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Leibniz and Sensible Qualities

Christian Leduc

Université de Montréal

This paper discusses the problem of sensible qualities, an important, but underestimated topic in Leibniz’s epistemology. In the first section, the confused character of sensible ideas is considered. Produced by the sensation alone, ideas of sensible qualities cannot be part of distinct descriptions of bodies. This is why Leibniz proposes to resolve sensible qualities by means of primary or mechanical qualities, a thesis which is analysed in the second section. Here, I discuss his conception of nominal definitions as distinct empirical representations. The provisional and modifiable status of nominal definitions is then explained in the third section. Since nominal descriptions always contain sensible determinations, Leibniz claims that empirical knowledge is indefinitely changeable according to progress in sciences. In the final section, I address the criterion of coherence that enables us to approve of hypotheses that are based on both sensible and empirical properties.

Like many of his contemporaries, Leibniz was interested in the theoretical

implications of sensible qualities. Several problems arise with the introduction of the

distinction between primary and secondary, or sensible, qualities. Within this discussion,

particular emphasis was placed upon two problems: the ontological status of sensible

qualities and the epistemological function of sensible notions in scientific explanations.

Leibniz addresses both. First, he is in partial agreement with the mechanist view, especially

maintained by the Cartesians, which had consisted mainly in denying any corporeal reality to

sensible qualities: sensory qualities such as heat, cold and colour do not exist in bodies, but

are rather only the sensible effect, on the mind, of corporeal primary qualities, such as size,

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shape or movement.1 But Leibniz’s view is less radical than the Cartesian one: even though

secondary qualities are not genuinely in bodies and must be understood as sensitive and not

real contents,2 their expressive relation to reality cannot be denied. In fact, both primary and

secondary properties, to varying ontological degrees, share a kind of resemblance with

reality.3

But to account for his conception of sensible qualities – in the Nouveaux Essais sur

l’entendement humain, just as, for example, in earlier texts such as the Meditationes de

cognitione, veritate et ideis or the Consilium de encyclopaedia nova conscribenda methodo

inventoria –, Leibniz also attributes importance to epistemological arguments, instead of only

to more metaphysical ones. The distinction between primary and secondary qualities does

quite naturally lead to the question of whether they possess a certain degree of corporeal

reality,4 but Leibniz is, in general, more interested in the theoretical consequences of sensible

qualities for experience. In fact, the cognitive function of sensible qualities is the focus of the

discussion in Leibniz’s works, and constitutes his main contribution to the debate. In the first

section of this paper, I will discuss Leibniz’s conception of sensible notions. In the second

1 Descartes was one of the first modern philosophers to give priority to mechanical qualities in the explanation of bodies. Although Cartesian philosophy does not explicitly address the ontological status of sensible qualities, some of Descartes’ assertions suggest that he had serious doubts about their corporeal reality. In his response to Arnauld, for example, Descartes clearly reduces sensory qualities to pure ideas of the mind, as in the case of the idea of cold: “Ita, si frigus fit tantum privatio, frigoris idea non est frigus ipsum, prout est objective in intellectu, sed aliud quid quod perperam pro ista privatione sumitur; nempe est sensu quidam nullum habens esse extra intellectum” AT, VII, 233. See Garber (1992) 75-80. 2 Letter to Conring of March 1678, A, II, 1, 604-5. See also: NE, 2.8.21, A, VI, 6, 132-33. 3 Sensible qualities are part of the order and interconnection of things. In the Nouveaux Essais sur l’entendement humain, a theological argument is presented. God could not have simply created superfluous sensible qualities without relating them in some way to the world. The principle of sufficient reason has, in this instance, convinced Leibniz of the inadequacy of the pure mechanist thesis in his treatment of sensible qualities: NE, 2.8.13, A, VI, 6, 131. 4 In some passages, Leibniz also expresses doubts about the reality of mechanical qualities: “On peut même démontrer que la notion de grandeur, de la figure et du mouvement n’est pas si distincte qu’on imagine, et qu’elle enferme quelque chose d’imaginaire, et de relatif à nos perceptions, comme le font encore (quoique bien davantage) la couleur, la chaleur, et d’autres qualités semblables dont on peut douter si elles se trouvent véritablement dans la nature des choses hors de nous” Discours de métaphysique § 12, A, VI, 4, 1545. Leibniz clearly tries here to substitute the mechanist ontology for his own conception of bodies as substantial unities whose main property is primitive force or action. See Adams (1994) 228-234.

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section, I will consider the role of sensible qualities in the establishment of nominal

definitions. In the final two sections, I will explain the provisional status of nominal

definitions and elucidate Leibniz’s view of empirical certainty.

1. Sensible ideas as confused notions

By the end of the 1670s, Leibniz’s views on sensible qualities were more or less

determined. Some of his earlier works discussed sensible qualities, but in them, Leibniz did

not elaborate his own particular theory, distinct from the Aristotelian or Cartesian theory.5

However, from 1677 onward, Leibniz wrote a series of texts which already contain the

fundamental elements of his position. It is interesting to note that Leibniz revisited what were

essentially the same epistemological theses in his later works. The doctrine proposed at the

end of the 1670s and in the 1680s is very similar to the one expressed in later texts, such as in

his correspondence with Sophie Charlotte, and in the Nouveaux Essais.6 Leibniz’s first

emphasis, of course, is on the particularity of sensible qualities in comparison with primary or

mechanical qualities. Leibniz affirms that concepts of sensible qualities are confused,

contrary to those of primary qualities, which are distinct. In the De synthesi et analysi

universali seu Arte inveniendi et judicandi, Leibniz makes the following distinction between

two kinds of concepts:

5 In the Nova methodus discendae docendaeque jurisprudentiae of 1667, Leibniz makes a distinction, probably influenced by the Aristotelian philosophy, between two types of sensible qualities, one internal, the other external: “Qualitates porro sensibiles in specie aliae mente tantum percipiuntur; aliae et phantasia, seu mediantibue organis corporeis” § 34, A, VI, 1, 286. Two years later, the Aristotelian thesis is refuted, both in the letter to Thomasius of April 1669 (A, II, 1, 35-36) and in the Confessio naturae contra atheistas (A, VI, 1, 490-91), in which Leibniz adopts an approach similar to Descartes’ one. Finally, sensible qualities are treated in a short essay of 1671 from a scientific point of view, without directly questioning their role in a theory of knowledge: De qualitatibus sensibilibus (A, VI, 2, 288-291). 6 Lettre touchant ce qui est indépendant des Sens et de la Matière to Sophie Charlotte, GP, VI, 491-95; NE, 2.2-3; 2.29; 3.4.

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The primary concepts, by the combination of which the rest are formed, are either distinct or confused. Those are distinct which are understood through themselves, such as being. Those are confused, and yet clear, which are perceived through themselves, such as coloured, which we cannot explain to another except by pointing. For though its nature is analyzable, since it has a cause, it cannot be sufficiently described and recognized by us by means of any marks which can be explained separately, but is only known confusedly, and therefore cannot be given a nominal definition.7

The criterion of distinctness is dependent upon the possibility of defining the

represented object.8 Leibniz is consistent on this point: without definition, a concept remains

indistinct, for it does not contain the sufficient requisites or marks to be distinguished and

recognized among other things. Thus, Leibniz proposes a gradation of knowledge, according

to the degrees of distinctness and perfection that are expressed in notions. There are several

versions of this typology in Leibniz’s work, from the Introductio ad encyclopaediam

arcanam to the Paraenesis de scientia generali tradenda and the Discours de métaphysique.9

But probably its most complete and comprehensible presentation is the one proposed in the

Meditationes de cognitione, veritate et ideis, on which my analysis will be mainly based.10

First, let us focus on lower levels of knowledge – the more distinct types will be discussed

subsequently. Leibniz claims that a notion can either be obscure or clear: 1) obscure ideas are

the most vague of representational contents, and relying upon them, human understanding

cannot even recognize a previously perceived object. Again, the Meditationes give two 7 De synthesi et analysi universali seu Arte inveniendi et judicandi, A, VI, 4, 539-40, trans. in MP, 11. 8 De synthesi et analysi universali, A, VI, 4, 541. A distinct notion provides explicit marks of a thing, as is the case of gold: ”At distincta notio est, qualem de auro habent docimastae, per notas scilicet et examina sufficienta ad rem ab aliis omnibus corporibus similibus discernendam: tales habere solemus circa notiones pluribus sensibus communes, ut numeri, magnitudinis, figurae, item circa multos affectus animi ut spem, metum, verbo circa omnia quorum habemus Definitionem nominalem, quae nihil aliud est, quam enumaratio notarum sufficientium” Meditationes cognitione, veritate et ideis, A, VI, 4, 586-87. 9 Introductio ad encyclopaediam arcanam, A, VI, 4, 528-29; Paraenesis, A, VI, 4, 973; Discours de métaphysique, A, VI, 4, 1567-69. 10 Leibniz seems to consider the Meditationes to be a definitive exposition of the levels of knowledge. In the Nouveaux Essais, for example, he refers directly to this work and uses the same arguments again: NE, 2.29.2-4, A, VI, 6, 254-55.

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examples of obscure ideas: the memory of a thing, such as a flower or an animal, that does

not enable us to distinguish it again in a different perceptual context, and equivocal terms,

such as those used by the Scholastic tradition – for example, the term cause, which signifies

simultaneously the material, formal, efficient and final causes.11 Not only are obscure ideas

indistinct, and thus unsuitable for defining and analyzing, they also do not allow us to

differentiate an object in our own actual representation. 2) Clear ideas contain more

conceptual perfection than obscure ones, but are still too confused to be expressed in

definitional form. Ideas of sensible qualities belong precisely to this category. Ideas that

originate from sensation are used in our experience to indicate and distinguish an object in its

actual appearance, but are nevertheless inadequate to define it. Leibniz discusses this

characteristic of confused notions in the Meditationes:

It is confused when I cannot enumerate one by one the marks which are sufficient to distinguish the thing from others, even though the thing may in truth have such marks and constituents into which its concept can be resolved. Thus we know colours, odours, flavours, and other particular objects of the senses clearly enough and discern them from each other but only by the simple evidence of the senses and not by marks that can be expressed. So we cannot explain to a blind man what red is, nor can we explain such quality to others except by bringing them into the presence of the thing and making them see, smell, or taste it, or at least by reminding them of some similar perception they have had in the past.12

Ideas of sensible qualities, like colours, sounds, or tastes are not sufficiently distinct to

provide a definition of corporeal phenomena. Either the object must be present so we can

perceive it through our sensory organs, or it must have been sensed at an earlier moment, so

that we may be able to remember its particular qualities, in a clear but not distinct way. The

human mind must be able to point out these qualities in order to differentiate them from each

11 Meditationes, A, VI, 4, 585-86. 12 Meditationes, A, VI, 4, 586, trans. in L, 291.

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other, otherwise there is no way to recognize sensible marks. For this reason, no scientific

knowledge can be established from confused notions. It is impossible to build judgments or

demonstrations upon ideas of sensation alone, for definitions of them cannot be given and

thus cannot be expressed in predicative form. In a well-known letter to Sophie Charlotte,

Leibniz goes so far as to claim that sensible qualities are occult qualities. In establishing

empirical knowledge, we would consequently need more distinct concepts than only confused

sensible ideas to explain the essence of bodies:

To use the analogy of an ancient writer, we use the external senses as a blind man uses his stick, and they help us to know their particular objects, which are colours, sounds, odours, tastes, and tactual qualities. But they do not help us to know what these sensible qualities are or in what they consist. […] So it can be said that sensible qualities are in fact occult qualities and that there must be others more manifest which could render them understandable.13

Although ideas of sensible qualities are clear enough to differentiate an object in actual

experience or to recall such differentiation in remembered experience, as cognitive contents

they still contain too much confusion to allow an object to attain an appropriate degree of

distinctness. Thus Leibniz calls them occult qualities. He states, at times, that it would be

possible to analyze ideas of sensible qualities in a minimal way – for example, green could be

resolved in terms of blue and yellow, since our sensible perception shows us that blue and

yellow are contained in green.14 But in the end this is not particularly helpful, because it is

13 Lettre touchant ce qui est indépendant des Sens et de la Matière to Sophie Charlotte, GP, VI, 492, trans. in L, 547. For a detailed analysis of this letter, see: Mugnai (1999). 14 Leibniz says that the discrimination of blue and yellow is similar to the one we make between the cogs of a turning wheel: “Comme maintenant que nous avons la parfaite analyse du vert, en bleu et jaune, et n’avons presque plus rien à demander à son égard que par rapport à ces ingrédients, nous ne sommes pourtant point capables de démêler les idées du bleu et du jaune dans notre idée sensitive du vert, pour cela même, que c’est une idée confuse. C’est à peu près, comme on ne saurait démêler l’idée des dents de la roue c’est-à-dire de la cause, dans la perception d’un transparent artificiel que j’ai remarqué chez les horlogers, fait par la prompte rotation d’une roue dentelée; ce qui en fait disparaître les dents, et paraître à leur place un transparent continuel

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still impossible to conceive of what is distinct in blue and yellow. Therefore, ideas of blue

and yellow are still confused and cannot be analyzed in their more distinct constituents. They

do not contain definitional marks, but rather are only the result of subjective and actual

perceptions.

The confusion of sensible ideas becomes explicit when we understand Leibniz’s

opposition to Locke’s thesis. Indeed, in the Nouveaux Essais, several passages are devoted to

the epistemological problem of sensible qualities. For his part, Locke claims in his Essay

concerning Human Understanding that certain sensible ideas constitute simple ideas in our

representation of reality. With external perception, as with internal perception, we should

analyze any representational content in its primitive elements, which are simple ideas.15 For

example, the idea of white is simple, because the mind is not able to analyze it into further

constituents: the idea of white is thus conceptually indivisible. For Locke, simple ideas are

clear and positive representations, since they do not contain any notional complexity. Locke

writes in the Essay:

Thus the Idea of Heat and Cold, Light and Darkness, White and Black, Motion and Rest, are equally clear and positive Ideas in the Mind; though, perhaps, some of the causes which produce them, are barely privations in those Subjects, from whence our Senses derive those Ideas. These the Understanding, in its view of them, considers all as distinct positive Ideas, without taking notice of the Causes that produce them.16

imaginaire composé des apparences successives des dents et de leur intervalles, mais où la succession est si prompte que notre fantaisie ne la saurait distinguer. On trouve donc bien ces dents dans la notion distincte de cette transparence, mais non pas dans cette perception sensitive confuse, dont la nature est d’être et demeurer confuse.” NE, 4.6.7, A, VI, 6, 403. 15 “Though the Qualities that affect our senses, are, in the things themselves, so united and blended, that there is no separation, no distance between them; yet ‘tis plain, the Ideas they produce in the Mind, enter by Sense simple and unmixed.” Essay, 2.2.1, 119. 16 Essay, 2.8.2, 132.

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By emphasizing the cognitive function of sensations, not their objective reality, Locke

concludes that certain ideas that arise through the senses are the simplest elements of our

external perception. Even though some simple ideas have no real external causes, Locke

considers them to be clear and positive representations. Of course, simple ideas can be

obscure, because of troubled sensory organs or weakness in the memory17, but when the mind

resolves an idea in an appropriate way, that is, when it renders it simple and uniform, it

avoids any obscurity and complexity.18

Leibniz has several criticisms of Locke’s thesis. Firstly, ideas of sensible qualities are

presented as simple in Locke’s work when they are epistemologically confused. It is correct

to state that sensible notions are clear – Leibniz never suggested the contrary –, but it is false

to maintain that they are simple. Again, in the Nouveaux Essais, Leibniz uses the example of

colours to illustrate the confused nature of sensible ideas:

It can be maintained, I believe, that these sensible ideas appear simple because they are confused and thus do not provide the mind with any way of making discriminations within what they contain; just like distant things which appear rounded because one cannot discern their angles, even though one is receiving some confused impression from them. It is obvious that green, for instance, comes from a mixture of blue and yellow; which makes it credible that the idea of green is composed of the ideas of those two colours, although the idea of green appears to us simple as that of blue, or as that of warmth. So these ideas of blue and of warmth should also be regarded as simple only in appearance.19

According to Locke, confusion is caused either by combining two or more ideas

inappropriately, or by naming them in an indistinct way.20 On the contrary, Leibniz insists

that confusion and complexity are intrinsic qualities of sensible ideas. Both Locke and

17 Essay, 2.29.3, 363. 18 On this aspect of Locke’s theory of ideas, see: Bolton (2004). 19 NE, 2.2.1, 120. 20 Essay, 2.29.6-13, 364-69.

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Leibniz believe that ideas of sensation are not analyzable, but for opposite reasons: for Locke,

we are not able to further resolve sensible ideas because they are simple – meaning clear,

uniform and positive ideas –, whereas for Leibniz, those same ideas of sensation are

indivisible into simpler notions purely because of their lack of distinctness. The Leibnizian

thesis can therefore be stated as follows: ideas of sensible qualities are confused because they

are not part of distinct conceptual analysis.21 Locke was mistaken, since sensible ideas are too

confused to be resolved into distinct notions. Simplicity is related to distinctness, not to

clearness alone, contrary to what Locke maintains in his Essay. As Leibniz clearly explains in

the previous passage, ideas of blue and yellow are basically as confused as the idea of green,

even though, in the sensation of it, green appears to be constituted by these two first ideas.

This aspect has consequences for the epistemological value of sensible qualities: because

sensible qualities are intrinsically confused marks, they alone cannot constitute the basis of

empirical knowledge. Phenomenal knowledge cannot be grounded in sensation alone.22 The

contribution of reason is therefore essential in establishing distinct empirical concepts, as

Leibniz claims in the Nouveaux Essais: “And the linking of phenomena which warrants the

truths of facts about sensible things outside us is itself verified by means of truths of reason,

just as optical appearances are explained by geometry”.23 Contrary to Locke, whose empirical

method is based mainly upon the perception of simple ideas, thus of sensible qualities,

Leibniz denies the intrinsic scientific value of such sensible ideas, because they alone are

insufficient for the establishment of distinct empirical knowledge. Sensible qualities cannot

be the sole theoretical foundations of experimental sciences and must therefore be

accompanied by more perfect and distinct kinds of representation.

21 NE, 2.20.6, A, VI, 6, 165-66. 22 Hidé Ishiguro also insists on this point: (1990) 62-63. 23 NE, 4.2.14, A, VI, 6, 374-75.

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2. Nominal definition

The second aspect of Leibniz’s view on sensible qualities is their role in establishing

empirical knowledge. We know that confused notions alone are insufficient to provide

definitional marks or properties of a represented object. Nevertheless, as I noted previously, it

would be less than completely true to say that Leibniz simply rejects the notion of sensible

qualities having a place in the scientific realm. Sensation cannot be the origin of distinct

empirical notions. However, when sensible notions are related to intelligible ones, they

become, in several cases, important theoretical sources for explaining phenomena. In fact,

sensible notions must be resolved through the help of mechanical or primary notions. It is

because mechanical notions are represented in our understanding as distinct marks that they

are given definitions and conceptual analysis. When applied to sensible qualities, those

mechanical notions make distinct empirical knowledge possible. Already in 1677, in a short

text entitled Revocatio qualitatum confusarum ad distinctas, Leibniz expresses this idea of

linking confused notions to distinct ones such as size, shape, and movement:

The secret of physical analysis consists in the only art of linking (revocare) sensible qualities (namely, heat and cold by touch; flavours by taste; odours by olfaction; sounds by hearing; colours by sight) to distinct qualities which accompany them, and which are number, magnitude, shape, movement, consistency, of which the latter two are particularly physical. […] it is also necessary that confused qualities result from such order, although we cannot otherwise demonstrate confused qualities by themselves, because no definition is given of confused qualities, and therefore no demonstration from them.24

24 Revocatio qualitatum confusarum ad distinctas, A, VI, 4, 1961-62 (my translation).

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Leibniz does not simply try to reduce sensible qualities to mechanical principles, but

rather attempts to explain why ideas of sensation cannot be described without the help of

those distinct principles. The excerpt clearly states that sensible qualities are accompanied by

qualities such as size, shape, and movement. Although sensible ideas are confused, they are

nevertheless related to primary notions which create the basis for conceptual distinctness.25

Again, the example of colours is explicit: our idea of a red object, for instance, is complex

and contains other qualities confusedly. Among these qualities are primary type ones which

enable us to perceive the object as being of a particular size, shape and movement. It is from

such qualities that a distinct description of bodies emerges, since it is possible to resolve the

mechanical qualities contained concretely in the idea of red into general and distinct

explanations. In fact, Leibniz never denied that general laws of nature could be explained by

purely rational principles – those of geometry and mechanics.26 The discursive treatment of

primary qualities often occurs prior to any sensible knowledge. But detailed descriptions of

bodies, applied to concrete reality, must account for sensible qualities. The Nouveaux Essais

clearly supports this claim:

But the ideas of sensible qualities such as colour, flavour etc. (which are really only illusory images) do come to us through the senses, i.e. from confused perceptions. And the truth about contingent singular things is grounded in success, whereby sensory phenomena are linked together in just the way required by truths of the intellect.27

Following the distinction between obscure and clear notions, and then between

confused and distinct notions, Leibniz divides distinct knowledge into two further types: a

25 Revocatio, A, VI, 4, 1962. 26 Leibniz repeats constantly that laws of nature can be explained by purely mechanical notions that are conceived a priori by the human reason: Elementa rationis, A, VI, 4, 722; Anima quomodo agat in corpus, A, VI, 6, 1367; NE, 1.1, A, VI, 6, 71-73. 27 NE, 4.4.1, 392.

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distinct notion can either be inadequate or adequate.28 Let us first consider the latter. 1) An

adequate notion is obtained when all requisites of a distinct notion are conceived of distinctly

in the same way and result in a real definition. Compared to empirical notions, adequate

notions are fully distinct representations that contain all the properties of a given essence. I

will not dwell on this aspect of Leibniz’s philosophy of knowledge, but it is clear that real

definitions signify necessary truth. For example, geometrical axioms or mechanical laws,

such as the law of inertia or of causality, belong to the adequate type of knowledge. Human

understanding proceeds a priori to formulate adequate notions, without the contribution of

sensible qualities.29 2) On the contrary, inadequate knowledge combines intelligible and

sensible concepts. To attain distinct empirical knowledge, sensible qualities must be

associated to mechanical qualities, and not simply be reduced to primary concepts. But

instead of leading to real and complete definitions, empirical concepts can only be conceived

of nominally. According to Leibniz, the main difference between adequate and inadequate

notions is their degree of distinctness – in other words, their conceptual completeness and

perfection.30 While complete and real definitions contain all the properties that express the

essence of a thing distinctly, inadequate and nominal definitions only give sufficient

requisites or marks to be able to recognize the thing among others. In the De synthesi et

analysi universali, Leibniz explains what a nominal definition is:

A nominal definition consists of the enumeration of the marks or requisites which are sufficient to distinguish a thing from all others; where, if we continue to seek the requisites of requisites, we shall finally arrive to primitive concepts, which are either absolutely without requisites, or are without requisites which can be explain

28 Meditationes, A, VI, 4, 587. 29 Meditationes, A, VI, 4, 587. 30 Complete and perfect concepts express primitive entities. See the Introductio ad encyclopaediam arcanam: A, VI, 4, 528.

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sufficiently. This is the art of handling distinct concepts. To the art of handling confused concepts there belongs the indication of the distinct concepts, into such, which accompany the confused ones. By means of these we can sometimes arrive at the cause of confused concepts, i.e. at some analysis of them.31

Nominal definition, as opposed to real definition, determines concepts by way of

sufficient requisites, and not by considering all properties signifying the complete possibility

of a thing. Even though empirical notions are incomplete, Leibniz considers them distinct, as

long as they enable us to identify the thing being defined from all the others.32 The complete

analysis of empirical concepts would indeed lead to full and adequate notions, of the

mechanical or geometrical sort, which are expressed in real definitions.33 However, a nominal

definition is sufficient to conceive of phenomena distinctly. Leibniz claims that nominal

definition allows us to give a partial explanation of the essence, and that it is distinct enough

to theoretically recognize an object. At this point it is important to differentiate three main

kinds of notions in Leibniz’s epistemology: 1) sensible ideas, which are confused perceptions

resulting from the sensation alone; 2) intelligible and necessary notions, which are obtained a

priori and for which real and complete definitions can be given; and finally, 3) empirical

notions, which are expressed by nominal definitions and which provide distinct descriptions

of phenomenal bodies. As we shall see, the notion of gold illustrates perfectly this mixed

mode of representation in which two kinds of concepts are implied. An empirical notion is

therefore neither a mere confused idea, such as colour or heat, nor a full distinct notion, such

as size, shape or movement, but a concept composed of the first two types of notions. And it

is through the help of both sensible and intelligible notions, not only of primary or

mechanical principles, that an appropriate knowledge of bodies can be established.

31 De synthesi et analysi universali, A, VI, 4, 540, trans. in MP, 11-12. 32 De thematum tractatione, A, VI, 4, 13. 33 Meditationes, A, VI, 4, 589.

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How exactly is the analysis of sensible qualities by means of distinct notions possible?

In other words, what differentiates distinct rational notions from empirical notions? This line

of questioning is significant, particularly in order to place Leibniz’s theory of experience

among modern thought. On one hand, Leibniz refutes the Cartesian view that our only source

of certainty is in intelligible notions. Indeed, Descartes believed that the demonstration of

mechanical principles was quite enough to conceive of bodies clearly and distinctly.

Sensation and imagination only confirm truths that were already known in the understanding

by rational principles.34 Thus, sensible qualities would not contribute to scientific learning,

but would rather testify in the particular to what was known universally by distinct general

principles.35 On the other hand, Leibniz does not simply return to a more empiricist

conception, such as Locke’s theory of experience, since he acknowledges that sensible ideas

are, by themselves, confused perceptions. Contrary to Locke, Leibniz does not claim that

sensible ideas are simple cognitive contents, but rather states that they are confused

perceptions unsuitable for notional analysis, and consequently for the establishment of

distinct experimental sciences.36

Leibniz therefore offers a third view that consists in coordinating both sensible and

primary concepts to establish distinct experience. To illustrate how sensible qualities

contribute to nominal knowledge, Leibniz often uses the example of gold. Firstly, he points

out that the notion of gold is neither a pure rational notion nor a mere sensible idea, but

belongs to the mixed mode of empirical concepts which combines the two other types of

34 Discours de la méthode, AT, VI, 61-65. 35 On this point, see: Garber (2001) 104-110. 36 Leibniz states against Locke that sensible qualities could not be described by real, nor by nominal definitions: NE, 3.3.18, A, VI, 6, 295.

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qualities.37 In other words, the mind could not express rationally the complete essence of gold

by enumerating a priori all its properties, nor is it limited to the confused perception of gold’s

sensible qualities. The essence of gold implies qualities such as shape, size and extension, but

also qualities that are partially explained by sensible ideas: heaviness, malleability and

fusibility are constituents of the nominal definition of gold which result from applying

mechanical to sensible notions. Leibniz writes the following in the Nouveaux Essais:

Gold can be nominally defined in various ways – it can be called the heaviest body we have, the most malleable, a fusible body which resists cupellation and aquafortis, etc. Each of these marks is sound, and suffices for the recognition of gold: provisionally, at least, and in the present state of the bodies around us, until the discovery of a still heavier one such as some alchemists claim their Philosopher’s Stone to be, or until we are shown that fixed silver, a silver-coloured metal with nearly all the other qualities of gold, which the Honourable Robert Boyle seems to say that he has made.38

I will set aside, for the moment, the provisional character of nominal definitions in

order to focus on the epistemological procedures that form the basis for empirical notions.

The cited passage clearly states that each empirical mark is sufficient to recognize gold

among other sorts of metal. If an assayer tries to determine whether a certain metal is made of

gold, he can do so by applying any one of the tests generally used to discern gold: if it resists

cupellation or if it is of the heaviest kind of metal, it is most likely made of gold.39 Each of

these properties constitutes a sufficient mark to define gold nominally, for they each

distinctly differentiate this specific essence from all the others. It must be noted here that

these properties belong to the previously identified composed type of empirical notions.40 To

conceive of heaviness, malleability or fusibility, it is necessary to combine mechanical

37 De tractatione rei, A, VI, 4, 547-48. 38 NE, 3.4.16, A, VI, 6, 299-300. 39 NE, 3.3.18, A, VI, 6, 294-95. 40 Meditationes, A, VI, 4, 587.

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principles and sensible qualities. For instance, the nominal definition of fusibility certainly

implies distinct qualities such as size, shape and movement, but it is also composed of

sensitive qualities, such as heat, that are obtained directly through tactile sensation. Without

the notion of heat, it would be impossible to express the essence of gold by way of the

property of fusibility. In nominal definitions, rational notions allow sensible qualities to be

formulated in a predicative form so that they become distinct marks. In other words,

mechanical and geometrical notions quantify sensitive qualities in order to explain

phenomena distinctly. This does not mean that, in this theoretical context, confused ideas of

sensation become distinct: the ideas of colours or heat remain confused, but when the mind

associates them to distinct notions, sensible ideas participate, in a certain way, in the

cognitive distinction of corporeal entities. In elaborating sensible qualities with the help of

mechanical or geometrical qualities, it becomes possible to obtain scientific definitions and

demonstrations. According to Leibniz, the mediation of distinct concepts is therefore always

necessary for the establishment of nominal definitions.

Moreover, in the Consilium de encyclopaedia nova conscribenda methodo inventoria,

Leibniz identifies a particular field of phenomenal science that is concerned exclusively with

sensible qualities. The Pœographia determinates how and which sensible qualities enable us

to obtain a distinct empirical conception of nature. Not only do primary qualities have a

major role to play in describing sensible qualities, but the analysis of sensible qualities is also

important in and of itself. In this text, Leibniz once again claims that strictly sensitive

qualities must be differentiated from composed and definable qualities:

The tenth is the Science of sensible qualities, which I use to call Poeographia. These qualities, insofar as they are to be defined, should be distinguished by their varieties and degrees, the subjects in which they exist and those which produce them, and finally the

17

consequences that follow from them. They are either simple or compounded. The former cannot be described; rather, in order to be cognized they must be sensed, which is the case with light, colour, sound, smell, taste, heat, cold. The latter can be explained by description: hence they are somehow intelligible, like solidity, fluidity, smoothness, viscosity, friability, fissility and other qualities of this kind; similarly, volatility, fixity, solubility, coagulation, precipitation. The preceding simple qualities cannot be subjected to reasoning, except insofar as they are usually associated with the compounded ones, as well as with the preceding ones which share with them magnitude, place and change. Hence, the simple qualities have to be treated historically, i.e., one should list in what manner they are usually associated with each other and with the intelligible qualities.41

The aim of the Pœographia is to enumerate which sensible qualities have a scientific

function in our description of bodies and to identify the degree to which they are related to

mechanical principles. Developing the theses begun in the Geometrica and the Mechanica,42

the science of sensible qualities in the Pœographia not only details our investigation of nature

by making the description of more particular phenomena possible, but also informs us of

essential generic and specific differentiations. For example, disciplines explaining the

chemical combinations of corporeal substances or describing the generation and the

corruption of organic bodies could not operate fully without the help of sensible qualities.

Research into chemical structures relies upon the use of multiple experimental marks and

distinctions that are obtained in part through sensible qualities. Ideas of colours or sounds, for

instance, would help to distinguish different kinds of metal, while those same ideas

associated to tactile perceptions would certainly contribute to the classification of vegetal and

animal species.43 As the passage also states, it is important to explain types of sensible

qualities, for this will determine their scientific function and epistemological potential. It is

essential to describe all sensible qualities in a historical way, namely by drawing as complete

41 Consilium de encyclopaedia nova conscribenda methodo inventoria, A, VI, 4, 347, trans. in D, 137-38. 42 Consilium, A, VI, 4, 346. On a detailed analysis of the Consilium and especially of the different kinds of scientific disciplines presented by Leibniz: Duchesneau (1993) 24-30. 43 Consilium, A, VI, 4, 348-49.

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a list of them as possible and by enumerating which of them could be related to intelligible

notions and which, on the contrary, are too confused to be put into a definitional form.44

However, once again, nominal definitions established by combining sensible and

primary qualities remain partial and incomplete descriptions. This is why Leibniz claims that

there could be several nominal notions of a same natural essence. Each of these marks is a

sufficient description to explain how a corporeal entity belongs to this or that kind, or species,

and then to recognize it among other essences. But these marks are far from offering

complete analyses of bodies. Despite the variety of distinct ways to grasp phenomena

conceptually, empirical representations could never express, even if they were all combined,

the full possibility of an object. The presence of confused concepts in scientific investigations

limits our empirical knowledge of phenomena. However, it remains relevant to draw upon

sensible qualities, for it is the only way to reach detailed descriptions of bodies. Without

predicates such as heaviness, malleability, and fusibility, which are partially obtained from

sensible ideas, it would be impossible to have a specific conception of gold. Mechanical and

geometrical notions are certainly more adequate, but are not meant to reach lower degrees of

explanations and classifications. For Leibniz, nominal notions provide distinct but multiple

and incomplete descriptions of natural kinds and species.45

3. Provisional knowledge

44 Following the Poeographia, Leibniz distinguishes the science that deals with phenomenal similarities, which he calls Homeographia. The Poeographia seems to be a prerequisite for this latter discipline, since the enumeration of simple and complex sensible qualities is necessary to the explanation of their degrees of difference and resemblance: “Horum corporum enumerandae sunt qualitates supra dictae secundum gradus differentiasque suas, tum eas quae in corpore rudi sponte sensui se offerunt, tum quae in eo per se tractato vel aliis corporibus commixto prodeunt” Consilium, A, VI, 347-48. 45 NE, 3.3.15-18, A, VI, 6, 293-95.

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The role of sensible qualities in Leibniz’s philosophy of knowledge seems clear at this

point. There is a type of notion in which sensible qualities appear to be essential, and by

which nominal definitions are established. Let us reconsider the two major points of

Leibniz’s position: 1) sensible ideas are confused concepts that appear simple, but are in fact

unsuitable for conceptual analysis. Conceptual analysis is possible when a notion can be

understood in its constituent parts, namely, by simpler and more distinct concepts. For

example, the term accident is more complex than the term substance, and by analyzing the

first we will arrive at the latter, since substance is more distinct and simple than accident.46 In

the case of sensible qualities, this analytical process cannot take place, because it is simply

impossible to determine the constituent parts of confused and sensitive indivisible ideas; 2) to

obtain distinct empirical knowledge, it is essential to combine sensible ideas with primary

ones. Conceptual analysis is based on this cognitive combination; otherwise sensible qualities

would be represented by confused perceptions. Subsequently, if an explanation of Leibniz’s

conception of empirical knowledge is to be proposed, the resolution of sensible qualities by

mechanical or geometrical qualities must be considered. Moreover, empirical knowledge,

contrary to rational knowledge, leads to multiple descriptions of bodies, to several nominal

definitions that express distinct but incomplete concepts of a same essence: gold can thus be

explained by different nominal requisites such as heaviness, malleability, and fusibility.

Another essential point must now be addressed: since nominal definitions are partial

descriptions of reality – each nominal mark being sufficient to distinguish one essence from

another –, it means that they might also be modified as progress is made in empirical

investigations. Leibniz claims that nominal definitions amount to a provisional knowledge of

46 De realitate accidentium, A, VI, 4, 995-96. See again the Introductio ad encyclopaediam arcanam, A, VI, 4, 529.

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reality. Each nominal definition is a partial explanation of bodies that could potentially be

replaced later on, depending on the evolution of scientific discoveries. The fact that empirical

notions are partially composed of confused ideas of sensible qualities limits our capacity to

completely describe natural essences. Hence, our conception of phenomena evolves

according to scientific progress.47 Nominal definitions, as we have seen, are incomplete

descriptions of phenomena. Even though it represents reality by means of clear and distinct

concepts, empirical knowledge remains inadequate, since it does not express all properties of

a given thing, but only the sufficient series of predicates to be able to recognize it as distinct

from all the others. Let us turn again to the example of gold: the description of gold is based

on both primary and secondary concepts and therefore constitutes a partial definition of this

specific essence. The term gold is not composed of only necessary and a priori truths – which

include geometrical and mechanical properties –, but also of contingent truths which cannot

be grasped completely by the human mind. This is why it is a provisional explanation of

gold, which is described by different distinct marks such as heaviness, malleability, and

fusibility. Each of these empirical marks constitutes a way to identify and classify gold

among phenomenal species, via a distinct, but incomplete, representation. Even if they were

all combined, the nominal properties of a given phenomena could not provide an adequate

and full description. In a short text of 1677, entitled De notionibus empiricis, Leibniz

explains the provisional, indeed artificial, character of nominal definitions by taking up again

the example of gold:

In empirical notions, such as gold and other things for which there is no possibility other than a posteriori, there can only be provisional definition. For example, if a certain gold was invented that possessed all the properties of natural gold known up

47 NE, 1.1.23, A, VI, 6, 85.

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until now and, if that gold responded to the usual tests, and if someone presented himself and indicated a new test to which this artificial gold would not stand up, this person would certainly have filled a gap in the definition of gold in use until now. Moreover, it must be noted that knowing the list of attributes of the thing is not sufficient grounding for definitions of empirical notions (as I call them), because if we were to concurrently unify all properties for definitional purposes, we would produce a definition longer than necessary; but it is necessary to see, by comparing them to other notions that share common attributes, which ones are sufficient to distinguish the thing (such as gold) from all those things that are known, and since this can be done in several ways, we are dealing with as many diverse definitions of gold. Thus, gold can be defined as the heaviest body, as the most ductile metal, as a yellow metal that we can cupellate or as a metal that we can cupellate and make malleable.48 As we have seen in a latter passage of the Nouveaux Essais,49 Leibniz clearly conceives

of nominal definitions as partial and provisional descriptions. But in this quotation, another

aspect is mentioned: nominal definitions are more similar to conceptual constructions than to

complete and adequate demonstrable requisites.50 The notion of gold gathers together

properties that identify and describe this metal according to our actual scientific learning.

Nominal knowledge is based on partially confirmed hypotheses and therefore provides a

provisional and sometimes artificial description of specific essences. It is by no means an

exact and final explanation of bodies, since there could always be subsequent, more distinct

concepts that would replace it. Empirical notions remain conjectural descriptions of

phenomena, and are constantly modified by and dependent upon the progress of science. The

essence of gold is therefore not conceived of by a unified definition, but by several partial

descriptions that identify and specify it in different ways. This does not mean that nominal

knowledge is, for Leibniz, indistinct, but only that it is indefinitely changeable and

modifiable. Empirical knowledge depends upon scientific experiences that must take

48 De notionibus empiricis, A, VI, 4, 16 (my translation). 49 NE, A, VI, 6, 3.4.16. 50 In the Recommandation pour instituer la science générale, Leibniz also emphasizes the hypothetical status of nominal definitions: A, VI, 4, 706.

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confused notions into account, such as those of sensible qualities, and is therefore limited in

its search for truth. This also means that nominal definitions are based on progressive

empirical procedures that could eventually alter the meaning of empirical concepts. The

hypothetical status of nominal definitions, contrary to real definitions, subsequently enables

conceptual adjustments that could in fact change our conception of reality. By modifying

nominal definitions, scientific experimentations also alter the meaning of our concepts, and

thus change the way we identify, describe and classify phenomenal properties. 51 If it happens

that we discover a heavier, more malleable or more fusible metal than gold – which indeed

has been the case –, our empirical concept of gold would be largely modified, to such a

degree that its meaning could radically change. Ultimately, we would not identify and

describe gold as we do at present.52 Leibniz later echoes the same postulation, as in the

following letter to Condi:

I support the method of drawing from phenomena what we can draw without supposing anything, even though this would sometimes only be to draw conjectural results, however when Data are not sufficient it is permitted (as we do sometimes in

51 For explanations about nominal definition and classification, see: Duchesneau (1988). 52 In her book, Leibniz’s Philosophy of Logic and Language, Hidé Ishiguro devotes an entire chapter to the problem of sensible qualities in Leibniz’s philosophy. She addresses in particular the problems of property identity and substitutability in relation to sensible qualities. In fact, the application of the principle of substitution salva veritate requires the extensional identity of represented objects. Property identity would then be based, according to Ishiguro’s interpretation, on the extension of concepts, not on their intension. Thus, the multiplicity of notions, of sensible, empirical or rational types, would enable us to have different comprehensions of the same extensional object or property. She writes: “We can learn to identify the heat, i.e. the fact that the object is hot, by having that sensation. But whenever an object is hot its constituent molecules might be in a particular state of motion of which we can learn in other ways, and predicates expressing such states of the object will be part of the concept or idea of heat. Learning such predicates of hot objects will not itself enable us to tell that a sensation we have is that of heat or that it is not, but from this it does not follow that these predicates are not constituents of the concept of heat. On the contrary, we would have a fuller concept of heat if we not merely identified it as a phenomenon which is there when we have a particular kind of sensation, but learned that the phenomenon which makes us have that kind of sensation has various properties which other people (who have no such sensations) will be able to detect as well” (1990) 65. The major problem with this interpretation is the lack of emphasis upon the important difference between adequate and inadequate concepts: in the latter, neither intensions nor extensions seem to be absolutely interchangeable.

23

deciphering) to imagine hypotheses, and if they are fortunate, we hold them provisionally while we are waiting for new experiences to bring us Nova Data […].53

In short, Leibniz never considered that complete and fixed conceptions of phenomenal

reality could be achieved. As the De notionibus empiricis clearly claims, empirical

knowledge is composed of alterable and modifiable conceptual constructions that depend

upon scientific progress and discoveries. A nominal notion is always a hypothetical

description that could later be replaced by a second, one that is more in agreement with our

actual knowledge of nature. Each empirical property is subject to experimental research

which aims to make our nominal definitions more distinct. In other words, the resolution of

sensible qualities by mechanical and geometrical qualities leads to more adequate

descriptions, which obviously has consequences for the establishment and evolution of

nominal definitions.54

4. Moral certainty

As I noted previously, intelligible principles, of geometrical and mechanical sorts,

must be conceived of as theoretical guides in the establishment of empirical notions. Without

them it would be impossible to discuss sensible qualities in a distinct way. But this does not

prevent nominal definitions from being relative and unfixed concepts. On the contrary, they

will always contain equivocal determinations. The support of intelligible principles in

empirical descriptions does not ensure full certainty. This is why Leibniz makes a final 53 Letter to Conti of December 1715, R, 42 (my translation). 54 It is probably for this reason that Leroy E. Loemker considers Leibniz’s philosophy of experience to be pragmatic: Loemker (1973) 170-71. There is obviously a theoretical link between Leibniz’s conception of experience based on provisional procedures and a more contemporary pragmatic theory of truth. But I believe that the term pragmatic is somewhat too historically marked and that Leibniz certainly not imply that truth is based on usefulness: NE, 4.1.1, A, VI, 6, 355-361.

24

distinction between two kinds of certainty that are based on types of representation, namely

between metaphysical and moral certainties. Instead of reaching metaphysical certainty – as

is the case with geometrical and mechanical notions expressed by real and complete

definitions –, empirical knowledge only reaches what Leibniz called moral certainty: the first

is completely demonstrable, while the second only partially is and requires the help of both

induction and demonstration. The presence of sensible qualities in a posteriori knowledge

accounts for this incompleteness. Contrary to a priori notions, we do not possess complete

and permanent explanations of empirical phenomena. In the text entitled De modo

distinguendi phaenomena realia ab imaginariis, Leibniz explains this difference:

We must admit it to be true that the criteria for real phenomena thus far offered, even when taken together, are not demonstrative, even though they have the greatest probability; or to speak popularly, that they provide a moral certainty but do not establish a metaphysical certainty, so that to affirm the contrary would involve a contradiction. Thus by no argument can it be demonstrated absolutely that bodies exist, nor is there anything to prevent certain well-ordered dreams from being the objects of our mind, which we judge to be true and which, because of their accord with each other, are equivalent to truth so far as practice is concerned. 55

The text from which this latter passage is drawn is primarily concerned with the

criterion of coherence which enables us to differentiate imaginary from real perceptions – an

idea on which I will not emphasize here.56 But in this excerpt, Leibniz clearly states that

judgments of facts, and therefore empirical knowledge, can never attain metaphysical

certainty. Since empirical notions contain confused marks – sensible qualities –, their

55 De modo distinguendi phaenomena realia ab imaginariis, A, VI, 4, 1502, trans. in L, 364. The Dissertatio de stylo philosophico Nizolii of 1670 already mentions the difference between metaphysical and moral certainties, namely between purely demonstrable knowledge and empirical knowledge which combines rational and sensible notions and requires the help of both induction and demonstration: A, VI, 3, 431. See also the De Veritatibus, de Mente, de Deo, de Universo of 1676 that emphasizes the relation between certainty and distinction: “Illud interea fatendum est demonstrationum longius pratractarum non esse nisi moralem certitudinem, quae scilicet uno mentis actu totae pervideri non possunt” A, VI, 3, 508. 56 See Poser (2000); Adams (2005).

25

demonstrability is largely limited. It is true that Leibniz considers, at times, the full

demonstrability of confused marks to be possible in the afterlife.57 At this moment, we could

attain a beatific vision that would enable us to have a perfect knowledge of the natural

world.58 Theoretically our previous hypothetical knowledge of bodies could reach

metaphysical certainty and complete distinction. However, with our present finite cognitive

capacities we could not attain the beatific level of knowledge. Therefore, the full

demonstrability of corporeal properties remains, in practice, impossible to achieve.59

Thus, the degree of certainty in empirical knowledge is not metaphysical, but moral,

since it is not concerned only with necessary truths, but also with contingent truths that the

human mind cannot completely analyze. Consequently Leibniz attempts to put forward an

empirical criterion in order to determine whether a nominal definition may be vindicated. In

fact, the validity of nominal notions is not only based on their level of distinctness – which

comes from primary notions –, but also on their general accordance with our actual scientific

knowledge of nature. Clearly, nominal definitions are not conceptually fixed and can

therefore be modified by more distinct descriptions. But this empirical procedure must be

done progressively, by creating a hypothesis with regard to other existing scientific

hypotheses and theories. Leibniz raises this point in the Nouveaux Essais:

If the ideas we combine are compatible, then the limits we assign to species are always exactly conformable to nature; and if we are careful to combine ideas which actually occur together, our notions are also conformable to experience. If we regard them as

57 This point was raised by an anonymous referee. 58 Examen religionis christianae, A, VI, 4, 2452. 59 See the letter to Foucher of 1675: “Cet accord perpétuel donne une grande assurance, mais après tout elle ne sera que morale, jusqu’à ce que quelque homme découvre a priori l’origine du monde que nous voyons, et qu’il puise dans le fond de l’essence pour quoi les choses sont de la manière qu’elles paraissent: car cela étant, il aura démontré que ce qui nous paraît, est une réalité, et qu’il est impossible que nous en soyons désabusés jamais. Mais je crois que cela approcherait fort de la vision béatifique, et qu’il est difficile d’y prétendre dans l’état où nous sommes” A, II, 1, 391.

26

only provisional with reference to actual bodies, and as subject to experiments which have been or will be made to discover more about them, and if we have recourse to the experts when fine points arise about whatever it is that the name is generally understood to stand for, then we shan’t be doing anything wrong. Thus, although nature can furnish more perfect and more convenient ideas, it will not give the lie to any ideas we have which are sound and natural even if they are perhaps not the soundest and most natural.60

Two important points are underlined in this excerpt. Firstly, within experimental

research it is possible to determine the soundness of a nominal definition or hypothesis by

considering its theoretical relation to other phenomenal explanations.61 Leibniz seems to

claim that an original description or a new discovery cannot be entirely in conflict with our

actual empirical conception of reality. Secondly, since nominal knowledge is provisional, the

empirical procedure leading to more perfect and more distinct definitions must be

progressive. Contrary to metaphysical certainty, moral certainty is attained by degrees,

assuring us of the validity of empirical hypotheses. Once again, the presence of sensible

qualities in our empirical descriptions – namely of confused and uncertain representations –

forces us to adopt a progressive method of investigation. If it were not for these sensible

qualities, our knowledge of phenomena could be completely resolved. Even though

intelligible principles of geometry and mechanics help to furnish distinct empirical

knowledge, the degree of certainty they provide will always be conjectural. In the Nouveaux

Essais, Leibniz supports this point once more:

But if experience supports these conclusions in a regular way, do you think that we can arrive in this at propositions which are certain? – as certain, at least, as those which affirm that the heaviest body we possess is fixed, and that the next heaviest is volatile. For it seems to me that, in the case of propositions which we have learned from

60 NE, 3.6.30, A, VI, 6, 322-23. 61 Letter to Conring of March 1678, A, II, 1, 600.

27

experience alone and not by the analysis and connections of ideas, we rightly attain to certainty (moral or physical, that is) but not to necessity (metaphysical certainty).62

In sum, Leibniz’s theory of experimental knowledge combines both primary conceptual

resources and more practical procedures in order to explain the way in which new discoveries

and hypotheses must be vindicated. Empirical knowledge leads to a kind of certainty, namely

moral certainty, that is grounded on progressive scientific procedures. These procedures are

essentially based on a criterion of cohesion between past and future nominal definitions.

Before being ratified, a new empirical description must be in general accordance with our

actual theories and hypotheses about reality.

5. Conclusion

It has been my intention in this paper to examine the treatment of sensible qualities in

Leibniz’s theory of experience. Four aspects have been considered in particular: 1) Leibniz

seems to have occupied a middle position regarding the epistemological function of sensible

ideas. Contrary to the Cartesian view, Leibniz does not reduce the role of sensible qualities to

one of confirming in the particular what was already known by rational principles. Sensible

qualities contribute to the elaboration of empirical knowledge. Contrary to the Lockean view,

Leibniz denies that sensible ideas could be the foundation of empirical knowledge. These

cognitive components alone remain confused representations of reality; 2) confused concepts

of sensation must be resolved by means of more distinct knowledge, namely by intelligible

notions of the geometrical or mechanical sort. This resolution does not mean complete

reduction, but rather associates confused with distinct notions in order to create nominal 62 NE, 4.6.13, A, VI, 6, 406.

28

definitions. Thus it becomes possible to express the sufficient marks in distinguishing

phenomenal entities. According to Leibniz, the essence of gold is described by a series of

nominal definitions, each of which is sufficient to differentiate this metal among others. This

is accomplished by composing empirical notions from both sensible and rational concepts; 3)

contrary to complete and adequate necessary concepts, empirical concepts are provisional and

partial explanations of reality. Moreover, the conceptual contents of empirical knowledge are

constantly being modified by new scientific discoveries. Each mark that identifies gold could

potentially be replaced by a more distinct one, meaning that our description of gold will most

likely progress; 4) finally, Leibniz proposes, mainly in the Nouveaux Essais, a

methodological criterion aimed at directing empirical investigations. For a new nominal

description to be vindicated, it must be in general accordance with our actual conceptions of

phenomenal reality. Leibniz puts forward this practical criterion because of the provisional

status of empirical notions which only lead to moral, and not metaphysical, certainty. Thus,

sensible qualities can be elements of our phenomenal knowledge of nature, but only under

certain essential conditions. Without these theoretical conditions – of distinction and cohesion

–, Leibniz posits that sensible qualities remain confused marks, unsuitable for any kinds of

conceptual analysis.

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