The identity theory
Thomas (Janice L.)
Source: Thomas (Janice L.) - Mind and Person in the Philosophy of Religion
Paper - Abstract

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  1. Essential reading – 53
  2. Further reading – 53
  3. Understanding the theory – 53
  4. An important qualification – 54
  5. Arguments in favour of the identity theory – 54
  6. Objections to the identity theory – 58
  7. Learning outcomes – 60
  8. Sample examination questions – 60
  9. Tips for answering sample questions – 60
Full Text
  1. Essential Reading
  2. Further reading
  3. Understanding the theory
    • The identity theory of mind is easy to understand by taking its title completely literally and seriously. The sort of identity being talked about here is strict, numerical identity1, such as the identity between Samuel Clemens and Mark Twain (the American author); Eric Blair and George Orwell (the British author); Clark Kent and Superman (the comic book character); Dr Jekyll and Mr Hyde (again a fictional character with two names) or, in the example made famous by the German logician, Gottlob Frege, between the morning star and the evening star (two names, in fact, for the planet Venus). These examples are all ones that turn up in the recommended reading.
    • Any pair of names which in fact name one and the same thing will provide us with cases of numerical or strict identity. There are not two stars: there is only one heavenly body seen sometimes in the morning and sometimes in the evening. Superman is never seen when Clark Kent is seen and recognised as himself; Clark Kent is never there to witness Superman performing a miraculous rescue. He could not be: Clark Kent is Superman. Superman and Clark Kent have strict numerical identity2 with each other. There is only one person although there are two distinct personalities or public images.
    • This is the meaning that the word ‘identity' has in the phrase ‘identity theory of mind'. The identity theory of mind says simply, ‘mind and brain are identical'; ‘mental states just are states of the brain and central nervous system'. If you are feeling angry or afraid or having a sensation of blue colour, you are in one or another or a further particular brain state. Your anger state is that brain state and nothing more. There is no immaterial/mental anger state causing your brain state. Your sensation of blue is not caused by the state of your brain brought about when light of a certain frequency enters your eye. Your sensation of blue colour just is that state your brain gets into when your eye is so irradiated.
  4. An important qualification
    • It was said above that the identity theory holds that the mind just is the brain and central nervous system and some writers claim just that in so many words (Patricia Smith Churchland, for example). But most thinkers accept that, since many brain states are not mental states, and since the mind is not a single entity in any straightforward sense, we should really regard the mind as an aggregation of mental states. The crucial identity would then be the identity between mental states and central nervous system states (and in particular, of course, brain states).
  5. Arguments in favour of the identity theory
    • There are four arguments commonly cited in support of the identity theory:
      … It is economical.
      … It improves on behaviourism.
      … Its analogy with accepted scientific reductions: water just is H2O.
      … It solves the mind/body interaction problem.
  6. The identity theory is economical
    • Ockham's razor is a principle, probably familiar from your study of Philosophy of Religion, which tells us that ‘we should not multiply entities beyond necessity'. That is, when we are evolving a theory to explain something (in this case the mental, inner lives of persons) we should, all other things being equal, try to get along with as few metaphysical categories as possible. We may be forced to posit both material and immaterial substances to explain all the phenomena adequately. If, however, we can explain everything about our psychological life, our sensations, emotions and beliefs, all in terms of events in, or states of, different parts of the brain and central nervous system we will be right not to subscribe to dualism. Parsimony (frugality) is not a proof, of course, but it is usually thought a virtue where explanatory theories are concerned.
    • It is probably worth stressing that the identity theory is not denying the existence of mental states. It is saying that mental states are real and existent but they are not immaterial. They are not extra, non-physical items over and above the physical parts and states of the thinker's body.
    • The economy of the identity theory — its positing of material substances alone — is, however, a virtue which could be claimed by any physicalist/materialist theory. The identity theory is only one version of materialism. What arguments do its defenders believe are special to it?
  7. It improves on behaviourism
    • As pointed out at the end of the previous chapter, if the identity theory can preserve the strengths of behaviourism while avoiding its faults then this certainly would speak well in its favour.
    • The earliest exponents of the identity theory were U.T. Place, J.J.C. Smart, Herbert Feigl, David Lewis and David Armstrong. Since most of these philosophers either were Australian or were writing in Australia in the 1950s and 1960s, the identity theory was sometimes called by its detractors ‘the Australian Heresy'. Its friends and practitioners called the theory ‘Central State Materialism' to mark its principal point of departure from Rylean behaviourism. You should try to read at least Place's classic article, "Place (U.T.) - Is Consciousness a Brain Process?", and one of the articles by Armstrong.
  8. Real inner states and dispositions
    • Behaviourism avoided positing mysterious, non-physical, unobservable inner states and emphasised instead the connection between our vocabulary for attributing mental states and the publicly observable behaviour characteristic of people said to be in pain or angry or perceiving certain sorts of things.
    • Exponents of the identity theory saw this behaviourist move as a case of ‘throwing the baby out with the bath water'. If a person is inclined or disposed to behave in certain characteristic ways, there must surely be something about that person, something in his inner constitution or states, which accounts for the characteristic behaviour he displays. Inner states are anathema when they are mysterious, magical and, in principle, unobservable. Specific inner physical states (some of the states of the brain and central nervous system) are a part of nature, under the sway of natural law and thus certainly preferable in point of concreteness and scientific comprehensibility to mysterious, ungrounded dispositions of the whole organism such as those appealed to by behaviourism.
    • Compare the dispositional property of brittleness in glass. We could just say that glass is inclined to fracture when lightly struck. But no scientist would be content to leave the explanation of the glass's brittleness at that. The brittleness of glass is grounded in its internal molecular structure. So, too, with the pain you feel when you burn your finger. Of course you are inclined to cry out and to try to cool the afflicted part, but the inclination towards this behaviour is rightly viewed as grounded in your inner physical state — whatever state of your brain was effected by the burn and in turn can command (if appropriate) the physical responses described. If you are, right now, apt to behave in a certain way, some actual present state of you makes you apt to behave that way — whether you do so or not in the event.
    • So the first of the major objections to behaviourism (listed in the previous chapter) is avoided by the identity theory.
  9. The inverted spectrum hypothesis
    • What of the second objection? This was the objection that there seems no logical barrier to two people's having different psychological states while being exactly similar in behavioural dispositions. Yet, if behaviourism is correct and behavioural dispositions are all there is to mental states, then this situation should be impossible.
    • The suggestion that it is possible that two people might differ systematically in the individual, private felt quality (sometimes called ‘phenomenological feel') of their sensations or inner states, although they are inclined to do and say exactly the same things (their behavioural dispositions are identical) is sometimes referred to as the inverted spectrum hypothesis.
    • There is serious debate among philosophers of mind about whether the inverted spectrum hypothesis expresses a genuine possibility or not. If it does, it clearly makes trouble for the behaviourist. Unfortunately, the same sort of trouble afflicts one version of the identity theory if the inverted spectrum is a real possibility. I will return to this point after I introduce the complication of distinct, significantly different identity theories (in fact, not until Chapter 7 below).
  10. The impossibility of the behaviourist programme of psychology
    • The third objection to behaviourism was that it seems to require something impossible. To specify the behavioural dispositions which might, in one person or another, constitute a particular sensation or belief (and to do so without getting into a circle by reinvoking mental states) seems potentially an endless task.
    • My believing it is going to rain (to take a favourite example from, among others, Smith and Jones) translates into ‘being inclined to bring in the washing' if I believe that is the way to keep the washing from getting wetter and if I also want the washing to get dry. Without reference to these other beliefs and desires, my behavioural intentions and dispositions do not come close to constituting a belief in impending rain.
    • The identity theory was certainly offered by its first exponents as affording a resolution of this predicament. On the identity theory a person's being in a particular psychological state just is that person's being in a particular neurophysiological state. Which mental state that neurophysiological state is depends in turn to some extent on what behaviour typically flows from that brain/central nervous system state. But the word ‘typically' is an important qualification. The cause of the neurophysiological state is also an important determinant of which sort of psychological state that brain state is identical with. If it is caused by injury then that brain state is pain; if it is caused by deprivation of liquid then it is thirst, and so on. Whatever behaviour issues (or does not) from the psychological state, you are really in it if and only if your brain is in the right state.
  11. The analogy with accepted scientific reductions: water just is H2O
    • Reading:
      "Churchland (Paul) - Matter & Consciousness", 1984, pp. 26-35.
    • One of the most powerfully persuasive considerations advanced by the defenders of the identity theory of mind is the analogy with other ‘nothing but' reductions or equivalences in physics and chemistry. Here is a summary of the examples Churchland offers:
      … sound is nothing but a train of compression waves travelling in the air
      … light is nothing but electromagnetic waves
      … colour is a triplet of reflectance efficiencies
      … warmth is nothing but a high average molecular kinetic energy
      … lightning is a sudden large scale discharge of electrons between clouds.
    • A frequently encountered example is ‘water is (nothing but) H2O': in just the same way, it is said, mental states are ‘nothing but' neurophysiological states.
    • These examples are intended to show the scientific respectability of the identity theory of mental states. It is just another in a long line of fruitful, ‘X is nothing but Y', scientific explanations.
    • We have introspection to tell us when we are experiencing this or that psychological state. And we have neuroscience to tell us what events in our brain and central nervous system correlate with those mental or psychological events. Why not identify, for example, a given pain reported by the subject with the neural event which is its correlate? Why not say, ‘the pain is nothing but the neural event'?
    • Churchland's examples are also intended to counter a possible objection to the proposed theory which could be summarised as follows.
    • Your mental states have very different properties from those which your neurophysiological states possess. In particular, your own mental states can be known by you immediately and directly without recourse to any observations or scientific instruments (you cannot really avoid knowing all about them!) On the other hand, your neurophysiological states take a lot of tricky scientific work to discover.
    • In contrast, your neurophysiological (i.e. physical) states and behaviour are there for anyone with the scientific expertise to observe or investigate whereas your mental states are private - you alone know what they are and what they feel like.
    • So someone might say, ‘surely two such different things (mental states and physiological states) could not be identical'. (Remember Leibniz' Law3 in the version employed in this guide: ‘if X has a property Y lacks or vice versa, X is not identical with Y'.)
    • The identity theorist will reply (as Churchland does, on page 33, about the example ‘temperature is knowable by feeling' etc.) that where science tells us some commonly observable phenomenon (like temperature or warmth) is in fact nothing but some precisely measurable or describable happening or structure (like mean molecular kinetic energy), we should realise that what is involved is a contingent fact, an equivalence or identity which science had to discover and which could have been different if the physical universe had had a different history of development and change.
    • But the surprisingness of the identity should not be a reason for doubting that it is an identity. It is worth remembering that many objects have properties that differ radically from each other. A collection of stones, wood and glass can constitute, be identical with, a cathedral - even though the materials have, say, each a particular melting point (a property which no cathedral has) and the cathedral itself might have a property (like that of being a beautiful building) which is not possessed by any of its material constituents.
    • Sound may be identical with a train of compression waves travelling in the air but the identity between the two does not prevent one of them (at least striking an observer as) having features the other lacks. The sound (but not the wave train) may be beautiful while the travelling waves (but not the sound heard) may be fast or slow4 relative to some other wave train.
  12. Interaction problem solved
    • Finally, as has been assumed throughout this section (but you would need to make the point explicitly in an exam answer), the great virtue of the identity theory is that it resolves the mind/body interaction problem which for so many people forms the fatal objection to dualism. As does behaviourism, the identity theory resolves the interaction problem by denying that mind and body are two things which could interact. Interaction requires, at the least, two things to have an effect on one another. If mental states just are brain states there is no problem about how a mental state and its corresponding brain state get any purchase on one another. When a mental state has a physical effect, that is because it is itself a physical state with the unmysterious power to produce physical effects.
  13. Parochialism/chauvinism
    • The first objection that the defenders of the identity theory had to combat was the protest that the theory as first expounded was chauvinist and parochial. If pain, for example, is to be defined as ‘the firing of a certain sort of fibre in a particular region of the afflicted person's brain', then what are we to say about the family dog or cat? Neither has a human brain, but pain was said to be nothing but a certain sort of human neurological event or state. We seem to have made pain something only humans can experience and this seems a view that needs a defence (rather than one to arrive at just by accident, so to speak).
    • Several responses to this objection are possible: perhaps there are different kinds of species-specific pain — human-pain, dog-pain, cat-pain, etc. But this response raises the difficult question, ‘What is it that makes them all types of pain?' The identity theorist began by telling us that what was essential to or definitive of pain was that it is identical with (it just is) a certain type of human brain state. But if, as common sense seems to tell us, dogs, cats and other nonhuman animals can feel pain too, then there is something more to being a pain than just being a brain state of a particular human kind or type. And that ‘something more' is what makes this human state and that dog state and that cat state all pain.
    • And then too, what about beings that have no brain? We may feel that it is reasonable to deny the capacity for pain to animals that have no brain or only an extremely rudimentary one (fish? insects?). And what about beings like sophisticated computers which undeniably exercise what (when humans exercise them) we would describe as mental or intellectual powers? They are not alive and possess no biological organs (therefore they literally have no brain of any kind) — shall we decide that no such entity could ever feel pain?
    • And what of creatures from other planets? Can we not imagine a creature with a wholly different biology, perhaps silicon5 based, which nonetheless (despite lacking anything like a brain) behaved in ways strongly suggestive of being in pain when it was injured?
    • Finally, what are we to say about disembodied6 persons, angels, God? If such beings exist and have minds, surely it makes no sense to say that mental states just are states of their brains: none has a brain.
    • The charge of chauvinism is very powerful, but most supporters of the identity theory were not persuaded by it to abandon their theory altogether. Instead, they adopted an amended theory which seemed to avoid the objection while retaining the virtues which identity theory claimed — of treating the mind as a part of nature suitable for natural science to investigate, and treating mental states as nothing but physical things. The name of this amended identity theory is ‘functionalism'. (Some think that the identity theory is just an off-shoot of functionalism — see "Braddon-Mitchell (David) & Jackson (Frank) - Philosophy of Mind and Cognition". Others find it very difficult to distinguish functionalism from identity theory, let alone decide which might form a subspecies of the other.)
    • Functionalism will be properly introduced and treated in detail in the next chapter. For the present, just think about the family-dog-in-pain case once again. I know nothing about canine neuroanatomy but if I were looking for the internal/brain state to identify with pain in Fido's case, I would have to compare his brain and a human's (say, my own) under different conditions and during different behaviours. I would try (perhaps by attaching electrodes to us both) to discover which bits of his brain and mine seemed activated by some injury (say a pinprick administered to each of us) and in turn coincided with my saying ‘ouch' and pulling away and the dog's yelping and pulling away. I would be inclined to call the type of activation of each brain which I had discovered in those circumstances ‘the pain brain state'. But this is because I would know that I had felt the pain just then and I would guess from his yelping/pulling away behaviour that the dog too felt his pain at that time.
    • In the example, I am identifying each brain state (the dog's and my own) as pain: each brain state occupies the right causal role in its possessor. Each seems to have been caused by injury and in turn to trigger or cause the outcry and withdrawal. Since each brain state performs the same causal role or function in its respective possessor it is given the name of the same mental state type.
  14. Type and token
    • There is, however, a more difficult objection to identity theory (or at least one kind of identity theory) which involves making a distinction between type and token. For any kind of thing there is both the kind to which it belongs (its type) and the individual instances which fall under that type (the type's tokens). So, for instance, the word ‘the' which appears many times on this page is both a type (which can have repeated instances) and a token (the one surrounded by quotation marks is itself a particular instance of the type). If you buy two books and write your name in the flyleaf of first one and then the other there will be one name-type and two tokens of it (one in each book).
    • Now think about the supposed identity between mental states and brain states. Are the things being identified types or tokens? When I prick my finger with a pin is the brain event caused by this injury and said to be identical with my pain a type or a token?
    • Notice first that the particular pain I feel and the particular brain state I am in are at least tokens. So if they are identical (if the identity theorist is right about mental state-brain state identity), then there is just one token which is both a brain state and a psychological state.
    • But earlier, when I introduced the identity theory in the terms used by Place, Smart and others, it was type-identity that seemed to be at issue. What was claimed was that certain sorts of identity would turn out to hold between types of brain activation and types of psychological event. The hope was that, for example, pain would turn out to be one kind of neural event (the firing of certain sorts of fibres or the activation of a specific region of the brain) and hope or fear would turn out to be identical with different kinds of neuronal events and locations.
    • Priest says roundly (113) that neurologists have found it ‘simply empirically false that the same sort of mental event has to be correlated always with the same sort of brain event'. Neither you and I, nor you yesterday and you today, need be in the same brain state in order to be in the same mental state. Or so neuroscience seems to have discovered.
    • The objection, then, is an objection to type-identity theories. This objection says that the parallel between ‘water just is H2O' and ‘pain just is a certain sort of brain state' breaks down and is known to do so on empirical grounds. A pang of fear might be identical with one type of brain state in you and identical with a quite different type of brain state in me. A pinprick pain might be identical with one sort of brain state at one point in your mental history and another pinprick pain you suffer might be constituted by a token of a quite different type of brain state on another occasion.
    • Of course, this way of putting the objection criticises the type-identity theory while at the same time affirming what has been called token-identity. The critic emphatically does not believe that any mental states are immaterial or non-physical. Each (token) mental state is numerically identical with a particular token physical state. But it is claimed that specific types of mental state are not to be identified with specific types of brain state because empirical brain research tells us that the correlations of psychological and physical types that would support a type-identity theory simply are not to be found.
  15. Learning outcomes: After reading through this chapter and the reading selections you should be able to:
    • Explain what kind of identity the identity theory is talking about
      … strict, numerical
      … contingent.
    • Explain how brain state identity is defended
      … on grounds of economy
      … as an improvement on behaviourism (by recognising real inner states and giving an account of them that isn't circular.)
      … by aligning itself with scientifically creditable inter-theoretic reductions (analogy with ‘Water is H2O')
      … as solving the mind/body interaction problem.
    • Outline these objections:
      … parochialism/chauvinism
      … empirical evidence against type-identity (explain type/token distinction).
    • Discuss the meaning of and objections to ‘contingent identity7'.
  16. Sample examination questions
    • Are types of mental states identical with types of brain states?
    • What arguments can be given to support the identity theory of mind and what serious criticisms have been addressed to it?
  17. Tips for answering sample questions
    • Remember, you are meant to evaluate, not just summarise. Always define key terms:
      … What is meant by ‘identity' and ‘identify'?
      … What is identified with what?
    • Since the identity theory and functionalism are so closely related, you should aim to study both topics if you are going to do either.


Part 1 (The Metaphysics of Mind and Body); Section 2 (Varieties of anti-dualism and materialism); Chapter 6. Hard Copy filed in "Various - Papers on Religion Boxes (Heythrop)".

In-Page Footnotes

Footnote 4: See also Chapter 8 of this subject guide - beauty as a supervenient property.

Text Colour Conventions (see disclaimer)

  1. Blue: Text by me; © Theo Todman, 2020
  2. Mauve: Text by correspondent(s) or other author(s); © the author(s)

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