The identity theory of mind holds that states and processes of the mindare identical to states and processes of the brain. Strictly speaking,it need not hold that the mind is identical to the brain. Idiomaticallywe do use ‘She has a good mind’ and ‘She has a goodbrain’ interchangeably but we would hardly say ‘Her mindweighs fifty ounces’. Here I take identifying mind and brain asbeing a matter of identifying processes and perhaps states of the mindand brain. Consider an experience of pain, or of seeing something, orof having a mental image. The identity theory of mind is to the effectthat these experiences justare brain processes, not merelycorrelated with brain processes.

Some philosophers hold that though experiences are brain processesthey nevertheless have fundamentally non-physical, psychical,properties, sometimes called ‘qualia’. Here I shall takethe identity theory as denying the existence of such irreduciblenon-physical properties. Some identity theorists give a behaviouristicanalysis of mentalstates, such as beliefs and desires, butothers, sometimes called ‘central state materialists’, saythat mental states are actual brain states. Identity theorists oftendescribe themselves as ‘materialists’ but‘physicalists’ may be a better word. That is, one might bea materialist about mind but nevertheless hold that there are entitiesreferred to in physics that are not happily described as‘material’.

In taking the identity theory (in its various forms) as a species ofphysicalism, I should say that this is an ontological, not atranslational physicalism. It would be absurd to try to translatesentences containing the word ‘brain’ or the word‘sensation’ into sentences about electrons, protons and soon. Nor can we so translate sentences containing the word‘tree’. After all ‘tree’ is largely learnedostensively, and is not even part of botanical classification. If wewere small enough a dandelion might count as a tree. Nevertheless aphysicalist could say that trees are complicated physical mechanisms.The physicalist will deny strong emergence in the sense of somephilosophers, such as Samuel Alexander and possibly C.D. Broad. Thelatter remarked (Broad 1937) that as far as was known at that time theproperties of common salt cannot be deduced from the properties ofsodium in isolation and of chlorine in isolation. (He put it tooepistemologically: chaos theory shows that even in a deterministictheory physical consequences can outrun predictability.) Of course thephysicalist will not deny the harmless sense of "emergence" in which anapparatus is not just a jumble of its parts (Smart 1981).

• 1. Historical Antecedents
• 2. The Nature of the Identity Theory
• 3. Phenomenal Properties and Topic-Neutral Analyses
• 4. Causal Role Theories
• 5. Functionalism and Identity Theory
• 6. Type and Token Identity Theories
• 7. Consciousness
• 8. Later Objections to the Identity Theory
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. Historical Antecedents

The identity theory as I understand it here goes back to U.T. Placeand Herbert Feigl in the 1950s. Historically philosophers andscientists, for example Leucippus, Hobbes, La Mettrie, and d'Holbach,as well as Karl Vogt who, following Pierre-Jean-Georges Cabanis, madethe preposterous remark (perhaps not meant to be taken too seriously)that the brain secretes thought as the liver secretes bile, haveembraced materialism. However, here I shall date interest in theidentity theory from the pioneering papers ‘Is Consciousness aBrain Process?’ by U.T. Place (Place 1956) and H. Feigl‘The "Mental" and the "Physical"’ (Feigl 1958).Nevertheless mention should be made of suggestions by Rudolf Carnap(1932, p. 127), H. Reichenbach (1938) and M. Schlick (1935).Reichenbach said that mental events can be identified by thecorresponding stimuli and responses much as the (possibly unknown)internal state of a photo-electric cell can be identified by thestimulus (light falling on it) and response (electric current flowing)from it. In both cases the internal states can be physical states.However Carnap did regard the identity as a linguistic recommendationrather than as asserting a question of fact. See his ‘HerbertFeigl on Physicalism’ in Schilpp (1963), especially p. 886. Thepsychologist E.G. Boring (1933) may well have been the first to usethe term ‘identity theory’. See Place (1990).

Place's very original and pioneering paper was written afterdiscussions at the University of Adelaide with J.J.C. Smart and C.B.Martin. For recollections of Martin's contributions to the discussionsee Place (1989) ‘Low Claim Assertions’ in Heil (1989).Smart at the time argued for a behaviourist position in which mentalevents were elucidated purely in terms of hypothetical propositionsabout behaviour, as well as first person reports of experiences whichGilbert Ryle regarded as ‘avowals’. Avowals were thought ofas mere pieces of behaviour, as if saying that one had a pain was justdoing a sophisticated sort of wince. Smart saw Ryle's theory asfriendly to physicalism though that was not part of Ryle's motivation.Smart hoped that the hypotheticals would ultimately be explained byneuroscience and cybernetics. Being unable to refute Place, andrecognizing the unsatisfactoriness of Ryle's treatment of innerexperience, to some extent recognized by Ryle himself (Ryle 1949, p.240), Smart soon became converted to Place's view (Smart 1959). In thishe was also encouraged and influenced by Feigl's ‘"The Mental"and the "Physical" ’ (Feigl 1958, 1967). Feigl's wide rangingcontribution covered many problems, including those connected withintentionality, and he introduced the useful term ‘nomologicaldanglers’ for the dualists' supposed mental-physicalcorrelations. They would dangle from the nomological net of physicalscience and should strike one as implausible excrescences on the fairface of science. Feigl (1967) contains a valuable‘Postscript’.

2. The Nature of the Identity Theory

Place spoke of constitution rather than of identity. One of hisexamples is ‘This table is an old packing case’. Another is‘lightning is an electric discharge’. Indeed this latterwas foreshadowed by Place in his earlier paper ‘The Concept ofHeed’ (Place 1954), in which he took issue with Ryle'sbehaviourism as it applied to concepts of consciousness, sensation andimagery. Place remarked (p. 255)

The logical objections which might be raised to thestatement ‘consciousness is a process in the brain’ are nogreater than the logical objections which might be raised to thestatement ‘lightning is a motion of electriccharges’.

It should be noticed that Place was using the word‘logical’ in the way that it was used at Oxford at thetime, not in the way that it is normally used now. One objection wasthat ‘sensation’ does not mean the same as ‘brainprocess’. Place's reply was to point out that ‘thistable’ does not mean the same as ‘this old packingcase’ and ‘lightning’ does not mean the same as‘motion of electric charges’. We find out whether this is atable in a different way from the way in which we find out that it isan old packing case. We find out whether a thing is lightning bylooking and that it is a motion of electric charges by theory andexperiment. This does not prevent the table being identical to the oldpacking case and the perceived lightning being nothing other than anelectric discharge. Feigl and Smart put the matter more in terms of thedistinction between meaning and reference. ‘Sensation’ and‘brain process’ may differ in meaning and yet have the samereference. ‘Very bright planet seen in the morning’ and‘very bright planet seen in the evening’ both refer to thesame entity Venus. (Of course these expressionscould beconstrued as referring to different things, different sequences oftemporal stages of Venus, but not necessarily or most naturally so.)

There did seem to be a tendency among philosophers to have thoughtthat identity statements needed to be necessary and a priori truths.However identity theorists have treated ‘sensations are brainprocesses’ as contingent. We had tofind out that theidentity holds. Aristotle, after all, thought that the brain was forcooling the blood. Descartes thought that consciousness isimmaterial.

It was sometimes objected that sensation statements are incorrigiblewhereas statements about brains are corrigible. The inference was madethat there must be something different about sensations. Ryle and ineffect Wittgenstein toyed with the attractive but quite implausiblenotion that ostensible reports of immediate experience are not reallyreports but are ‘avowals’, as if my report that I havetoothache is just a sophisticated sort of wince. Place, influenced byMartin, was able to explain the relative incorrigibility of sensationstatements by their low claims: ‘I see a bent oar’ makes abigger claim than ‘It looks to me that there is a bentoar’. Nevertheless my sensation and my putative awareness of thesensation are distinct existences and so, by Hume's principle, it mustbe possible for one to occur without the other. One should denyanything other than a relative incorrigibility (Place 1989).

As remarked above, Place preferred to express the theory by thenotion of constitution, whereas Smart preferred to make prominent thenotion of identity as it occurs in the axioms of identity in logic. SoSmart had to say that if sensation X is identical to brain process Ythen if Y is between my ears and is straight or circular (absurdly tooversimplify) then the sensation X is between my ears and is straightor circular. Of course it is not presented to us as such in experience.Perhaps only the neuroscientist could know that it is straight orcircular. The professor of anatomy might be identical with the dean ofthe medical school. A visitor might know that the professor hiccups inlectures but not know that the dean hiccups in lectures.

3. Phenomenal Properties and Topic-Neutral Analyses

Someone might object that the dean of the medical school does notqua dean hiccup in lectures.Qua dean he goes tomeetings with the vice-chancellor. This is not to the point but thereis a point behind it. This is thatthe property of being theprofessor of anatomy is not identical withthe property ofbeing the dean of the medical school. The question might be asked, thateven if sensations are identical with brain processes, are there notintrospected non-physical properties of sensations that are notidentical with properties of brain processes? How would a physicalistidentity theorist deal with this? The answer (Smart 1959) is that theproperties of experiences are ‘topic neutral’. Smartadapted the words ‘topic-neutral’ from Ryle, who used themto characterise words such as ‘if, ‘or’,‘and’, ‘not’, ‘because’. If youoverheard only these words in a conversation you would not be able totell whether the conversation was one of mathematics, physics, geology,history, theology, or any other subject. Smart used the words‘topic neutral’ in the narrower sense of being neutralbetween physicalism and dualism. For example ‘going on’,‘occurring’, ‘intermittent’,‘waxing’, ‘waning’ are topic neutral. So is‘me’ in so far as it refers to the utterer of the sentencein question. Thus to say that a sensation is caused by lightning or thepresence of a cabbage before my eyes leaves it open as to whether thesensation is non-physical as the dualist believes or is physical as thematerialist believes. This sentence also is neutral as to whether theproperties of the sensation are physical or whether some of them areirreducibly psychical. To see how this idea can be applied to thepresent purpose let us consider the following example.

Suppose that I have a yellow, green and purple striped mental image.We may also introduce the philosophical term ‘sense datum’to cover the case of seeing or seeming to see something yellow, greenand purple: we say that we have a yellow, green and purple sense datum.That is I would see or seem to see, for example, a flag or an array oflamps which is green, yellow and purple striped. Suppose also, as seemsplausible, that there is nothing yellow, green and purple striped inthe brain. Thus it is important for identity theorists to say (asindeed they have done) that sense data and images are not part of thefurniture of the world. ‘I have a green sense datum’ isreally just a way of saying that I see or seem to see something thatreally is green. This move should not be seen as merely anadhoc device, since Ryle and J.L. Austin, in effect Wittgenstein,and others had provided arguments, as when Ryle argued that mentalimages were not a sort of ghostly picture postcard. Place characterisedthe fallacy of thinking that when we perceive something green we areperceiving something green in the mind as ‘the phenomenologicalfallacy’. He characterizes this fallacy (Place 1956):

the mistake of supposing that when the subject describeshis experience, when he describes how things look, sound, smell, taste,or feel to him, he is describing the literal properties of objects andevents on a peculiar sort of internal cinema or television screen,usually referred to in the modern psychological literature as the‘phenomenal field’.

Of course, as Smart recognised, this leaves the identity theorydependent on a physicalist account of colour . His early account ofcolour (1961) was too behaviourist, and could not deal, for example,with the reversed spectrum problem, but he later gave a realist andobjectivist account (Smart 1975). Armstrong had been realist aboutcolour but Smart worried that if so colour would be a veryidiosyncratic and disjunctive concept, of no cosmic importance, of nointerest to extraterrestrials (for instance) who had different visualsystems. Prompted by Lewis in conversation Smart came to realize thatthis was no objection to colours being objective properties.

One first gives the notion of a normal human percipient with respectto colour for which there are objective tests in terms of ability tomake discriminations with respect to colour. This can be done withoutcircularity. Thus ‘discriminate with respect to colour’ isa more primitive notion than is that of colour. (Compare the way thatin set theory ‘equinumerous’ is antecedent to‘number’.) Then Smart elucidated the notion of colour interms of the discriminations with respect to colour of normal humanpercipients in normal conditions (say cloudy Scottish daylight). Thisaccount of colour may be disjunctive and idiosyncratic. (Maxwell'sequations might be of interest to Alpha Centaurians but hardly ourcolour concepts.) Anthropocentric and disjunctive they may be, butobjective none the less. David R. Hilbert (1987) identifies colourswith reflectances, thus reducing the idiosyncrasy and disjunctiveness.A few epicycles are easily added to deal with radiated light, thecolours of rainbows or the sun at sunset and the colours due todiffraction from feathers. John Locke was on the right track in makingthe secondary qualities objective as powers in the object, but erred inmaking these powers to be powers to produce ideas in the mind ratherthan to make behavioural discriminations. (Also Smart would say that ifpowers are dispositions we should treat the secondary qualities as thecategorical bases of these powers, e.g. in the case of coloursproperties of the surfaces of objects.) Locke's view suggested that theideas have mysterious qualia observed on the screen of an internalmental theatre. However to do Locke justice he does not talk in effectof ‘red ideas’ but of ‘ideas of red’.Philosophers who elucidate ‘is red’ in terms of‘looks red’ have the matter the wrong way round (Smart1995).

Let us return to the issue of us having a yellow, purple and greenstriped sense datum or mental image and yet there being no yellow,purple and green striped thing in the brain. The identity theorist(Smart 1959) can say that sense data and images are not real things inthe world: they are like the average plumber. Sentences ostensiblyabout the average plumber can be translated into, or elucidated interms of, sentences about plumbers. So also there is having a greensense datum or image but not sense data or images, and the having of agreen sense datum or image is not itself green. So it can, so far asthis goes, easily be a brain process which is not green either.

Thus Place (1956, p. 49):

When we describe the after-image as green... we are sayingthat we are having the sort of experience which we normally have when,and which we have learned to described as, looking at a green patch oflight.

and Smart (1959) says:

When a person says ‘I see a yellowish-orangeafter-image’ he is saying something like this: "There issomething going on which is like what is going on when I have myeyes open, am awake, and there is an orange illuminated in good lightin front of me".

Quoting these passages, David Chalmers (1996, p. 360) objects thatif ‘something is going on’ is construed broadly enough itis inadequate, and if it is construed narrowly enough to cover onlyexperiential states (or processes) it is not sufficient for theconclusion. Smart would counter this by stressing the word‘typically’. Of course a lot of things go on in me when Ihave a yellow after image (for example my heart is pumping bloodthrough my brain). However they do nottypically go on then:they go on at other times too. Against Place Chalmers says that theword ‘experience’ is unanalysed and so Place's analysis isinsufficient towards establishing an identity between sensations andbrain processes. As against Smart he says that leaving the word‘experience’ out of the analysis renders it inadequate.That is, he does not accept the ‘topic-neutral’ analysis.Smart hopes, and Chalmers denies, that the account in terms of‘typically of’ saves the topic-neutral analysis. In defenceof Place one might perhaps say that it is not clear that the word‘experience’ cannot be given a topic neutral analysis,perhaps building on Farrell (1950). If we do not need the word‘experience’ neither do we need the word‘mental’. Rosenthal (1994) complains (against the identitytheorist) that experiences have some characteristically mentalproperties, and that ‘We inevitably lose the distinctively mentalif we construe these properties as neither physical nor mental’.Of course to be topic neutral is to be able to be both physical andmental, just as arithmetic is. There is no need for the word‘mental’ itself to occur in the topic neutral formula.‘Mental’, as Ryle (1949) suggests, in its ordinary use is arather grab-bag term, ‘mental arithmetic’, ‘mentalillness’, etc. with which an identity theorist finds notrouble.

4. Causal Role Theories

In their accounts of mind, David Lewis and D.M. Armstrong emphasise thenotion of causality. Lewis's 1966 was a particularly clear headedpresentation of the identity theory in which he says (I here refer tothe reprint in Lewis 1983, p. 100):

My argument is this: The definitive characteristic of any(sort of) experience as such is its causal role, its syndrome of mosttypical causes and effects. But we materialists believe that thesecausal roles which belong by analytic necessity to experiences belongin fact to certain physical states. Since these physical states possessthe definitive character of experiences, they must beexperiences.

Similarly, Robert Kirk (1999) has argued for the impossibility ofzombies. If the supposed zombie has all the behavioural and neuralproperties ascribed to it by those who argue from the possibility ofzombies against materialism, then the zombie is conscious and so not azombie.

Thus there is no need for explicit use of Ockham's Razor as in Smart(1959) though not in Place (1956). (See Place 1960.) Lewis's paper wasextremely valuable and already there are hints of a marriage betweenthe identity theory of mind and so-called ‘functionalist’ideas that are explicit in Lewis 1972 and 1994. In his 1972(‘Psychophysical and Theoretical Identifications’) heapplies ideas in his more formal paper ‘How to Define TheoreticalTerms’ (1970). Folk psychology contains words such as‘sensation’, ‘perceive’, ‘belief,‘desire’, ‘emotion’, etc. which we recognise aspsychological. Words for colours, smells, sounds, tastes and so on alsooccur. One can regard common sense platitudes containing both thesesorts of these words as constituting a theory and we can take them astheoretical terms of common sense psychology and thus as denotingwhatever entities or sorts of entities uniquely realise the theory.Then if certain neural states do so too (as we believe) then the mentalstates must be these neural states. In his 1994 he allows for tact inextracting a consistent theory from common sense. One cannotuncritically collect platitudes, just as in producing a grammar,implicit in our speech patterns, one must allow for departures fromwhat on our best theory would constitute grammaticality.

A great advantage of this approach over the early identity theory isits holism. Two features of this holism should be noted. One is thatthe approach is able to allow for the causal interactions between brainstates and processes themselves, as well as in the case of externalstimuli and responses. Another is the ability to draw on the notion ofRamseyfication of a theory. F.P. Ramsey had shown how to replace thetheoretical terms of a theory such as ‘the property of being anelectron’ by ‘the property X such that...’. so thatwhen this is done for all the theoretical terms, we are left only with‘property X such that’, ‘property Y such that’etc. Take the terms describing behaviour as the observation terms andpsychological terms as the theoretical ones of folk psychology. ThenRamseyfication shows that folk psychology is compatible withmaterialism. This seems right, though perhaps the earlier identitytheory deals more directly with reports of immediate experience.

The causal approach was also characteristic of D.M. Armstrong'scareful conceptual analysis of mental states and processes, such asperception and the secondary qualities, sensation, consciousness,belief, desire, emotion, voluntary action, in hisA MaterialistTheory of the Mind (1968a) with a second edition (1993) containinga valuable new preface. Parts I and II of this book are concerned withconceptual analysis, paving the way for a contingent identification ofmental states and processes with material ones. As had Brian Medlin, inan impressive critique of Ryle and defence of materialism (Medlin1967), Armstrong preferred to describe the identity theory as‘Central State Materialism’. Independently of Armstrong andLewis, Medlin's central state materialism depended, as theirs did, on acausal analysis of concepts of mental states and processes. See Medlin1967, and 1969 (including endnote 1).

Mention should particularly be made here of two of Armstrong's otherbooks, one on perception (1961), and one on bodily sensations, (1962).Armstrong thought of perception ascoming to believe by means ofthe senses (compare also Pitcher 1971). This combines theadvantages of Direct Realism with hospitality towards the scientificcausal story which had been thought to have supported the earlierrepresentative theory of perception. Armstrong regarded bodilysensations as perceptions of states of our body. Of course the lattermay be mixed up with emotional states, as an itch may include apropensity to scratch, and contrariwise in exceptional circumstancespain may be felt without distress. However, Armstrong sees the centralnotion here as that of perception. This suggests a terminologicalproblem. Smart had talked of visual sensations. These were notperceptions but something which occurred in perception. So inthis sense of ‘sensation’ there should be bodilysensation sensations. The ambiguity could perhaps be resolved by usingthe word ‘sensing’ in the context of ‘visual’,‘auditory’, ‘tactile’ and ‘bodily’,so that bodily sensations would be perceivings which involvedintrospectible ‘sensings’. These bodily sensations areperceptions and there can be misperceptions as when a person with hisfoot amputated can think that he has a pain in the foot. He has asensing ‘having a pain in the foot’ but the world does notcontain a pain in the foot, just as it does not contain sense data orimages but does contain havings of sense data and of images.

Armstrong's central state materialism involved identifying beliefsand desires with states of the brain (1968a). Smart came to agree withthis. On the other hand Place resisted the proposal to extend theidentity theory to dispositional states such as beliefs and desires. Hestressed that we do not have privileged access to our beliefs anddesires. Like Ryle he thought of beliefs and desires as to beelucidated by means of hypothetical statements about behaviour and gavethe analogy of the horsepower of a car (Place 1967). However he heldthat the dispute here is not so much about the neural basis of mentalstates as about the nature of dispositions. His views on dispositionsare argued at length in his debate with Armstrong and Martin(Armstrong, Martin and Place, T. Crane (ed.) 1996). Perhaps we can berelaxed about whether mental states such as beliefs and desires aredispositions or are topic neutrally described neurophysiological statesand return to what seems to be the more difficult issue ofconsciousness. Causal identity theories are closely related toFunctionalism, to be discussed in the next section. Smart had been waryof the notion of causality in metaphysics believing that it had noplace in theoretical physics. However even so he should have admittedit in folk psychology and also in scientific psychology and biologygenerally, in which physics and chemistry are applied to explaingeneralisations rather than strict laws. If folk psychology uses thenotion of causality, it is no matter if it is what Quine has calledsecond grade discourse, involving the very contextual notions ofmodality.

5. Functionalism and Identity Theory

It has commonly been thought that the identity theory has beensuperseded by a theory called ‘functionalism’. It could beargued that functionalists greatly exaggerate their difference fromidentity theorists. Indeed some philosophers, such as Lewis (1972 and1994) and Jackson, Pargetter and Prior (1982), have seen functionalismas a route towards an identity theory.

Like Lewis and Armstrong, functionalists define mental states andprocesses in terms of their causal relations to behaviour but stopshort of identifying them with their neural realisations. Of course theterm ‘functionalism’ has been used vaguely and in differentways, and it could be argued that even the theories of Place, Smart andArmstrong were at bottom functionalist. The word‘functionalist’ has affinities with that of‘function’ in mathematics and also with that of‘function’ in biology. In mathematics a function is a setof ordered n-tuples. Similarly if mental processes are defined directlyor indirectly by sets of stimulus-response pairs the definitions couldbe seen as ‘functional’ in the mathematical sense. Howeverthere is probably a closer connection with the term as it is used inbiology, as one might define ‘eye’ by its function eventhough a fly's eye and a dog's eye are anatomically and physiologicallyvery different. Functionalism identifies mental states and processes bymeans of their causal roles, and as noted above in connection withLewis, we know that the functional roles are possessed by neural statesand processes. (There are teleological and homuncular forms offunctionalism, which I do not consider here.) Nevertheless aninteractionist dualist such as the eminent neurophysiologist Sir JohnEccles would (implausibly for most of us) deny that all functionalroles are so possessed. One might think of folk psychology, and indeedmuch of cognitive science too, as analogous to a ‘blockdiagram’ in electronics. A box in the diagram might be labelled(say) ‘intermediate frequency amplifier’ while remaining)neutral as to the exact circuit and whether the amplification iscarried out by a thermionic valve or by a transistor. Using terminologyof F. Jackson and P. Pettit (1988, pp. 381–400) the ‘rolestate’ would be given by ‘amplifier’, the‘realiser state’ would be given by ‘thermionicvalve’, say. So we can think of functionalism as a ‘blackbox’ theory. This line of thought will be pursued in the nextsection.

Thinking very much in causal terms about beliefs and desires fits invery well not only with folk psychology but also with Humean ideasabout the motives of action. Though this point of view has beencriticised by some philosophers it does seem to be right, as can beseen if we consider a possible robot aeroplane designed to find its wayfrom Melbourne to Sydney. The designer would have to include anelectronic version of something like a map of south-eastern Australia.This would provide the ‘belief’ side. One would also haveto program in an electronic equivalent of ‘go to Sydney’.This program would provide the ‘desire’ side. If wind andweather pushed the aeroplane off course then negative feedback wouldpush the aeroplane back on to the right course for Sydney. Theexistence of purposive mechanisms has at last (I hope) shown tophilosophers that there is nothing mysterious about teleology. Nor arethere any great semantic problems over intentionality (with a‘t’). Consider the sentence ‘Joe desires aunicorn’. This is not like ‘Joe kicks a football’.For Joe to kick a football there must be a football to be kicked, butthere are no unicorns. However we can say ‘Joe desires-true ofhimself "possesses a unicorn" ’. Or more generally ‘Joebelieves-true S’ or ‘Joe desires-true S’ where S isan appropriate sentence (Quine 1960, pp. 206–16). Of course if one doesnot want to relativise to a language one needs to insert ‘or somesamesayer of S’ or use the word ‘proposition’, andthis involves the notion of proposition or intertranslatability. Evenif one does not accept Quine's notion of indeterminacy of translation,there is still fuzziness in the notions of ‘belief’ and‘desire’ arising from the fuzziness of‘analyticity’ and ‘synonymy’. The identitytheorist could say that on any occasion this fuzziness is matched bythe fuzziness of the brain state that constitutes the belief or desire.Just how many interconnections are involved in a belief or desire? On aholistic account such as Lewis's one need not suppose thatindividuation of beliefs and desires is precise, even though goodenough for folk psychology and Humean metaethics. Thus the way in whichthe brain represents the world might not be like a language. Therepresentation might be like a map. A map relates every feature on itto every other feature. Nevertheless maps contain a finite amount ofinformation. They have not infinitely many parts, still less continuummany. We can think of beliefs as expressing the different bits ofinformation that could be extracted from the map. Thinking in this waybeliefs would correspond near enough to the individualist beliefscharacteristic of folk and Humean psychology.

6. Type and Token Identity Theories

The notion ‘type’ and ‘token’ here comes byanalogy from ‘type’ and ‘token’ as applied towords. A telegram ‘love and love and love’ contains onlytwo type words but in another sense, as the telegraph clerk wouldinsist, it contains five words (‘token words’). Similarly aparticular pain (more exactly a having a pain) according to the tokenidentity theory is identical to a particular brain process. Afunctionalist could agree to this. Functionalism came to be seen as animprovement on the identity theory, and as inconsistent with it,because of the correct assertion that a functional state can berealised by quite different brain states: thus a functional state mightbe realised by a silicon based brain as well as by a carbon basedbrain, and leaving robotics or science fiction aside, my feeling oftoothache could be realised by a different neural process from whatrealises your toothache.

As far as this goes a functionalist can at any rate accept tokenidentities. Functionalists commonly deny type identities. HoweverJackson, Pargetter and Prior (1982) and Braddon-Mitchell and Jackson(1996) argue that this is an over-reaction on the part of thefunctionalist. (Indeed they see functionalism as a route to theidentity theory.) The functionalist may define mental states as havingsome state or other (e.g., carbon based or silicon based) whichaccounts for the functional properties. The functionalist second orderstate is a state of having some first order state or other which causesor is caused by the behaviour to which the functionalist alludes. Inthis way we have a second order type theory. Compare brittleness. Thebrittleness of glass and the brittleness of biscuits are both the stateof having some property which explains their breaking, though the firstorder physical property may be different in the two cases. This way oflooking at the matter is perhaps more plausible in relation to mentalstates such as beliefs and desires than it is to immediately reportedexperiences. When I report a toothache I do seem to be concerned withfirst order properties, even though topic neutral ones.

If we continue to concern ourselves with first order properties, wecould say that the type-token distinction is not an all or nothingaffair. We could say that human experiences are brain processes of onelot of sorts and Alpha Centaurian experiences are brain processes ofanother lot of sorts. We could indeed propose much finerclassifications without going to the limit of mere tokenidentities.

How restricted should be the restriction of a restricted typetheory? How many hairs must a bald man have no more of? An identitytheorist would expect his toothache today to be very similar to histoothache yesterday. He would expect his toothache to be quite similarto his wife's toothache. He would expect his toothache to be somewhatsimilar to his cat's toothache. He would not be confident aboutsimilarity to an extra-terrestrial's pain. Even here, however, he mightexpect some similarities of wave form or the like.

Even in the case of the similarity of my pain now to my pain tenminutes ago, there will be unimportant dissimilarities, and alsobetween my pain and your pain. Compare topiary, making use of ananalogy exploited by Quine in a different connection. In Englishcountry gardens the tops of box hedges are often cut in various shapes,for example peacock shapes. One might make generalizations aboutpeacock shapes on box hedges, and one might say that all the imitationpeacocks on a particular hedge have the same shape. However if weapproach the two imitation peacocks and peer into them to note theprecise shapes of the twigs that make them up we will find differences.Whether we say that two things are similar or not is a matter ofabstractness of description. If we were to go to the limit ofconcreteness the types would shrink to single membered types, but therewould still be no ontological difference between identity theory andfunctionalism.

An interesting form of token identity theory is the anomalous monismof Davidson 1980. Davidson argues that causal relations occur under theneural descriptions but not under the descriptions of psychologicallanguage. The latter descriptions use intentional predicates, butbecause of indeterminacy of translation and of interpretation, thesepredicates do not occur in law statements. It follows that mind-brainidentities can occur only on the level of individual (token) events. Itwould be beyond the scope of the present essay to consider Davidson'singenious approach, since it differs importantly from the more usualforms of identity theory.

7. Consciousness

Place answered the question ‘Is Consciousness a BrainProcess?’ in the affirmative. But what sort of brain process? Itis natural to feel that there is something ineffable about which nomere neurophysiological process (with only physical intrinsicproperties) could have. There is a challenge to the identity theoristto dispel this feeling.

Suppose that I am riding my bicycle from my home to the university.Suddenly I realise that I have crossed a bridge over a creek, gonealong a twisty path for half a mile, avoided oncoming traffic, and soon, and yet have no memories of all this. In one sense I was conscious:I was perceiving, getting information about my position and speed, thestate of the bicycle track and the road, the positions and speeds ofapproaching cars, the width of the familiar narrow bridge. But inanother sense I was not conscious: I was on ‘automaticpilot’. So let me use the word ‘awareness’ for thisautomatic or subconscious sort of consciousness. Perhaps I am not onehundred percent on automatic pilot. For one thing I might be absentminded and thinking about philosophy. Still, this would not be relevantto my bicycle riding. One might indeed wonder whether one is ever onehundred percent on automatic pilot, and perhaps one hopes that oneisn't, especially in Armstrong's example of the long distance truckdriver (Armstrong 1962). Still it probably does happen, and if it doesthe driver is conscious only in the sense that he or she is alert tothe route, of oncoming traffic etc., i.e. is perceiving in the sense of‘coming to believe by means of the senses’. The driver getsthe beliefs but is not aware of doing so. There is no suggestion ofineffability in this sense of ‘consciousness’, for which Ishall reserve the term ‘awareness’.

For the full consciousness, the one that puzzles us and suggestsineffability, we need the sense elucidated by Armstrong in a debatewith Norman Malcolm (Armstrong and Malcolm 1962, p. 110). Somewhatsimilar views have been expressed by other philosophers, such as Savage(1976), Dennett (1991), Lycan (1996), Rosenthal (1996). A recentpresentation of it is in Smart (2004). In the debate with NormanMalcolm, Armstrong compared consciousness with proprioception. A caseof proprioception occurs when with our eyes shut and without touch weare immediately aware of the angle at which one of our elbows is bent.That is, proprioception is a special sense, different from that ofbodily sensation, in which we become aware of parts of our body. Nowthe brain is part of our body and so perhaps immediate awareness of aprocess in, or a state of, our brain may here for present purposes becalled ‘proprioception’. Thus the proprioception eventhough the neuroanatomy is different. Thus the proprioception whichconstitutes consciousness, as distinguished from mere awareness, is ahigher order awareness, a perception of one part of (or configurationin) our brain by the brain itself. Some may sense circularity here. Ifso let them suppose that the proprioception occurs in an in practicenegligible time after the process propriocepted. Then perhaps there canbe proprioceptions of proprioceptions, proprioceptions ofproprioceptions of proprioceptions, and so on up, though in fact thesequence will probably not go up more than two or three steps. The lastproprioception in the sequence will not be propriocepted, and this mayhelp to explain our sense of the ineffability of consciousness. CompareGilbert Ryle inThe Concept of Mind on the systematicelusiveness of ‘I’ (Ryle 1949, pp. 195–8).

Place has argued that the function of the ‘automaticpilot’, to which he refers as ‘the zombie within’, isto alert consciousness to inputs which it identifies as problematic,while it ignores non-problematic inputs or re-routes them to outputwithout the need for conscious awareness. For this view ofconsciousness see Place (1999).

8. Later Objections to the Identity Theory

Mention should here be made of influential criticisms of the identitytheory by Saul Kripke and David Chalmers respectively. It will not bepossible to discuss them in great detail, partly because of the factthat Kripke's remarks rely on views about modality, possible worldssemantics, and essentialism which some philosophers would want tocontest, and because Chalmers' long and rich book would deserve alengthy answer. Kripke (1980) calls an expression a rigid designator ifit refers to the same object in every possible world. Or in counterparttheory it would have an exactly similar counterpart in every possibleworld. It seems to me that what we count as counterparts is highlycontextual. Take the example ‘water is H₂O’. Inanother world, or in a twin earth in our world as Putnam imagines(1975), the stuff found in rivers, lakes, the sea would not beH₂O but XYZ and so would not be water. This is certainlygiving preference to real chemistry over folk chemistry, and so far Iapplaud this. There are therefore contexts in which we say that on twinearth or the envisaged possible world the stuff found in rivers wouldnot be water. Nevertheless there are contexts in which we couldenvisage a possible world (write a science fiction novel) in whichbeing found in rivers and lakes and the sea, assuaging thirst andsustaining life was more important than the chemical composition and soXYZ would be the counterpart of H₂O.

Kripke considers the identity ‘heat = molecular motion’,and holds that this is true in every possible world and so is anecessary truth. Actually the proposition is not quite true, for whatabout radiant heat? What about heat as defined in classicalthermodynamics which is ‘topic neutral’ compared withstatistical thermodynamics? Still, suppose that heat has an essence andthat it is molecular motion, or at least is in the context envisaged.Kripke says (1980, p. 151) that when we think that molecular motionmight exist in the absence of heat we are confusing this with thinkingthat the molecular motion might have existed without beingfelt as heat. He asks whether it is analogously possible thatif pain is a certain sort of brain process that it has existed withoutbeingfelt as pain. He suggests that the answer is‘No’. An identity theorist who accepted the account ofconsciousness as a higher order perception could answer‘Yes’. We might be aware of a damaged tooth and also ofbeing in an agitation condition (to use Ryle's term for emotionalstates) without being aware of our awareness. An identity theorist suchas Smart would prefer talk of ‘having a pain’ rather thanof ‘pain’: pain is not part of the furniture of the worldany more than a sense datum or the average plumber is. Kripke concludes(p. 152) that the

apparent contingency of the connection between the mentalstate and the corresponding brain state thus cannot be explained bysome sort of qualitative analogue as in the case of heat.

Smart would say that there is a sense in which the connection ofsensations (sensings) and brain processes is only half contingent. Acomplete description of the brain state or process (including causesand effects of it) would imply the report of inner experience, but thelatter, being topic neutral and so very abstract would not imply theneurological description.

Chalmers (1996) in the course of his exhaustive study ofconsciousness developed a theory of non-physical qualia which to someextent avoids the worry about nomological danglers. The worry expressedby Smart (1959) is that if there were non-physical qualia there would,most implausibly, have to be laws relating neurophysiological processesto apparently simple properties, and the correlation laws would have tobe fundamental, mere danglers from the nomological net (as Feigl calledit) of science. Chalmers counters this by supposing that the qualia arenot simple but unknown to us, are made up of simple proto-qualia, andthat the fundamental laws relating these to physical entities relatethem tofundamental physical entities. His view comes to arather interesting panpsychism. On the other hand if the topic neutralaccount is correct, then qualia are no more than points in amultidimensional similarity space, and the overwhelming plausibilitywill fall on the side of the identity theorist.

On Chalmers' view how are we aware of non-physical qualia? It hasbeen suggested above that this inner awareness is proprioception of thebrain by the brain. But what sort of story is possible in the case ofawareness of a quale? Chalmers could have some sort of answer to thisby means of his principle of coherence according to which the causalneurological story parallels the story of succession of qualia. It isnot clear however that this would make us aware of the qualia. Thequalia do not seem to be needed in the physiological story of how anantelope avoids a tiger.

People often think that even if a robot could scan its ownperceptual processes this would not mean that the robot was conscious.This appeals to our intuitions, but perhaps we could reverse theargument and say that because the robot can be aware of its awarenessthe robotis conscious. I have given reason above to distrustintuitions, but in any case Chalmers comes some of the way in that hetoys with the idea that a thermostat has a sort of proto-qualia. Thedispute between identity theorists (and physicalists generally) andChalmers comes down to our attitude to phenomenology. Certainly walkingin a forest, seeing the blue of the sky, the green of the trees, thered of the track, one may find it hard to believe that our qualia aremerely points in a multidimensional similarity space. But perhaps thatis whatit is like (to use a phrase that can be distrusted) tobe aware of a point in a multidimensional similarity space. One mayalso, as Place would suggest, be subject to ‘the phenomenologicalfallacy’. At the end of his book Chalmers makes some speculationsabout the interpretation of quantum mechanics. If they succeed thenperhaps we could envisage Chalmers' theory as integrated into physicsand him as a physicalist after all. However it could be doubted whetherwe need to go down to the quantum level to understand consciousness orwhether consciousness is relevant to quantum mechanics.

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Acknowledgments

I would like to express my thanks to David Armstrong, Frank Jackson andUllin Place for comments on an earlier draft of this article and DavidChalmers for careful editorial suggestions.