The cognitive functions of language

 

Peter Carruthers
Department of Philosophy
University of Maryland
College Park, MD 20742
pc154@umd.edu
www.shef.ac.uk/~phil/department/staff/Carruthers.html

http://http://www.philosophy.umd.edu/people/faculty/pcarruthers/

 

Abstract: This paper explores a variety of different versions of the thesis that natural language is involved in human thinking. It distinguishes amongst strong and weak forms of this thesis, dismissing some as implausibly strong and others as uninterestingly weak. Strong forms dismissed include the view that language is conceptually necessary for thought (endorsed by many philosophers) and the view that language is de facto the medium of all human conceptual thinking (endorsed by many philosophers and social scientists). Weak forms include the view that language is necessary for the acquisition of many human concepts, and the view that language can serve to scaffold human thought processes. The paper also discusses the thesis that language may be the medium of conscious propositional thinking, but argues that this cannot be its most fundamental cognitive role. The idea is then proposed that natural language is the medium for non-domain-specific thinking, serving to integrate the outputs of a variety of domain-specific conceptual faculties (or central-cognitive ‘quasi-modules’). Recent experimental evidence in support of this idea is reviewed, and the implications of the idea are discussed, especially for our conception of the architecture of human cognition. Finally, some further kinds of evidence which might serve to corroborate or refute the hypothesis are mentioned. The overall goal of the paper is to review a wide variety of accounts of the cognitive function of natural language, integrating a number of different kinds of evidence and theoretical consideration in order to propose and elaborate the most plausible candidate.

 

Keywords: cognitive evolution, conceptual module, consciousness, domain-general, inner speech, logical form (LF), language, thought.

 

1            Introduction

Natural language looms large in the cognitive lives of ordinary folk. Although proportions vary, many people seem to spend a good deal of their waking activity engaged in ‘inner speech’, with imaged natural language sentences occupying a significant proportion of the stream of their conscious mentality.

This bit of folk-wisdom has been corroborated by Hurlburt (1990, 1993), who devised a method for sampling people’s inner experience. Subjects wore headphones during the course of the day, through which they heard, at various intervals, a randomly generated series of bleeps. When they heard a bleep, they were instructed to immediately ‘freeze’ what was passing through their consciousness at that exact moment and then make a note of it, before elaborating on it later in a follow-up interview. Although frequency varied widely, all normal (as opposed to schizophrenic) subjects reported experiencing inner speech on some occasions – with the minimum being 7% of occasions sampled, and the maximum being 80%. Most subjects reported inner speech on more than half of the occasions sampled. (The majority of subjects also reported the occurrence of visual images and emotional feelings – on between 0% and 50% of occasions sampled in each case). Think about it: more than half of the total set of moments which go to make up someone’s conscious waking life occupied with inner speech - that’s well nigh continuous!

            Admittedly, the sample-sizes in Hurlburt’s studies were small; and other interpretations of the data are possible. (Perhaps the reports of linguistically-clothed thoughts occurring at the time of the beep were a product of confabulation, for example, reflecting people’s naïve theory that thought must be in natural language. If so, this should be testable.) But let us suppose that inner verbalization is as ubiquitous as common-sense belief and Hurlburt’s data would suggest. Just what would all this inner verbalization be doing? What would be its function, or cognitive role? The naïve common-sense answer is that inner verbalization is constitutive of our thinking – it is that we think by talking to ourselves in inner speech (as well as by manipulating visual images etc.). Anyone who holds such a view endorses a version of what I shall call ‘the cognitive conception of language’, which maintains that, besides its obvious communicative functions, language also has a direct role to play in normal human cognition (in thinking and reasoning).

            Quite a different answer would be returned by most members of the cognitive science community, however. For they endorse what I shall call ‘the (purely) communicative conception of language’, according to which language is but an input–output system for central cognition. Believing that language is only a channel, or conduit, for transferring thoughts into and out of the mind, they are then obliged to claim that the stream of inner verbalization is more-or-less epiphenomenal in character. (Some possible minor cognitive roles for inner speech, which should nevertheless be acceptable to those adopting this perspective, will be canvassed later.) The real thinking will be going on elsewhere, in some other medium of representation.

            One reason for the popularity of the communicative conception amongst cognitive scientists is that almost all now believe that language is a distinct input–output module of the mind (at least in some sense of ‘module’, if not quite in Fodor’s classic - 1983 – sense). And they find it difficult to see how the language faculty could both have this status and be importantly implicated in central cognition. But this reasoning is fallacious. For compare the case of visual imagination. Almost everyone now thinks that the visual system is a distinct input-module of the mind, containing a good deal of innate structure. But equally, most cognitive scientists now accept that visual imagination re-deploys the resources of the visual module for purposes of reasoning – for example, many of the same areas of the visual cortex are active when imagining as when seeing. (For a review of the evidence, see Kosslyn, 1994.)

What is apparent is that central cognition can co-opt the resources of peripheral modules, activating some of their representations to subserve central cognitive functions of thinking and reasoning. The same is then possible in connection with language. It is quite consistent with language being an innately structured input and output module, that central cognition should access and deploy the resources of that module when engaging in certain kinds of reasoning and problem solving.

Note, too, that hardly anyone is likely to maintain that visual imagery is a mere epiphenomenon of central cognitive reasoning processes, playing no real role in those processes in its own right. On the contrary, it seems likely that there are many tasks which we cannot easily solve without deploying a visual (or other) image. For example, suppose you are asked (orally) to describe the shape which is enclosed within the capital letter ‘A’. It seems entirely plausible that success in this task should require the generation of a visual image of that letter, from which the answer (‘a triangle’) can then be read off. So it appears that central cognition operates, in part, by co-opting the resources of the visual system to generate visual representations, which can be of use in solving a variety of spatial-reasoning tasks. And this then opens up the very real possibility that central cognition may also deploy the resources of the language system to generate representations of natural language sentences (in ‘inner speech’), which can similarly be of use in a variety of conceptual reasoning tasks.

There is at least one further reason why the cognitive conception of language has had a bad press within the cognitive science community in recent decades. (It continues to be popular in some areas of the social sciences and humanities, including philosophy.) This is that many of the forms of the thesis which have been defended by philosophers and by social scientists are implausibly strong, as we shall see in section 3 below. The unacceptability of these strong views has then resulted in all forms of the cognitive conception being tarred with the same brush.

A crucial liberalizing move, therefore, is to realize that the cognitive conception of language can come in many different strengths, each one of which needs to be considered separately on its own merits. In this paper I shall distinguish between some of the many different versions of the cognitive conception. I shall begin (in section 2) by discussing some weak claims concerning the cognitive functions of language which are largely uncontroversial. This will help to clarify just what (any interesting form of) the cognitive conception is, by way of contrast. I shall then (in section 3) consider some claims which are so strong that cognitive scientists are clearly right in rejecting them, before zeroing in on those which are both interesting and plausible (in sections 4 and 5). I shall come to focus, in particular, on the thesis that natural language is the medium of inter-modular integration. This is a theoretical idea which has now begun to gather independent empirical support. Finally (in sections 6 and 7) some additional implications, elaborations, and possible further empirical tests of this idea are discussed.

I should explain at the outset, however, that the thesis I shall be working towards is that it is natural language syntax which is crucially necessary for inter-modular integration. The hypothesis is that non-domain-specific thinking operates by accessing and manipulating the representations of the language faculty. More specifically, the claim is that non-domain-specific thoughts implicate representations in what Chomsky (1995) calls ‘Logical Form’ (LF). Where these representations are only in LF, the thoughts in question will be non-conscious ones. But where the LF representation is used to generate a full-blown phonological representation (an imaged sentence), the thought will generally be conscious.[1]

I should emphasize that I shall not be claiming that syntax is logically required for inter-modular integration, of course. Nor shall I be claiming that only natural language syntax – with its associated recursive and hierarchical structures, compositionality, and generativity – could possibly play such a role in any form of cognition, human or not. (In fact it is the phrase-structure element of syntax which does the work in my account; see section 6.1 below.) Rather, my claim will be that syntax does play this role in human beings. It is a factual claim about the way in which our cognition happens to be structured, not an unrestricted modal claim arrived at by some sort of task-analysis.

I should also declare at the outset how I shall be using the word ‘thought’ in this paper. Unless I signal otherwise, I intend all references to thought and thinking to be construed realistically. Thoughts are discrete, semantically-evaluable, causally-effective states, possessing component structure, and where those structures bear systematic relations to the structures of other, related, thoughts. So distinct thoughts have distinct physical realizations, which may be true or false, and which cause other such thoughts and behavior. And thoughts are built up out of component parts, where those parts belong to types which can be shared with other thoughts. It is not presupposed, however, that thoughts are borne by sentence-like structures. Although I shall be arguing that some thoughts are carried by sentences (viz. non-domain-specific thoughts which are carried by sentences of natural language), others might be carried by mental models or mental images of various kinds.

It is hugely controversial that there are such things as thoughts, thus construed, of course. And while I shall say a little in defense of this assumption below (in section 3.3), for the most part it is just that – an assumption – for present purposes. I can only plead that one can’t do everything in one paper, and that one has to start somewhere. Those who don’t want to share this assumption should read what follows conditionally: if we were to accept that there are such things as realistically-construed thoughts, then how, if at all, should they be seen as related to natural language sentences?

Finally, a word about the nature of the exercise before we proceed further. This paper ranges over a great many specialist topics and literatures in a number of distinct disciplines. Of necessity, therefore, our discussion of any given subject must be relatively superficial, with most of the detail, together with many of the required qualifications and caveats, being omitted. Similarly, my arguments against some of the competitor theories are going to have to be extremely brisk, and some quite large assumptions will have to get taken on board without proper examination. My goal, here, is just to map out an hypothesis space, using quite broad strokes, and then to motivate and discuss what I take to be the most plausible proposal within it.

 

2          Weak claims

Everyone will allow that language makes some cognitive difference. For example, everyone accepts that a human being with language and a human being without language would be very different, cognitively speaking. In this section I shall outline some of the reasons why.

 

2.1            Language as the conduit of belief

Everyone should agree that natural language is a necessary condition for human beings to be capable of entertaining at least some kinds of thought. For language is the conduit through which we acquire many of our beliefs and concepts, and in many of these cases we could hardly have acquired the component concepts in any other way. So concepts which have emerged out of many years of collective labor by scientists, for example – such as electron, neutrino, and DNA – would de facto be inaccessible to someone deprived of language. This much, at any rate, should be obvious. But all it really shows is that language is required for certain kinds of thought; not that language is actually involved in or is the representational vehicle of those thoughts.

            It is often remarked, too, that the linguistic and cognitive abilities of young children will normally develop together. If children’s language is advanced, then so will be their abilities across a range of tasks; and if children’s language is delayed, then so will be their cognitive capacities. To cite just one item from a wealth of empirical evidence: Astington (1996) and Peterson and Siegal (1998) report finding a high correlation between language-ability and children’s capacity to pass false-belief tasks, whose solution requires them to attribute, and reason from, the false belief of another person. Does this and similar data show that language is actually involved in children’s thinking?

In the same spirit, we might be tempted to cite the immense cognitive deficits which can be observed in those rare cases where children grow up without exposure to natural language. Consider, for example, the cases of so-called ‘wolf children’, who have survived in the wild in the company of animals, or of children kept by their parents locked away from all human contact (Malson, 1972; Curtiss, 1977). Consider, also, the cognitive limitations of profoundly deaf children born of hearing parents, who have not yet learned to sign (Sachs, 1989; Schaller, 1991). These examples might be thought to show that human cognition is constructed in such a way as to require the presence of natural language if it is to function properly.

            But all that such data really show is, again, that language is a necessary condition for certain kinds of thought and types of cognitive process; not that it is actually implicated in those forms of thinking. And this is easily explicable from the standpoint of someone who endorses the standard cognitive science conception of language, as being but an input–output system for central cognition, or a mere communicative device. For language, in human beings, is a necessary condition of normal enculturation. Without language, there are many things which children cannot learn; and with delayed language, there are many things which children will only learn later. It is only to be expected, then, that cognitive and linguistic development should proceed in parallel. It does not follow that language is itself actually used in children’s central cognition.

            Another way of putting the point is that this proposed cognitive function of language is purely developmental - or diachronic - rather than synchronic. Nothing is said about the role of language in the cognition of adults, once a normal set of beliefs and concepts has been acquired. And the evidence from aphasia suggests that at least many aspects of cognition can continue to operate normally once language has been removed.

            Aphasias come in many forms, of course, and in many different degrees of severity. And it is generally hard to know the extent of any collateral damage – that is, to know which other cognitive systems besides the language faculty may have been disabled as a result of the aphasia-causing brain-damage. But many patients with severe aphasia continue to be adept at visuo-spatial thinking, at least (Kertesz, 1988), and many continue to manage quite well for themselves in their daily lives.

            Consider, for example, the a-grammatic aphasic man studied in detail by Varley (1998, 2002). He is incapable of either producing or comprehending sentences, and he also has considerable difficulty with vocabulary, particularly verbs. He has lost all mentalistic vocabulary (‘belief’, ‘wants’, etc.), and his language system is essentially limited to nouns. Note that there is not a lot of explicit thinking that you can do using just nouns! (It should also be stressed that he has matching deficits of input and output, suggesting that it is the underlying system of linguistic knowledge which has been damaged). Yet he continues to drive, and to have responsibility for the family finances. He is adept at communicating, using a mixture of single-word utterances and pantomime. And he has passed a range of tests of theory of mind (the standard battery of false-belief and deception tasks, explained using nouns and pantomime), as well as various tests of causal thinking and reasoning. It appears that, once language has done its developmental work of loading the mind with information, a good deal of adult cognition can thereafter survive its loss.

            Since natural language is the conduit for many of our beliefs and for much of our enculturation, everyone should accept that language is immensely important for normal cognitive development. That language has this sort of cognitive function should be no news to anyone.[2]

 

2.2            Language as sculpting cognition

A stronger and more controversial thesis has been proposed and defended by some researchers over recent decades. This is that the process of language acquisition and enculturation does not merely serve to load the mind with beliefs and concepts, but actually sculpts our cognitive processes to some degree (Lucy, 1992a, 1992b; Nelson, 1996; Bowerman and Levinson, 2001).[3]

For example, acquisition of Yucatec (as opposed to English) – in which plurals are rarely marked and many more nouns are treated grammatically as substance-terms like ‘mud’ and ‘water’ – leads subjects to see similarities amongst objects on the basis of material composition rather than shape (Lucy, 1992b; Lucy and Gaskins, 2001). And children brought up speaking Korean (as opposed to English) – in which verbs are highly inflected and massive noun ellipsis is permissible in informal speech – leads children to be much weaker at categorization tasks, but much better at means–ends tasks such as using a rake to pull a distant object towards them (Choi and Gopnik, 1995; Gopnik et al., 1996; Gopnik, 2001).

            Fascinating as these data are, they do not, in themselves, support any version of the cognitive conception of language. This is because the reported effects of language on cognition are still entirely diachronic and developmental, rather than synchronic. The fact that acquiring one language as opposed to another causes subjects to attend to different things and to reason somewhat differently doesn’t show that language itself is actually involved in people’s thinking. Indeed, on the hypothesis proposed by Gopnik (2001), language-acquisition has these effects by providing evidence for a pre-linguistic theorizing capacity, which operates throughout development to construct children’s systems of belief and inference.

 

2.3            Language as a cognitive scaffold

Other claims can be extracted from the work of Vygotsky (1934/1986), who argues that language and speech serve to scaffold the development of cognitive capacities in the growing child. Researchers working in this tradition have studied the self-directed verbalizations of young children – for example, observing the effects of their soliloquies on their behavior (Diaz and Berk, 1992). They have found that children tend to verbalize more when task demands are greater, and that those who verbalize most tend to be more successful in problem-solving.

This claim of linguistic scaffolding of cognition admits of a spectrum of readings, however. At its weakest, it says no more than has already been conceded above, that language may be a necessary condition for the acquisition of certain cognitive skills. At its strongest, on the other hand, the idea could be that language forms part of the functioning of the highest-level executive system – which would then make it a variant of the ideas to be discussed in sections 4 and 5 below.

            Clark (1998) argues for a sort of intermediate-strength version of the Vygotskian idea, defending a conception of language as a cognitive tool. (Chomsky, too, has argued for an account of this sort. See his 1976, ch.2.) According to this view – which Clark labels ‘the supra-communicative conception of language’ – certain extended processes of thinking and reasoning constitutively involve natural language. The idea is that language gets used, not just for communication, but also to augment human cognitive powers.

Thus by writing an idea down, for example, I can off-load the demands on memory, presenting myself with an object of further leisured reflection; and by performing arithmetic calculations on a piece of paper, I may be able to handle computational tasks which would otherwise be too much for me (and my short-term memory). In similar fashion, it may be that inner speech serves to enhance memory, since it is now well-established that the powers of human memory systems can be greatly extended by association (Baddeley, 1988). Inner speech may thus facilitate complex trains of reasoning (Varley, 1998).

Notice that on this supra-communicative account, the involvement of language in thought only arises when we focus on a process of thinking or reasoning extended over time. So far as any given individual (token) thought goes, the account can (and does) buy into the standard input–output conception of language. It maintains that there is a neural episode which carries the content of the thought in question, where an episode of that type can exist in the absence of any natural language sentence and can have a causal role distinctive of the thought, but which in the case in question causes the production of a natural language representation. This representation can then have further benefits for the system of the sort which Clark explores (off-loading or enhancing memory).

            According to stronger forms of the cognitive conception to be explored in later sections, in contrast, a particular tokening of an inner sentence is (sometimes) an inseparable part of the mental episode which carries the content of the thought-token in question. So there is no neural or mental event at the time which can exist distinct from that sentence, which can occupy a causal role distinctive of that sort of thought, and which carries the content in question; and so language is actually involved in (certain types of) cognition, even when our focus is on individual (token) thinkings.

In this section I have discussed two weak claims about the role of language (that language is necessary for the acquisition of many beliefs and concepts; and that language may serve as a cognitive tool, enhancing the range and complexity of our reasoning processes). These claims should be readily acceptable to most cognitive scientists. In addition, I have briefly introduced a more controversial thesis, namely that the acquisition of one or another natural language can sculpt our cognitive processes, to some degree. But this thesis relates only to the developmental, or diachronic, role of language. It says nothing about the role of language in adult cognition. We will in future focus on more challenging versions of the cognitive conception of language.

 

3          Strong claims

As is starting to emerge, the thesis that language has a cognitive function admits of a spectrum of readings. In this section I shall jump to the other end of that spectrum, considering forms of the cognitive conception of language which are too strong to be acceptable.

 

3.1            Language as necessarily required for thought

When the question of the place of natural language in cognition has been debated by philosophers the discussion has, almost always, been conducted a priori in universalist terms. Various arguments have been proposed for the claim that it is a conceptually necessary truth that all thought requires language, for example (Wittgenstein, 1921, 1953; Davidson, 1975, 1982; Dummett, 1981, 1989; McDowell, 1994). But these arguments all depend, in one way or another, upon an anti-realist conception of the mind – claiming, for instance, that since we cannot interpret anyone as entertaining any given fine-grained thought in the absence of linguistic behavior, such thoughts cannot even exist in the absence of such behavior (Davidson, 1975). Since the view adopted in this paper – and shared by most cognitive psychologists – is quite strongly realist about thought, I do not propose to devote any time to such arguments.

            Notice, too, that Davidson et al. are committed to denying that any non-human animals can entertain genuine thoughts, given that it is very doubtful whether any such animals are capable of understanding and using a natural language (in the relevant sense of ‘language’, that is; see Premack, 1986; Pinker, 1994). This conclusion conflicts, not just with common-sense belief, but also with what can be discovered about animal cognition, both experimentally and by observation of their behavior in the wild (de Waal, 1982, 1996; Walker, 1983; Gallistel, 1990; Savage-Rumbaugh and Lewin, 1994; Byrne, 1995; Dickinson and Shanks, 1995; Allen and Bekoff, 1997; Hauser, 2000; Povinelli, 2000). So not only are the arguments of Davidson et al. unsound, but we have independent reasons to think that their conclusion is false.

            Dummett (1994) makes some attempt to accommodate this sort of point by distinguishing between concept-involving thoughts (which are held to be necessarily dependent upon language) and what he calls ‘proto-thoughts’, which are what animals are allowed to possess. Proto-thoughts are said to consist of ‘visual images superimposed on the visually perceived scene’, and are said to be possible only when tied to current circumstances and behavior. But such an account vastly under-estimates the cognitive capacities of non-human animals, I believe. If an animal can decide whom to form an alliance with, or can calculate rates of return from different sources of food, or can notice and exploit the ignorance of another, then these things cannot be accounted for in Dummett’s terms. And given that conceptual thinking of this sort is possible for animals, then he will be left without any principled distinction between animal thought and human thought.

            I do not expect that these brief considerations will convince any of my philosophical opponents, of course; and they aren’t meant to. Given the intended readership of this target-paper, their position is not really one that I need to take seriously. It is mentioned here just to set it aside, and (most importantly) in order that other, more plausible, versions of the cognitive conception of language shouldn’t be confused with it.

I propose, therefore, to take it for granted that thought is conceptually independent of natural language, and that thoughts of many types can actually occur in the absence of such language. But this leaves open the possibility that some types of thought might de facto involve language, given the way in which human cognition is structured. It is on this – weaker but nevertheless still controversial – set of claims that I shall focus. Claims of this type seem to me to have been unjustly under-explored by researchers in the cognitive sciences; partly, no doubt, because they have been run together with the a priori and universalist claims of some philosophers, which have been rightly rejected.

 

3.2       The Joycean machine

Another overly-strong form of cognitive conception of language – which has been endorsed by some philosophers and by many social scientists – is that language is, as a matter of fact, the medium of all human conceptual thinking. Most often it has been associated with a radical empiricism about the mind, according to which virtually all human concepts and ways of thinking, and indeed much of the very structure of the human mind itself, are acquired by young children from adults when they learn their native language – these concepts and structures differing widely depending upon the conceptual resources and structures of the natural language in question. This mind-structuring and social-relativist view of language is still dominant in the social sciences, following the writings early in this century of the amateur linguist Whorf (many of whose papers have been collected together in his 1956) – indeed, Pinker (1994) refers to it disparagingly as ‘the Standard Social Science Model’ of the mind.

Perhaps Dennett (1991) provides one of the clearest exponents of this view. He argues that human cognitive powers were utterly transformed following the appearance of natural language, as the mind became colonized by memes (ideas, or concepts, which are transmitted, retained and selected in a manner supposedly analogous to genes; see Dawkins, 1976). Prior to the evolution of language, on this picture, the mind was a bundle of distributed connectionist processors – which conferred on early hominids some degree of flexibility and intelligence, but which were quite limited in their computational powers. The arrival of language then meant that a whole new – serial and compositionally structured – cognitive architecture could be programmed into the system.

This is what Dennett calls the Joycean machine (named after James Joyce’s ‘stream of consciousness’ writing). The idea is that there is a highest-level processor which runs on a stream of natural-language representations, utilizing learned connections between ideas, and patterns of reasoning acquired in and through the acquisition of linguistic memes. On this account, then, the concept-wielding mind is a kind of social construction, brought into existence through the absorption of memes from the surrounding culture. And on this view, the conceptual mind is both dependent upon, and constitutively involves, natural language.

Admittedly, what Dennett will actually say is that animals and pre-linguistic hominids are capable of thought, and engage in much intelligent thinking. But this is because he is not (in my sense) a realist about thoughts. On the contrary, he (like Davidson) is what is sometimes called an ‘interpretationalist’ – he thinks that there is nothing more to thinking than engaging in behavior which is interpretable as thinking. Yet he does seem committed to saying that it is only with the advent of natural language that you get a kind of thinking which involves discrete, structured, semantically-evaluable, causally-effective states – that is, thoughts realistically construed.

            Bickerton’s proposals (1990, 1995) are somewhat similar, but more biological in flavor. He thinks that, before the evolution of language, hominid cognition was extremely limited in its powers. On his view these early forms of hominid cognition consisted largely of a set of relatively simple computational systems, underpinning an array of flexible but essentially behavioristic conditioned responses to stimuli. But then the evolution of language some 100,000 years ago involved a dramatic re-wiring of the hominid brain, giving rise to distinctively human intelligence and conceptual powers.[4]

Bickerton, like Dennett, allows that subsequent to the evolution of language the human mind would have undergone further transformations, as the stock of socially transmitted ideas and concepts changed and increased. But the basic alteration was coincident with, and constituted by, a biological alteration – namely, the appearance of an innately-structured language-faculty. For Bickerton is a nativist about language. (Indeed, his earlier work on the creolization of pidgin languages – 1981 – is often cited as part of an argument for the biological basis of language; see Pinker, 1994.) And it is language which, he supposes, conferred on us the capacity for ‘off-line thinking’ – that is, the capacity to think and reason about topics and problems in the abstract, independent of any particular sensory stimulus.

            These strong views seem very unlikely to be correct. This is so for two reasons. First, they undervalue the cognitive powers of pre-linguistic children, animals, and earlier forms of hominid. Thus Homo erectus and archaic forms of Homo sapiens, for example, were able to survive in extremely harsh tundra environments, presumably without language (see below). It is hard to see how this could have been possible without a capacity for quite sophisticated planning and a good deal of complex social interaction (Mithen, 1996). Second, the views of Dennett and Bickerton are inconsistent with the sort of central-process modularism which has been gaining increasing support in recent decades. On this account the mind contains a variety of conceptual modules – for mind-reading, for doing naïve physics, for reasoning about social contracts, and so on – which are probably of considerable ancestry, pre-dating the appearance of a modular language-faculty.[5] So hominids were already capable of conceptual thought, and of reasoning in a complex, and presumably ‘off-line’, fashion before the arrival of language.

In sections 3.3 and 3.4 which follow I shall elaborate briefly on these points. But first, I want to consider a potential reply which might be made by someone sympathetic to Bickerton’s position. For Bickerton actually thinks that earlier hominids probably used a form of ‘proto-language’ prior to the evolution of syntax, similar to the language used by young children and to pidgin languages. (This is, in fact, a very plausible intermediate stage in the evolution of natural language.) It might be claimed, then, that insofar as hominids are capable of intelligent thought, this is only because those thoughts are framed in proto-language. So the view that thought is dependent upon language can be preserved.

Such a reply would, indeed, give Bickerton a little extra wiggle-room; but only a little. For as we shall see in section 3.3 below, a good deal of the evidence for hominid thinking is provided by the capacities of our nearest relatives, the great apes, who are known to lack even a proto-language (without a good deal of human enculturation and explicit training, at any rate; Savage-Rumbaugh and Lewin, 1994). And some of the other evidence – e.g. provided by hominid stone knapping – is not plausibly seen as underpinned by proto-language. Moreover, the various thought-generating central modules, to be discussed in section 3.4 below, are almost surely independent both of language and proto-language. So it remains the case that much hominid thought is independent even of proto-language.

 

3.3            Hominid intelligence

Since social intelligence is something which we share with the other great apes (especially chimpanzees), it is reasonable to conclude that the common ancestor of all apes - and so, by implication, all earlier forms of hominid - will also have excelled in the social domain. While it is still disputed whether chimpanzees have full-blown mind-reading, or ‘theory of mind’, abilities, of the sort attained by a normal four-year-old child, it is not in dispute that the social behavior of great apes can be extremely subtle and sophisticated (Byrne and Whiten, 1988, 1998; Byrne, 1995; Povinelli, 2000).

Two points are worth stressing in this context. One is that it is well-nigh impossible to see how apes can be capable of representing multiple, complex, and constantly changing social relationships (who is friends with whom, who has recently groomed whom, who has recently fallen out with whom, and so on) unless they are capable of structured propositional thought.[6] This is a development of what Horgan and Tienson (1996) call ‘the tracking argument for Mentalese’ (i.e. an argument in support of the claim that thoughts are structured out of recombinable components). Unless the social thoughts of apes were composed out of elements variously representing individuals and their properties and relationships, then it is very hard indeed to see how they could do the sort of one-off learning of which they are manifestly capable. This surely requires separate representations for individuals and their properties and relations, so that the latter can be varied while the former are held constant. So (contra Dennett and Bickerton) we have reason to think that all earlier forms of hominid would have been capable of sophisticated conceptual thought (realistically construed), at least in the social domain.[7]

            The second point to note is that the social thinking of apes seems sometimes to be genuinely strategic in nature, apparently involving plans which are executed over the course of days or months. Consider, for example, the way in which a band of male chimpanzees will set out quietly and in an organized and purposive manner towards the territory of a neighboring group, apparently with the intention, either of killing some of the males of that group, or of capturing some of its females, or both (Byrne, 1995). Or consider the way in which a lower-ranking male will, over the course of a number of months, build up a relationship with the beta male, until the alliance is strong enough for them to co-operate in ousting the alpha male from his position (de Waal, 1982). Presumably the thinking which would generate such long-term plans and strategies would have to be ‘off-line’, in the sense of not being tied to or driven by current perceptions of the environment.

            We can conclude, then, that all of our hominid ancestors would have had a sophisticated social intelligence. In addition, the stone-tool-making abilities of later species of Homo erectus indicate a sophisticated grasp of fracture dynamics and the properties of stone materials. Making stone tools isn’t easy. It requires judgment, as well as considerable hand-eye co-ordination and upper-body strength. And since it uses a reductive technology (starting from a larger stone and reducing it to the required shape) it cannot be routinized in the way that (presumably) nest-building by weaver birds and dam-building by beavers can be. Stone knappers have to hold in mind the desired shape and plan two or more strikes ahead in order to work towards it using variable and unpredictable materials (Pelegrin, 1993; Mithen, 1996). Moreover, some of the very fine three-dimensional symmetries produced from about half-a-million years ago would almost certainly have required significant capacities for visual imagination - in particular, an ability to mentally rotate an image of the stone product which will result if a particular flake is struck off (Wynn, 2000). And this is surely ‘off-line’ thinking if anything is!

            We can also conclude that early humans were capable of learning and reasoning about their natural environments with a considerable degree of sophistication. They were able to colonize much of the globe, ranging from Southern Africa to North-Western Europe to South-East Asia. And they were able to thrive in a wide variety of habitats (including extremely harsh marginal tundra environments), adapting their life-style to local - and sometimes rapidly changing - circumstances (Mithen, 1990, 1996). This again serves as a premise for a version of the ‘tracking argument’, suggesting that early humans were capable of compositionally-structured thoughts about the biological as well as the social worlds.

 

3.4       The modular mind

The above claims about the cognitive powers of our early ancestors both support, and are in turn supported by, the evidence of modular organization in the minds of contemporary humans. On this account, besides a variety of input and output modules (including early vision, face-recognition, and language, for example), the mind also contains a number of innately channeled conceptual modules, designed to process conceptual information concerning particular domains. Although these would not be modules in Fodor’s classic (1983) sense, in that they wouldn’t have proprietary transducers, might not have dedicated neural hardware, and might not be fully encapsulated, they would still be innately channeled dedicated computational systems, generating information in accordance with algorithms which are not shared with, nor accessible to, other systems.

Plausible candidates for such conceptual modules might include a naïve physics system (Leslie, 1994; Spelke, 1994; Spelke et al., 1995; Baillargeon, 1995), a naïve psychology or ‘mind-reading’ system (Carey, 1985; Leslie, 1994; Baron-Cohen, 1995), a folk-biology system (Atran, 1990, 1998, 2002), an intuitive number system (Wynn, 1990, 1995; Gallistel and Gelman, 1992; Dehaerne, 1997), a geometrical system for re-orienting and navigating in unusual environments (Cheng, 1986; Hermer and Spelke, 1994, 1996) and a system for processing and keeping track of social contracts (Cosmides and Tooby, 1992; Fiddick et al., 2000).

Evidence supporting the existence of at least the first two of these systems (folk-physics and folk-psychology) is now pretty robust. Very young infants already have a set of expectations concerning the behaviors and movements of physical objects, and their understanding of this form of causality develops very rapidly over the first year or two of life. And folk-psychological concepts and expectations also develop very early, and follow a characteristic developmental profile. Indeed, recent evidence from the study of twins suggests that three-quarters of the variance in mind-reading abilities amongst three year olds is both genetic in origin and largely independent of the genes responsible for verbal intelligence, with only one quarter of the variance being contributed by the environment (Hughes and Plomin, 2000).[8]

            Now, of course the thesis of conceptual modularity is still highly controversial, and disputed by many cognitive scientists. And I cannot pretend to have said enough to have established it here; nor is there the space to attempt to do so. This is going to be one of the large assumptions which I need to ask my readers to take on board as background to what follows. However, there is one sort of objection to conceptual modularity which I should like to respond to briefly here. This is that there simply hasn’t been time for all of these modular systems to have evolved (or at any rate, not those of them that are distinctively human – geometry and folk-physics might be the exceptions).

            Tomasello (1999) argues that the mere six million years or so since the hominid line diverged from the common ancestor of ourselves and chimpanzees is just too short a time for the processes of evolution to have sculpted a whole suite of conceptual modules. He thinks that explanations of distinctively-human cognition need to postulate just one – at most two – biological adaptations, in terms of which all the other cognitive differences between us and chimpanzees should be explained. His preferred option is theory of mind ability, which underpins processes of cultural learning and cultural accumulation and transmission. Others might argue in similar fashion that the only major biological difference is the language faculty (Perner, personal communication).

            The premise of this argument is false, however; six million years is a lot of time, particularly if the selection pressures are powerful ones. (Only 10,000 years separate polar bears and grizzlies, for example.) And this is especially so when, as in the present case, many of the systems in question don’t have to be built ab initio, but can result from a deepening and strengthening of pre-existing faculties. Thus theory of mind would surely have developed from some pre-existing social-cognition module; folk-biology from a pre-existing foraging system; and so on. In order to reinforce the point, one just has to reflect on the major, and multiple, physical differences between ourselves and chimpanzees – including upright gait, arm-length, physical stature, brain size, nasal shape, hairlessness, whites of eyes, and so on and so forth. These, too, have all evolved – many of them independently, plainly – over the last six million years.

 

3.5       Taking stock

What has happened in the cognitive sciences in recent decades, then, is this. Many researchers have become increasingly convinced, by neuropsychological and other evidence, that the mind is more or less modular in structure, built up out of isolable, and largely isolated, components (Fodor, 1983; Sachs, 1985; Shallice, 1988; Gallistel, 1990; Barkow et al., 1992; Hirschfeld and Gelman, 1994; Sperber et al., 1995; Pinker, 1997). They have also become convinced that the structure and contents of the mind are substantially innate (Fodor, 1981, 1983; Carey, 1985; Spelke, 1994), and that language is one such isolable and largely innate module (Fodor, 1983; Chomsky, 1988; Pinker, 1994). There has then been, amongst cognitive scientists, a near-universal reaction against the cognitive conception of language, by running it together with the Whorfian hypothesis. Most researchers have assumed, without argument, that if they were to accept any form of cognitive conception of language, then that would commit them to Whorfian linguistic relativism and radical empiricism, and would hence be inconsistent with their well-founded beliefs in modularity and nativism (Pinker, 1994).

            It is important to see, however, that someone endorsing the cognitive conception of language does not have to regard language and the mind as cultural constructs, either socially determined or culturally relative. In fact, some form of cognitive conception of language can equally well be deployed along with a modularist and nativist view of language and mind. There are a range of positions intermediate between the input–output conception of language on the one hand, and Whorfian relativism (the Standard Social Science Model) on the other, which deserve the attention of philosophers and cognitive scientists alike. These views are nativist as opposed to empiricist about language and much of the structure of the mind, but nevertheless hold that language is constitutively employed in many of our thoughts.

 

4            Language and conscious thinking

What is at stake, then, is the question whether language might be constitutively involved in some forms of human thinking. But which forms? In previous work I suggested that language might be the medium in which we conduct our conscious propositional thinking - claiming, that is, that inner speech might be the vehicle of conscious-conceptual (as opposed to conscious visuo-spatial) thinking (Carruthers, 1996). This view takes seriously and literally the bit of folk-wisdom with which this paper began - namely, that much of our conscious thinking (viz. our propositional thinking) is conducted in inner speech.

            Now, if the thesis here is that the cognitive role of language is confined to conscious thinking, then it will have to be allowed that much propositional thinking also takes place independently of natural language - for it would hardly be very plausible to maintain that there is no thinking but conscious thinking. And there are then two significant options regarding the relations between non-conscious language-independent thought, on the one hand, and conscious language-involving thinking, on the other. For either we would have to say that anything which we can think consciously, in language, can also be thought non-consciously, independently of language; or we would have to say that there are some thought-types which can only be entertained at all, by us, when tokened consciously in the form of an imaged natural language sentence.

            Suppose that it is the first - weaker and more plausible - of these options which is taken. Then we had better be able to identify some element of the distinctive causal role of an imaged sentence which is sufficiently thought-like or inference-like for us to be able to say that the sentence in question is partly constitutive of the (conscious) tokening of the thought-type in question, rather than being merely expressive of it. For otherwise - if everything which we can think consciously, in language, we can also think non-consciously, without language - what is to block the conclusion that inner speech is merely the means by which we have access to our occurrent thoughts, without inner speech being in any sense constitutive of our thinking? (On this, at length, see Carruthers 1998b.)

            There would seem to be just two distinct (albeit mutually consistent) possibilities here. One (implicit in Carruthers, 1996) would be to propose a suitably weakened version of Dennett’s Joycean machine hypothesis. While allowing (contra Dennett) that much conceptual thinking (realistically construed) and all conceptual thought-types are independent of language (in the sense of not being constituted by it), we could claim that there are certain learned habits and patterns of thinking and reasoning which are acquired linguistically, and which are then restricted to linguistic (and conscious) tokenings of the thoughts which they govern. It is surely plausible, for example, that exact long-division or multiplication can only be conducted consciously, in imaged manipulations of numerical symbols. Similarly, it may be that the result of taking a course in logic is that one becomes disposed to make transitions between sentences, consciously in language, where one would otherwise not have been disposed to make the corresponding transitions between the thoughts expressed. If these sorts of possibilities are realized, then we would have good reason to say of a token application of a particular inference-form, that the imaged natural language sentences involved are constitutive of the inference in question, since it could not have taken place without them.

A second possibility is proposed and defended by Frankish (1998a, 1998b, and forthcoming; see also Cohen, 1993). This is that the distinctive causal role of inner speech is partly a function of our decisions to accept, reject, or act on the propositions which our imaged sentences express. I can frame a hitherto unconsidered proposition in inner speech and decide that it is worthy of acceptance, thereby committing myself to thinking and acting thereafter as if that sentence were true. Then provided that I remember my commitments and execute them, it will be just as if I believed the proposition in question. (In his published work Frankish describes this level of mentality as the ‘virtual mind’ and the beliefs in question as ‘virtual beliefs’.) But, by hypothesis, I would never have come to believe what I do, nor to reason as I do reason, except via the tokening of sentences in inner speech. Frankish argues, in effect, that there is a whole level of mentality (which he now dubs ‘supermind’) which is constituted by our higher-order decisions and commitments to accept or reject propositions; and that language is constitutive of the thoughts and beliefs which we entertain at this level.

Such views have considerable plausibility; and it may well be that one, or other, or both of these accounts of the causal role of inner speech is correct. Indeed, the dual process theory of human reasoning developed over the years by Evans and colleagues (Wason and Evans, 1975; Evans and Over, 1996), and more recently by Stanovich (1999), combines elements of each of them. On this account, in addition to a suite of computationally powerful, fast, and implicit reasoning systems (from our perspective, a set of conceptual modules), the mind also contains a slow, serial, and explicit reasoning capacity, whose operations are conscious and under personal control, and which is said (by some theorists at least; e.g. Evans and Over, 1996) to involve natural language. The emphasis here on learned rules in the operations of the explicit system is reminiscent of Dennett’s ‘Joycean machine’, whereas the stress on our having personal control over the operations of that system seems very similar to Frankish’s conception of ‘supermind’.

Not only is some form of dual-process theory plausible, but it should also be stressed that these accounts are independent of central-process modularism. Those who deny the existence of any conceptual modules can still accept that there is a level of thinking and reasoning which is both language-involving and conscious. It is surely plain, however, that none of the above accounts can amount to the most fundamental cognitive function of language once conceptual modularity is assumed.

Given conceptual modularity, then unless the above views are held together with the thesis to be developed in section 5 below – namely, that language provides the medium for inter-modular communication and non-domain-specific thinking – then we can set their proponents a dilemma. Either they must claim that a domain-general architecture was in place prior to the evolution of language. Or they must allow that there was no significant domain-general cognition amongst hominids prior to the appearance of language and language-involving conscious thinking; and they must claim that such cognition still evolved as a distinct development, either at the same time or later. Since contemporary humans are manifestly capable of conjoining information across different domains in both their theoretical thought and their planning, then either pre-linguistic humans must already have had domain-general theoretical and practical reasoning faculties, or they must have evolved them separately at the same time or after the evolution of the language faculty (that is, if it isn’t language itself which enables us to combine information across modules).

The problem with the first alternative, however – namely, that domain-general reasoning capacities pre-dated language – is that the evidence from cognitive archaeology suggests that this was not the case. For although the various sub-species of Homo erectus and archaic forms of Homo sapiens were smart, they were not that smart. Let me briefly elaborate.

As Mithen (1996) demonstrates at length and in detail, the evidence from archaeology is that the minds of early humans were in important respects quite unlike our own. While they successfully colonized diverse and rapidly changing environments, the evidence suggests that they were incapable of bringing together information across different cognitive domains. It seems that they could not (or did not) mix information from the biological world (utilized in hunting and gathering) with information about the physical world (used in tool making); and that neither of these sorts of information interacted with their social intelligence. Although they made sophisticated stone tools, they did not use those tools for specialized purposes (with different kinds of arrow-head being used for different kinds of game, for example); and they did not make tools out of animal products such as antler and bone. There is no sign of the use of artifacts as social signals, in the form of body ornaments and such-like, which is so ubiquitous in modern human cultures. And there is no indication of totemization or other sorts of linkages between social and animal domains, such as lion-man figurines, cave-paintings, or the burying of the dead with (presumably symbolic) animal parts - which all emerge onto the scene for the first time with modern humans. As Mithen summarizes the evidence, it would appear that early humans had sophisticated special intelligences, but that these faculties remained largely isolated from one another.

The problem with the second horn of the dilemma sketched above is just that it is hard to believe, either that a domain-general reasoning faculty might have evolved after the appearance of language some 100,000 years ago (in just the 20,000 years or so before the beginning of the dispersal of modern humans around the globe), or that language and domain-general capacities might have co-evolved as distinct faculties. For as we shall see in section 5, the evolution of language would in any case have involved the language faculty taking inputs from, and sending outputs to, the various modular systems, if there wasn’t already a domain-general system for it to be linked to. And it is hard to discern what the separate selection pressures might have been, which would have led to the development of two distinct faculties at about the same time (language and domain-general thought), when just one would serve.

 

5            Language as the medium of non-domain-specific thinking

The hypothesis which I particularly want to explore, then, is that natural language is the medium of non-domain-specific thought and inference. Versions of this hypothesis have been previously proposed by Carruthers (1996, 1998a), by Mithen (1996), and by Spelke and colleagues (Hermer-Vazquez et al., 1999; Spelke and Tsivkin, 2001; Spelke, forthcoming). I shall sketch the thesis itself, outline the existing experimental evidence in its support, and then (in the section following) consider some of its ramifications and possible elaborations. Finally (in section 7) I shall discuss what further evidence needs to be sought as a test of our thesis.

 

5.1       The thesis

The hypothesis in question assumes a form of central-process modularism. That is, it assumes that in addition to the various input and output modules (vision, face-recognition, hearing, language, systems for motor-control, etc.), the mind also contains a range of conceptual modules, which take conceptual inputs and deliver conceptual outputs. Evidence of various sorts has been accumulating in support of central-process modularism in recent decades (some of which has already been noted above). One line of support is provided by evolutionary psychologists, who have argued on both theoretical and empirical grounds that the mind contains a suite of domain-specific cognitive adaptations (Barkow et al., 1992; Sperber, 1996; Pinker, 1997). But many who would not describe themselves as ‘evolutionary psychologists’ have argued for a modular organization of central cognition, on developmental, psychological, and/or neuro-pathological grounds (Carey, 1985; Shallice, 1988; Gallistel, 1990; Carey and Spelke, 1994; Leslie, 1994; Spelke, 1994; Baron-Cohen, 1995; Smith and Tsimpli, 1995; Hauser and Carey, 1998).

            What cognitive resources were antecedently available, then, prior to the evolution of the language faculty? Taking the ubiquitous laboratory rat as a representative example, I shall assume that all mammals, at least, are capable of thought – in the sense that they engage in computations which deliver structured (propositional) belief-like states and desire-like states (Dickinson, 1994; Dickinson and Balleine, 2000). I shall also assume that these computations are largely carried out within modular systems of one sort or another (Gallistel, 1990) – after all, if the project here is to show how cross-modular thinking in humans can emerge out of modular components, then we had better assume that the initial starting-state was a modular one. Furthermore, I shall assume that mammals possess some sort of simple non-domain-specific practical reasoning system, which can take beliefs and desires as input, and figure out what to do.

            I shall assume that the practical reasoning system in animals (and perhaps also in us) is a relatively simple and limited-channel one. Perhaps it receives as input the currently-strongest desire and searches amongst the outputs of the various belief-generating modules for something which can be done in relation to the perceived environment which will satisfy that desire. So its inputs have the form DESIRE [Y] and BELIEF [IF X THEN Y], where X should be something for which an existing motor-program exists. I assume that the practical reasoning system is not capable of engaging in other forms of inference (generating new beliefs from old), nor of combining together beliefs from different modules; though perhaps it is capable of chaining together conditionals to generate a simple plan – e.g. BELIEF [IF W THEN X], BELIEF [IF X THEN Y] ® BELIEF [IF W THEN Y].

            The central modules will take inputs from perception, of course. And my guess is that many of the beliefs and desires generated by the central modules will have partially indexical contents – thus a desire produced as output by the sex module might have the form, ‘I want to mate with that female’, and a belief produced by the causal-reasoning module might have the form, ‘That caused that’. So if the practical reasoning system is to be able to do anything with such contents, then it, too, would need to have access to the outputs of perception, to provide anchoring for the various indexicals. The outputs of the practical reasoning system are often likely to be indexical too, such as an intention of the form, ‘I’ll go that way’.

The inputs to central-process modules can presumably include not only conceptualized perceptions but also propositional descriptions (in the latter case deriving from linguistic input - for we surely use our mind-reading system, for example, when processing a description of someone’s state of mind as well as when observing their behavior). And in some cases, too, the inputs to a module will include the outputs of other central-process modules; for we might expect that there will be cases in which modules are organized into some sort of hierarchy. But what of the outputs from central-process modules? Besides being directed to other modules (in some instances), and also to the practical reasoning system, where is the information which is generated by central-process modules normally sent? And in particular, is there some non-domain-specific central arena where all such information is collated and processed?

            The hypothesis being proposed here is that there is such an arena, but one which crucially implicates natural language, and which cannot operate in the absence of such language. Moreover, the hypothesis is not just that our conscious propositional thinking involves language (as sketched in section 4 above), but that all non-domain-specific reasoning of a non-practical sort (whether conscious or non-conscious) is conducted in language. And as for the question of what a non-conscious tokening of a natural language sentence would be like, we can propose that it would be a representation stripped of all imagistic-phonological features, but still consisting of natural language lexical items and syntactic structures. (The role of syntax in the present account will be further explored in section 6.1 below.)

Chomsky (1995) has maintained, for example, that there is a level of linguistic representation which he calls ‘Logical Form’ (LF), which is where the language faculty interfaces with central cognitive systems. We can then claim that all cross-modular thinking consists in the formation and manipulation of these LF representations. The hypothesis can be that all such thinking operates by accessing and manipulating the representations of the language faculty. Where these representations are only in LF, the thoughts in question will be non-conscious ones. But where the LF representation is used to generate a full-blown phonological representation (an imaged sentence), the thought will nomally be conscious. And crucially for my purposes, the hypothesis is that the language faculty has access to the outputs of the various central-process modules, in such a way that it can build LF representations which combine information across domains.

Let me say a just little more about the conscious / non-conscious distinction as it operates here. As I shall mention again in a moment (and as I shall return to at some length in section 6.2) language is both an input and an output module. Its production sub-system must be capable of receiving outputs from the conceptual modules in order to transform their creations into speech. And its comprehension sub-system must be capable of transforming heard speech into a format suitable for processing by those same conceptual modules. Now when LF representations built by the production sub-system are used to generate a phonological representation, in ‘inner speech’, that representation will be consumed by the comprehension sub-system, and made available to central systems. One of these systems is a theory of mind module. And on the sort of higher-order theory of consciousness which I favor (Carruthers, 2000), perceptual and imagistic states get to be phenomenally conscious by virtue of their availability to the higher-order thoughts generated by the theory of mind system (i.e. thoughts about those perceptual and imagistic states). So this is why inner speech of this sort is conscious: it is because it is available to higher-order thought.

            The hypothesis, then, is that non-domain-specific, cross-modular, propositional thought depends upon natural language - and not just in the sense that language is a necessary condition for us to entertain such thoughts, but in the stronger sense that natural language representations are the bearers of those propositional thought-contents. So language is constitutively involved in (some kinds of) human thinking. Specifically, language is the vehicle of non-modular, non-domain-specific, conceptual thinking which integrates the results of modular thinking.

            Before moving on to discuss the evidence in support of our thesis, consider one further question. Why does it have to be language, and not, for example, visual imagery which serves the integrative function? For visual images, too, can carry contents which cross modular domains. But such visual thinking will access and deploy the resources of a peripheral input module. It cannot, therefore, play a role in integrating information across conceptual modules, because the latter exist down-stream of the input-systems. Vision provides input to conceptual modules, and doesn’t receive output from them. The language faculty, in contrast, while also ‘peripheral’, has both input and output functions. (I shall return to this point again in section 6.2 below.) I would hypothesize, therefore, that in cases where visual images have cross-modular contents (and aren’t memory images), they are always generated from some linguistic representation which originally served to integrate those contents.

 

5.2       The evidence

What evidence is there to support the hypothesis that natural language is the medium of inter-modular communication, or of non-domain-specific integrated thinking? Until recently, the evidence was mostly circumstantial. For example, one indirect line of argument in support of our thesis derives from cognitive archaeology, when combined with the evidence of contemporary central-process modularism (Mithen, 1996). For as we noted above, it seems that we only have significant evidence of cross-modular thought following the emergence of contemporary humans some 100,000 years ago; whereas independent evidence suggests that language, too, was a late evolutionary adaptation, only finally emerging at about the same time (perhaps from an earlier stage of ‘proto-language’ - Bickerton, 1990, 1995). So the simplest hypothesis is that it is language which actually enables cross-modular thinking.

            Another strand of indirect evidence can be provided if we take seriously the idea that the stream of inner verbalization is constitutive of (some forms of) thinking (Carruthers, 1996). For as we saw in section 4 above, such views can only plausibly be held (given the truth of central-process modularism) together with the present hypothesis that language is the main medium of inter-modular communication.

            Much more importantly, however, direct tests of (limited forms of) our hypothesis have now begun to be conducted. The most important of these is Hermer-Vazquez et al. (1999), which provides strong evidence that the integration of geometric properties with other sorts of information (color, smell, patterning, etc.) is dependent upon natural language. The background to their studies with human adults is the apparent discovery of a geometric module in rats by Cheng (1986), as well as the discovery of a similar system in pre-linguistic human children (Hermer and Spelke, 1994, 1996).

            Cheng (1986) placed rats in a rectagonal chamber, and allowed them to discover the location of a food source. They were then removed from the chamber and disoriented, before being placed back into the box with the food now hidden. In each case there were multiple cues available - both geometric and non-geometric - to guide the rats in their search. For example, the different walls might be distinctively colored or patterned, one corner might be heavily scented, and so on. In fact in these circumstances the rats relied exclusively on geometric information, searching with equal frequency, for example, in the two geometrically-equivalent corners having a long wall on the left and a short wall on the right. Yet rats are perfectly well capable of noticing and remembering non-geometric properties of the environment and using them to solve other tasks. So it appears that, not only are they incapable of integrating geometric with non-geometric information in these circumstances, but that geometric information takes priority.

            (This makes perfectly good ecological-evolutionary sense. For in the rat’s natural environment, overall geometrical symmetries in the landscape are extremely rare, and geometrical properties generally change only slowly with time; whereas object-properties of color, scent-markings, and so on will change with the weather and seasons. So a strong preference to orient by geometrical properties is just what one might predict.)

            Hermer and Spelke (1994, 1996) found exactly the same phenomenon in pre-linguistic human children. Young children, too, rely exclusively on geometric information when disoriented in a rectangular room, and appear incapable of integrating geometrical with non-geometrical properties when searching for a previously seen but now-hidden object. Older children and adults are able to solve these problems without difficulty - for example, they go straight to the corner formed with a long wall to the left and a short blue wall to the right. It turns out that success in these tasks isn’t directly correlated with age, nonverbal IQ, verbal working-memory capacity, vocabulary size, or comprehension of spatial vocabulary. In contrast, the only significant predictor of success in these tasks which could be discovered, was spontaneous use of spatial vocabulary conjoined with object-properties (e.g. ‘It’s left of the red one’). Even by themselves, these data strongly suggest that it is language which enables older children and adults to integrate geometric with non-geometric information into a single thought or memory.

            Hermer-Vazquez et al. (1999) set out to test this idea with a series of dual-task experiments with adults. In one condition, subjects were required to solve one of these orientation problems while shadowing (i.e. repeating back) speech played to them through a set of headphones. In another condition, they were set the same problems while shadowing (with their hands) a rhythm played to them in their headphones. The hypothesis was that speech-shadowing would tie up the resources of the language faculty, whereas the rhythm-shadowing tasks would not; and great care was taken to ensure that the latter tasks were equally if not more demanding of the resources of working memory.

            The results of these experiments were striking. Shadowing of speech severely disrupted subjects’ capacity to solve tasks requiring integration of geometric with non-geometric properties. In contrast, shadowing of rhythm disrupted subjects’ performance relatively little. Moreover, a follow-up experiment demonstrated that shadowing of speech didn’t disrupt subjects’ capacities to utilize non-geometric information per se - they were easily able to solve tasks requiring only memory for object-properties. So it would appear that it is language itself which enables subjects to conjoin geometric with non-geometric properties, just as the hypothesis that language is the medium of cross-modular thinking predicts.

            Of course, this is just one set of experiments