levels explanation

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Levels of Explanation Vindicated

Vctor M. Verdejo & Daniel Quesada

Published online: 25 September 2010

# Springer Science+Business Media B.V. 2010

Abstract Marrs celebrated contribution to cognitive science (Marr 1982, chap. 1)

was the introduction of (at least) three levels of description/explanation. However,

most contemporary research has relegated the distinction between levels to a rather

dispensable remark. Ignoring such an important contribution comes at a price, or so

we shall argue. In the present paper, first we review Marrs main points and

motivations regarding levels of explanation. Second, we examine two cases in which

the distinction between levels has been neglected when considering the structure of

mental representations: Cummins et al.s distinction between structural representa-

tion and encodings (Cummins in Journal of Philosophy, 93(12):591614, 1996;

Cummins et al. in Journal of Philosophical Research, 30:405408, 2001) and

Fodors account of iconic representation (Fodor2008). These two cases illustrate the

kind of problems in which researchers can find themselves if they overlook

distinctions between levels and how easily these problems can be solved when levels

are carefully examined. The analysis of these cases allows us to conclude that

researchers in the cognitive sciences are well advised to avoid risks of confusion by

respecting Marrs old lesson.

Ever since Marrs influential work on the computational account of vision (Marr

1982) it has been a familiar idea that computational research can be taken to

involve (at least) three levels of description/explanation. These are: the level of the

function to be computed; the level of the algorithm that computes the given

function; and finally, the level of realization of the function in hardware structures.

Rev.Phil.Psych. (2011) 2:7788

DOI 10.1007/s13164-010-0041-0

V. M. Verdejo (*)

Department of Linguistics, Logic and Philosophy of Science, Universidad Autnoma de Madrid,Ctra. Colmenar Km. 15,6, Cantoblanco 28049 Madrid, Spain

e-mail: [email protected]

D. Quesada

Department of Philosophy, Universitat Autnoma de Barcelona, Campus UAB, Edifici B, Bellaterra

08193 Barcelona, Spain

e-mail: [email protected]


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This distinction between levels, which is perhaps best viewed as a fundamental

point that clarifies successful explanations in cognitive psychology, seems to have

been relegated to a rather dispensable remark in most of the contemporary

literature in the philosophy of cognitive science. In our view, the tendency to

unreflectively presuppose or even to simply ignore the distinction between levelsin cognitive explanation has had dramatic consequences in the elucidation of

crucial philosophical and psychological topics. In the present paper, we first briefly

revisit the classic Marrian distinction in order to emphasize its main motivation and

essence. As is well known, each level is supposed to explain a particular range of

facts, but it must be emphasized that the full account of a particular phenomenon

should be seen as an empirical account of that phenomenon at all levels of

description/explanation. Second, we apply this lesson to two particular develop-

ments regarding the putative structure of mental representation: 1) the analysis of

systematicity phenomena as proposed by Cummins et al. in both linguistic andnonlinguistic domains (Cummins 1996; Cummins et al. 2001, 2005); and 2)

Fodors account of the structure of iconic representation (Fodor 2008, chap. 6). In

both cases, the distinction between levels of explanation is carelessly neglected

with very undesirable results. A final concluding remark will encourage theorists

working in the cognitive sciences to specify and reflect on the particular level or

levels of explanation at which their contribution is meant to figure.

1 Inter-level explanations

As is well known, David Marr (1982, pp. 1929) provided a model for

understanding proper explanation in cognitive research. According to that model, a

cognitive process should be accounted for at three levels. Level 1 states the function

that is computed (and why) what Marr called the computational theory. Level 2

specifies the algorithm that implements this function, together with the representa-

tions required for the algorithm to work on. Finally, level 3 explains the realization

in hardware of the algorithm specified at level 2.

Marr was not alone in distinguishing levels of explanation. For instance, Newell

(1986) and Pylyshyn (1984) also introduced closely related level distinctions. On the

other hand, Marrs levels raise several issues regarding, inter alia, the need of

distinguishing further levels (e.g., Peacocke 1986), the individualistic (e.g., Segal

1989) or anti-individualistic (e.g., Burge 1986) nature of computational contents, or

the possibility of a nonintentional conception of computational approaches (e.g.,

Egan 1996). For ease of exposition, we will abstract from all these polemical

territories. Nonetheless, it is worth noting that we will be assuming that Marrs

topmost level involves representational contents and therefore an intentional

characterization of the Marrian function.1

1 On this assumption, Newells knowledge level and Pylyshyns semantic level are closely related to (and

arguably can be identified with) Marrs level 1. We thank an anonymous referee for this journal for

bringing this point to our attention. Among many others, authors that have provided an intentional/

semantic reading of Marrs topmost level include Bermdez (1995), Burge (1986), Davies (1991), Kitcher

(1988), Morton (1993) and Shapiro (1997).

78 V.M. Verdejo, D. Quesada


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consideration of level 1 should precede, as a matter of principle, research at the

other levels. Marrwho, for instance, included a section titled Importance of

computational theory (1982, p. 27)contributed perhaps greatly to give this

impression. Moreover, some authors have explicitly endorsed the view that Marrs

topmost level has methodological priority (e.g., Egan 1996; Segal 1989; Shapiro1997). However, it is important to stress that the essence of Marrs levels is a point

about explanation, not heuristicsas Garca-Carpintero (1995) emphasized. In

other words, in Marrs distinction of levels there is no methodological constraint

whatsoever about which level comes first. Therefore, which level takes

precedence will depend on the specifics of the phenomena to be accounted for.

What matters is that all three levels are levels at which an information

processing device must be understood before one can be said to have understood

it completely (Marr 1982, p. 24). Therefore, whatever the precise order followed

in your preferred account of informational processes, Marrs fundamentalcontention is that it had better result in an account at all levels if it is to lead to

full understanding of those processes.2

2 Systematicity and structure

Authors from the connectionist tradition (Cummins 1996; Cummins et al. 2001,

2005) have vehemently argued that, even if systematicity phenomena in language

require mental representation (MR from now on) with a certain specifiablestructure, it is not the case that they require MR in a language of thought (LOT

henceforth). More precisely, these authors argue that, whereas MR in a LOTor

classical MRentails that MR shares structure with (is a structural representation

of) the linguistic domain, all that linguistic systematicity requires is that MR

preserves the structure of language, something that can be doneas Smolensky et

al. (1992) demonstrated mathematically with tensor-product networks by means

of MR that merely encodes (is an encoding of) linguistic structure. The distinction

between MR that shares structure with language and MR that merely encodes that

structure therefore becomes crucial. On the one hand, it shows that encodingand

not only LOT MRis a good candidate for figuring in the account of primary

(inputoutput) linguistic systematicity effects.

[]Paul Smolensky, Graldine Legendre, and Yoshiro Miyata have proven

that for every classical parserthat is, a parser defined over classical

representationsthere exists a tensor-product network that is weakly (input

output) equivalent to the classical parser but does not employ classical

representations. Thus, it appears that the classical explanation and the tensor-

product explanation explain the systematicity of thought equally well

[Cummins et al. (2001), p. 170].

2 According to this, cognitive research may, consistently with Marrs programme, begin with the lowest

implementation level. This is so even if it is not possible to make progress when research remains confined

to this level; cf. Marrs analogy: trying to understand perception by studying only neurons is like trying

to understand bird flight by studying only feathers: it just cannot be done (Marr 1982, p. 27).



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On the other hand, it shows that, in the case of nonlinguistic systematicity, LOT

MR can only encode the relevant structure at most (because LOT MR shares

structure with language) and is therefore, on explanatory grounds, analogous to

nonclassical representation.

Since a tensor-product scheme like that employed by Smolensky, Legendre,

and Miyata need not share structure with the domains it represents in order

to account for primary systematicity effects, such a scheme could, in

principle, account for primary systematicity effects in a variety of

structurally distinct domains. The same point can be made about classical

representation: since it can be used structurally to encode domains such as

music that are structurally unlike language, a classical scheme could

account for primary systematicity effects in nonlinguistic domains. Since

classical and nonclassical structural encodings are evidently on equal

footing in this respect, we conclude again that there is no sound argument

from primary systematicity effects in language to classical representation

[Cummins et al. (2001), p. 184].

In short, if Cummins et al.s considerations are sound, the classical argument

from systematicity to structured MR in a LOT is certainly blocked. There are

many aspects of Cummins et al.s developments that deserve careful attention.

However, for present purposes, we would like to stress that these developments

constitute a good example of the difficulties that a theorist may face when

ignoring the distinction between levels of explanation. In particular, we willshow that without such a distinction, Cummins et al.s own distinction between

MR that shares structure with a given domain and MR that encodes that

structure, is either unintelligible or else cognitively irrelevant. The dramatic

consequence will be that their account amounts at best to a highly undesired

oversimplification, and at worst to a serious misunderstanding of the system-

aticity issues they are concerned with.

The key point made by Cummins and allies, to repeat, is that the debate between

classicist and connectionist models of structure can be accounted for in terms of the

difference between classical schemas of MR in which structure (S) is shared with a

(linguistic) domain (D) and connectionist schemas of MR in which that (linguistic)

structure is encoded rather than directly shared. On reflection however, it is not very

clear what it is for a (schema of) MR to share S with D; and, correspondingly, what

it is for a (schema of) MR to encode the S of D, since they define an encoding

negatively as not being a sharing of structure (Cummins 1996, p. 599; Cummins et

al. 2001, pp. 180 and 182). What does it mean then that MR shares S with D?

Indeed, we submit that there are only two straightforward interpretations of the claim

that MR shares S with D.

According to the first, D consists of a set of cognitive states whose systematic

properties are to be explained. However, if D consists of cognitive states, we cannot

make sense of the idea that MR (which is of course MR of a cognitive state) shares S

with the members of D because MR is already part of the cognitive states that it is

allegedly sharing S with. According to this interpretation, we can say that MR shows

or manifests the structure of D but not, intelligibly, that it shares S with D. Consider

Levels of Explanation Vindicated 81


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that our domain is the pair of sentences John loves Mary and Mary loves John.

We come to know that those sentences are systematic variants of one another

because, respecting grammatical constraints of combination, they are made out of the

same constituents: John, Mary and loves. The scheme of representations in

this case corresponds to words. Does the scheme of representationsconsisting ofwordsshare structure with the domainconsisting of sentences? Not intelligibly.

The right thing to say is that the scheme of representations shows or manifests the

structure of the domain but not, meaningfully, that it shares structure with the

domain.

On the second straightforward interpretation, D consists of a set of external

objects or properties. This interpretationwhich seems to be in accordance with

the authors contention that language, colour, algebra and the capitals of twenty

states in the USA are different domains (cf. Cummins et al. 2001)makes sense

because now we do have two things whose structure we can compare. However,there is a price to be paid: we are evaluating the structure of MR in relation to

extra-mental objects or properties. That is, we are presupposing that, in order to

explain cognitive systematicity phenomena, MR must preserve noncognitive

structure, which is certainly at odds with Cummins resistance to LOT

representational schemes on the basis that, instead of being derived from the

systematicity of language, the systematicity of thought ought to depend only on

the structure of the mind (Cummins 1996, p. 595). If language as such is no

good in elucidating the structure of the mind, extra-mental objects generally are

no good either, and for exactly the same reason. In short, the distinction betweenthe sharing and encoding of structure now makes sense but is cognitively

irrelevant.

What Cummins et al. need in order for their distinction to work is to consider

levels of explanation. Following this approach, we can assume that the members

of D are cognitive states as accounted for at a high level of description . Thus, the

relation of sharing S can be taken to hold between that high level of description

and a lower level of description. The consideration of two different levels of

descripti on wit h a given struc ture Sw hic h can be s een as ro ug hly

corresponding to Marrs level 1 and level 2makes the distinction between

MR that shares S with D and MR that encodes S of D both intelligible and

cognitively relevant. It is intelligible because we now have two different levels

whose structure can meaningfully be shared. It is cognitively relevant because we

are evaluating the structure of cognitive states as such (and not as dependent on

outer reality).

To illustrate, following paradigmatic (linguistic) characterizations of system-

aticity, the high level function will be based upon cases such as that anyone capable

of producing/understanding John loves Mary is also capable of producing/

understanding Mary loves John. More precisely, the (Marrian) topmost function

will be a function that goes from a given propositional representation (e.g., the

representation that John loves Mary) to its systematic variants obtained through

recombination of constituents (e.g., the representation that Mary loves John). The

structure identified at this high level is linguistic structure: the constituents of a

propositional representation are the grammatical constituents of its associated



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sentence.3 Now, at the algorithmic level (Marrs level 2), we can meaningfully

postulate representations that share or, alternatively, that merely encode the structure

found at the highest level. For instance, algorithmic representation in terms of LISP

programming language just mimics linguistic structure and accounts for system-

aticity phenomena via programmed permutation of linguistic constituents (in termsof car, cdr and cons operations). In contrast, algorithmic representations

consisting of tensor-product networks or Gdel numbering are instances of

representations that merely encode linguistic structurevia superimposable activa-

tion vectors or via products of uniquely factorable numbers (see Cummins et al.

2001 for details). The systematicity function at level 1 would in this case be

implemented by mathematical operations. Thus, the claim that LISP representation

shares structure with high-level representation (while tensor-product networks or

Gdel numbering do not) is now both intelligible and cognitively relevant.

Notoriously, the analysis in terms of Marrian levels helps to clarify (part of)the real disagreement between the LOT and the connectionist explanations of

cognitive systematicity: they both agree that certain cognitive states are

systematic because of (empirically tested) facts having to do with the structure

of those states identified at a high level of description. However, they disagree as

to whether that structure at a high level of description is reproduced in MR at a

lower level of description. The failure to see the relevance of the distinction

between levels thus amounts to a fatal misunderstanding of the connectionist/

classical MR debate. The debate is about what the proper inter-level explanatory

relations really are, and not only about what particular algorithmic representationwould explain systematicity. This has been clear at least since Fodor and

Pylyshyns (1988) original presentation of the challenge for connectionist

developments, to wit: either connectionist networks cannot explain systematicity

or, if they do, they are implementations of LOT models. However, if this is a

challenge at all, to grant the appeal of representational schemes that do not share

their structure with languagesuch as a suitable sort of encodingis not to grant

that the representational scheme at hand cannot be considered to consist of LOT

representation. The key point is precisely that high-level phenomenasuch as,

paradigmatically, systematicitymay have more than one lower-level implemen-

tation. Something must be said therefore to justify Cummins et al.s outright

presupposition that encodings do not implement a LOT.

Cummins et al. may be right that connectionist explanations of systematicity are

on an equal evidential footing with LOT explanations. However, our point here is

that they have not even started to show that this is indeed the case. Our diagnosis is

clear: they fail to distinguish levels of explanation and in so doing they make it

3 Note that, in this context, the specified Marrian function is not merely a function in extension, that is, it

not only defines an input/output relation. Systematicity phenomena require also the identification of the

(linguistic) structural information that makes an input/output relation a systematic relation. A function that,

e.g., goes from the representation of Rab to the representation of Rba would not capture systematicity if

Rab and Rba were primitive representations in the system. One such intensional function can be seen as a

Marrian function that not only states what the system does but also why the system does it. In Peacockean

terms, the function is seen as specifying, at Peacockes level 1.5, the information on which the algorithm

draws (cf. Peacocke 1986).



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impossible to correctly identify the fundamental traits of the dialectics they are

concerned with. As we shall see, Cummins et al. are not alone in underestimating the

need to distinguish between levels in cognitive explanation.

3 Structure in iconic representation

The second case on which we wish to focus is Fodors recent articulation of the view

that iconic representation is of a distinctive kind (Fodor 2008). In particular, and

unlike discursive or linguistic representation, icons are said to be a homogeneous

kind of symbol from both the syntactic and the semantic point of view (Fodor2008,

p. 174). By introducing this kind of representation, Fodor aims to show that there is

empirical evidence for a kind of nonconceptual representation in perceptual

experience. Similarly to the case of Cummins et al.s developments, it is our viewthat Fodors strategy in accomplishing his aim is utterly inadequate from the outset.

Furthermore, in this case also the mistaken approach results in undesired results as

regards the intended line of reasoning. Fodor may be right that there is a distinctive

kind of representation whose nature is nonconceptual; the point we wish to make

here is that even so, Fodors attempt is not successful because he simply ignores the

need to distinguish between levels of explanation in his account of the structure of a

mental representation.

Fodor takes pictures to be paradigmatic of iconic representation. For the familiar facts

having to do with systematicity and productivity, Fodor claims that iconic representa-tion, like discursive representation, is compositional (cf. Fodor 2008, pp. 171 and

173).4 As it happens, the kind of compositionality at stake in the case of iconic

representation is homogeneous, that is, it offers no way of distinguishing canonical

parts from mere parts. Thus, Fodor describes iconic compositionality as follows:

Picture principle: If P is a picture of X, then parts of P are pictures of parts

of X.

Pictures and the like differ from sentences and the like in that icons dont have

canonical decompositions into parts, all the parts of an icon are ipso facto

constituents. [Fodor (2008), p. 173, his emphasis]

Even if this characterization of the structure of an icon may have some intuitive

appeal, it is, we submit, certainly unsatisfactory. There are two reasons for this.

Firstly, Fodors picture principle misleadingly suggests that an icons structure is

dependent upon the structure of some worldly X. This is because, unlike Cummins

et al.s account of the relation of MR sharing S with D, the relation of MR

4

This is, on reflection, very striking. The distinctive feature of pictures is that they are structurallyhomogeneous symbols (see below). But it is very controversial to suppose that homogeneous symbols

that is, symbols with no canonical constituentscan figure in systematicity phenomena, because what

systematicity demands, if anything, is that the distinctive (semantic-plus-syntactic) contribution of parts of

a symbol can be identified in a large number of other symbolssay, the semantic-plus-syntactic

contribution ofa found in Fa, Ga, Rab, Rba,). That is impossible if, as homogeneity demands, symbols

do not have distinctive (semantic-plus-syntactic) parts. This puzzle, however, does not affect our present

discussion.



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iconically representing/being a picture of X is clearly a relation between (part of) a

cognitive state and some extra-mental reality, albeit described quite abstractly. Thus,

a first problem with the picture principle is the characterization it offers of an icons

structure; namely, it says that an iconic representation of X is structurally constituted

by homogeneous iconic representations of parts of X. In other words, the parts of anicon are described by reference to the parts of some X. One very undesirable

consequence of this characterization of the structure of an icon is therefore that it is

apparently dependent upon an extra-mental reality. It may be true that the iconic

representation of, say, a giraffe, is such that the parts of the icon represent parts of

the giraffe. It does not follow, and it is certainly false, that the structure of the

(mental) picture of a giraffe is defined in terms of the structure (whatever that may

be) of the giraffe. To put it crudely, the giraffe consists to a large extent of molecules

of water, whereas the structure of the picture of the giraffe is determined, at the very

minimum, by the relevant sort of computational process that leads to the picturesconfiguration.5 In short, the relation of iconically representing X in the sense of

the picture principle is, as it stands, useless in the account of the icons structure as

such.6

The second and more important reason for questioning Fodors characterization of

the structure of an icon is that it is patently ambiguous as to whether it belongs to a

high-level or to an algorithmic-level of explanation. In being ambiguous about this,

it runs the risk of being absolutely irrelevant to the conceptualist/nonconceptualist

debate to which it purportedly contributes. According to Fodor, the candidates for

iconic representation are structurally homogeneous representations through whichsubjects subpersonally register certain information that is personally available to

them only when their conceptual equipment is put to work. Examples include

Sperlings (1960) recognition of tachistoscopically presented letters and Julesz

(1971) illusion of three-dimensionality in stereoscopically presented dots. However,

the debate surrounding nonconceptual content is (primarily) a debate about content

at a high and personal level of description; a level that is capable of entering into the

account of a subjects intentional behaviour. Fodors failure is thus a failure to

5 Examples of accounts of the structure in visual perception abound in cognitive science. To mention just

one, Hummel (2001) offers a synthetic account in terms of static and dynamic binding. In Hummels

model, only when dynamic binding (which is understood as requiring attention) is involved does explicit

representation of (parts of) objects and their spatial relations take place. When the visual information is

processed quickly, no explicit representation of the structure is used by the algorithm. The present point is

that, which structure (if any) a given representation actually has, is never accounted for in terms of outer

reality, but only in terms of inner computational processes. Fodors apparent mistake seems thus to be

Pylyshyns vigorously denounced intentional fallacy, that is, the fallacy of attributing properties of what

is being represented to the representation itself (as if our representation of a red square were itself red and

square). Yet so long as we assume that the form of some mental representation must account for the

content of the perceptual experience we are inevitably led to postulate a picture-like representation to

match a picture-like experience (Pylyshyn 2007, p. 122).6 Not of course in the account of the icons structure as a representation of X. The problem is that the

specification of the structure of a given representation is something over and above what the representation

represents. The point is even clearer in the case of discursive representation. When we say that the

discursive representation John loves Mary is constituted by John, loves and Mary it is not on the ground

that John loves Mary represents John, the relation of loving and Mary. In fact, that this is exactly what

John loves Mary actually does represent is perfectly compatible with this discursive representation having

some other constituents; indeed, it is compatible with the discursive representation being primitive.



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appropriately link the empirical cases to which he appeals at a subpersonal (roughly

algorithmic) level with the kind of (nonconceptual) high-level content that he needs

in order for his account to bear on the debate between conceptualists and

nonconceptualists.7 In fact, Fodor fails to see that the characterization of an icons

structure that he offers fatally confuses the high-level and the low-level account ofthat structure. This becomes perfectly clear if we consider that it does not even begin

to follow from the picture principlethat is, from the principle that if P is a picture

of X, then parts of P are pictures of parts of Xthat icons are such thatall the parts

of an icon are ipso facto constituents, i.e., that they are structurally homogeneous

symbols. For instance, a system that delivers a picture of a red ball respects the

picture principle. However, the system may consist of the combination of the

information from two detectorsone for colour and another for shapeor else it

may just take a picture of the object. In the latter case the representation is

structurally homogeneous, but in the former, it is not. Clearly, what Fodor does is tomisleadingly conflate a point about the algorithmic structure of a mental iconic

representation with considerations regarding a claim about the intuitive high-level

characterization of that representation.

Fodors troubles can be avoided by quite a simple explanatory remark; one that

introduces levels of explanation. Thus, a much more adequate account of the

structure of an icon would take the picture principle in the context of the

specification of a high-level cognitive function (a computational theory in Marrs

terminology) from whatever proximal stimuli are involved in the identification of

elements in a systems environment

some Xs as primitively identified in the perceptual mechanismto an iconic (nonconceptual) representation of those

elements in accordance with the principle. In addition, a way of performing the

required function so that parts of an icon represent parts of an X, is via structurally

homogeneous symbols at an algorithmic level. The key point would be that low-

level structurally-homogeneous representation involves the existence of nonconcep-

tual high-level content as defined by the picture principle. What Fodor needsand

7 As an anonymous referee rightly points out, since Bermdez 1995 paper, the debate surrounding

nonconceptual content has also taken into account nonconceptual content at a subpersonal level. Thisfact, however, does not affect the point just made about Fodors failure to link subpersonal cases with

intentional content of perceptual experience. For one thing, Fodors contribution to the debate aims to

offer an empirical argument against the a priori approaches of philosophers in the Sellars tradition

(Fodor2008, p. 193). That tradition be it on the conceptualist or on the nonconceptualist sidehas

invariably been concerned with conceptual/nonconceptual contents of personal perceptual experience.

For another thing, the conceptualist/nonconceptualist debate seems far from being settled even if we

accept the existence of genuine nonconceptual content at a subpersonal level. This seems to be

Bermdez own view, who acknowledges for instance that it may well turn out that the

nonconceptual content of subpersonal information systems is rather different from the nonconceptual

content of perception (Bermdez 2007, p. 69). Finally, we are here following Toribio (forthcoming),

who presents the so-called subpersonal worry, that is, the worry that Fodors line of argument is

threatened by considering that iconic representation can only be subpersonal algorithmic representation,

and hence, over and above the neo-Fregean conceptualist position focused on the nature of content from

the subjects own point of view. Note also that, under the assumption that Marrs level 1 is intentional

(see section 1 above), the high and personal level of description can be accounted for in terms of Marrs

topmost level (thereby arguably equating in this context Marrs level 1 with Newells knowledge level

or Pylyshyns semantic level).



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what he is far from providingis therefore a sound inference from semantic/

syntactic features of low-level MR to semantic/syntactic features of high-level

representation as captured by the picture principle. Finally, it would be crucial to

look for good empirical evidence that such structurally-homogeneous representations

do, in fact, exist.8

Note that the proposed account frees us from the problems just discussed. On

the one hand, the relation between icons as mental representations, and extra-

mental realities is clarified as a cognitive function leading from primitively

identified elements in the environment to their iconic representation. In particular,

this is not an account of the icons structure in terms of outer realities. On the

other hand, it becomes manifest why empirical research has a bearing on the

determination of the nature of high-level (nonconceptual) content; that is, it

becomes manifest that a low-level finding of structurally homogeneous

representation would confirm and explain the high-level characterization of iconsvia the picture principle.

4 Conclusion

There are surely many other examples in which the consideration of levels of

explanation would make discussion in cognitive science much more precise and

revealing. Here we have provided a detailed analysis of two particular cases in order

to suggest that the distinction between levels is especially illuminating when we arefaced with the task of elucidating the structure of mental representations. In the first

case, the distinction between levels is needed in order for Cummins et al.s key

notion of MR sharing structure S with a domain D to be both intelligible and

cognitively relevant. In addition, the consideration of levels of explanation shows

that the assumption that encodings are not some form of LOT representation does

not take account of the classical challenge to connectionist models. In the second

case, we have shown that, without an analysis in terms of high and low levels of

description, Fodors notion ofMR iconically representing X is of very little use in

accounting for the structure of iconic representation. Absent such an analysis, the

notion suggests an unpalatable dependence upon extra-mental realities in the

elucidation of that structure and, more importantly, it messily conflates high-level

features of iconic (nonconceptual) representation with empirical data regarding a

particular algorithmic implementation of that representation. One lesson to be

learned from all this is that theorists should be aware of which level they are working

at so that our research into cognitive phenomena is as clear and precise as it actually

can be.

8 It seems clear to us that, even if Fodor is right that some iconic representation is structurally

homogeneous, this may be far from being the general case. To mention one well-known example,

according to Marr (1982), representations in the earliest stage of visual processing consist of primal

sketches which are certainly structurally heterogeneous: they involve the processing of geometrical

information and intensity changes in light from the two-dimensional retinal image so as to detect edges,

bars, ends and blobs. Therefore, as far as Marrs classic model is concerned, there are distinctive semantic/

syntactic parts in iconic representation right from the start.



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Acknowledgements We would like to thank Josefa Toribio, Christopher Evans and two anonymous

referees for their helpful comments and suggestions on earlier drafts. This research has been partially

funded by the MICINN, Spanish government, under the research project FFI2008-06164-C02-02, the

CONSOLIDER INGENIO 2010 Program, grant CSD2009-0056, and the Catalan government, via the

consolidated research group GRECC, SGR2009-1528.

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