiate various sorts of radiating virtue-light, magnetism, and virtus motrix, for example. But this was a development, rather than a repudia- tion, of Neoplatonic doctrine.

Finally, Kepler's theory of the nature of light was surely of Neoplatonic inspi- ration. Kepler was confronted with a variety of ancient and medieval speculations about the nature of light. The atomistic theory, according to which light is itself body, had been virtually without medieval defenders but was regaining respectability in Kepler's own day.118 The Aristotelian opinion, according to which light is a property or state of a corporeal medium and thus corporeal by participation, flourished among peripatetics of course, but it had also been adopted by the Neoplatonizing Aristotelians of the Middle Ages-Grosseteste, Bonaventure, and Bacon-and through combined Aristotelian and Baconian influence had become a philosophical premise with nearly self-evidential status and a cornerstone of optical thought. Against this backdrop, Ficino reintroduced the incorporealism of Plotinus, which treated light as an incorporeal self-existent and the medium as an obstacle rather than a necessity. Faced with these alternatives, Kepler joined Plotinus and the incorporealists.

Why did he do so? A complete answer, of course, is out of reach. We will never know enough of Kepler's upbringing, formal education, personal relationships, professional circumstances, religious outlook, and emotional make-up to judge precisely why he believed what he believed. We can assert, however, that Neoplatonic philosophy in the Christianized form given it by Ficino was a force to be reckoned with at the end of the sixteenth century, that for the deeply religious Kepler its apparent compatibility with Christian theology was no insig- nificant consideration, and that one of the attractions of the incorporealist theory of light was surely its affiliation with Neoplatonic metaphysics. However, Kepler possessed an extraordinarily rigorous intellect, and any belief also had to pass rational and empirical muster. In the case of light, it appears that its acknowledged swiftness and the irrefutable fact that refraction was exclusively a surface phenomenon provided powerful support for the incorporealist theory.

While choosing the incorporealist theory of light, Kepler also creatively modi- fied it. Although Kepler's theory is fundamentally Plotinian, it is not merely Plotinian. While adopting Plotinus's position on the relationship of light to the medium, Kepler combined it with the geometrical optics of the Baconian tradition. This set the stage for two accomplishments. First, Kepler surpassed all of

118 On Thomas Harriot's attempt to convert Kepler to the corpuscular theory, see Straker, "Kepler's Optics" (cit. n. 2), pp. 507-508.




his predecessors in the mastery of this geometrical tradition and thereby equipped himself to solve several problems that had perplexed generations of optical practitioners-specifically, to formulate successful theories of vision and the camera obscura."19 Second, and more important for our purposes, Kepler carried this mathematical program to the heart of the science of optics. His claim was not simply that light can be described mathematically but that the very nature of light is mathematical. Plotinus denied all corporeality to light; Kepler took the next step and made light a mathematical substance. Devoid of all matter or corporeal substance, light belongs inevitably to the intelligible realm of mathematics. To be sure, Kepler admits that it may be "impossible to discover the innermost nature of light," but he quickly adds that "it is nevertheless a noble undertaking to probe things that pertain closely to the nature of light before proceeding to the behavior of light."'120 And when Kepler does probe the nature of light, he finds it to be a mathematical surface-not some corporeal thing pos- sessing a mathematical surface, but a mathematical surface in and of itself, an immaterial, self-subsistent, geometrical entity. The ultimate realities, here as in the search for cosmic order, prove to be mathematical.

How, then, shall we view the portrait of Kepler as empiricist, modern mathematical physicist, and mechanizer of optics? This picture is not so much false as incomplete. Kepler surely had an awesome commitment to the empirical data, which drove him on occasion to give up deeply held philosophical presuppositions rather than violate what he took to be the truth of the phenomena. He was a mathematical physicist of uncommon talent, and, if he would have been sur- prised to discover that he was a "mechanist," his work may have prepared for the mechanization of optics.'2' With a little touching up, then, the "modernist" portrait of Kepler can be judged a tolerable rendition of one of his aspects. It reveals a profile that we can acknowledge as his, although in its isolation, its lack of context, it fails to convey the full, variegated richness of the man himself. We will draw closer to the true Kepler as we see his various aspects in relation to one another, as we begin to grasp the topography of his mind and personality. There is no need, then, to renounce the portrayal of Kepler as a founder of modern science; there is only the necessity of seeing his scientific achievements in context.

119 On this mathematical tradition and Kepler's relationship to it, see Lindberg, Theories of Vision (cit. n. 3), Chs. 6, 7, 9; Lindberg, Pecham and the Science of Optics (cit. n. 55); and Lindberg, "Laying the Foundations of Geometrical Optics" (cit. n. 93).

120 Kepler, Paralipomena, p. 39. 121 J use the expression "mechanization of optics" to denote in general the effort to reduce optics to

the laws of mechanics-more specifically, the reduction of optics to the laws of motion of corpuscles. Crombie and Straker, who employ the same expression, define "mechanization" much more broadly. See Crombie, "Mechanistic Hypothesis," and Straker, "Kepler's Optics" (both cit. n. 2).



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