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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: Victor.verdejo@uab.cat
D. Quesada
Department of Philosophy, Universitat Autnoma de Barcelona, Campus UAB, Edifici B, Bellaterra
08193 Barcelona, Spain
e-mail: Daniel.quesada@uab.cat
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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).
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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
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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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