Identifying the Identity Theory

Cross-Posted @ Philosophy Sucks!

While I was perusing the new entries over at PhilPapers yesterday I came across Tom Polger‘s forthcoming paper inPhilosophical Psychology Are Sensations Still Brain Processes? The paper is very interesting (disclaimer: I have a special interest in this stuff; see for instance The Identity Theory in 2-D) and I thought I would summarize its main points and then say something about where we disagree towards the end. 

The first part of the paper Polger identifies eight theses that Smart defended in his celebrated paper. These are,

1. Sensation reports are genuine reports

2. Sensation reports do not refer to anything irreducibly psychical

3. Sensations are “nothing over and above” brain processes

4. Sensations are identical to brain processes

5. The identity theory is a metaphysical theory, not a semantic proposal or an
empirical hypothesis

6. Metaphysical theories of the nature of the mind do not make competing
empirical predictions; so they should be evaluated by their theoretical virtues,
e.g., simplicity and parsimony

7. For any thing or kind x, there are “logically” necessary conditions for being a
thing of that kind.

8. Sensation expressions are topic-neutral.

The first claim is simply an endorsement of realism about phenomenal consciousness. Claims 2-4 spell out commitments to physicaism and the identity theory. Claims 5 and 6 spell out Smart’s distinctive views about the identity theory. Claim 7  basically asserts that we can know a priori what pains are essentially. Claim 8 amounts to the idea that concepts like ‘pain’ etc do not entail a commitment to any kind of ontology all by themselves.

Polger goes on to argue that of these every one but 7 should be accepted by contemporary identity theorists. Claim 6 should be accepted but not interpreted too narrowly. The identity theory should be accepted for broadly ‘inference to the best explanation’ reasons. Parsimony and simplicity play a role in that inference but there are other things that also play a role; As Polger says, “There are also what Jaegwon Kim has called explanatory and causal arguments for the identity theory”. The reason that 7 should be rejected according to Polger is the kind of resources that Kripkean arguments give to the identity theorist. In place of 7 above Polger suggests 7*

7*. A Posteriori. The identity of sensations and brain processes is a posteriori

7* is then an updated version of Smart’s claim that mind/brain identities are to be construed as ordinary scientific identities. We now have a post-Kripkean understanding of these kinds of identities and the contemporary identity theory should reflect that.

In the second part of the paper Polger goes on to formulate a master argument against the identity theory that he thinks subsumes all arguments against it and then responds to the various particular objections. The master argument goes as follows;

(P1) If the identity theory is true, then there is a necessary one-to-one relation between sensations and brain processes.15 (necessity of identity)

(P2) If VARIATION then there is not a necessary one-to-one relation between sensations and brain processes. (definition of VARIATION)


(P4) There is not a necessary one-to-one relation between sensations and brain processes. (P2, P3)

(C2) The identity theory is false. (P1, P4)

The particular objections that we find spell out varieties of variation claims: actual, nomological, metaphysical, logical. Polger identifies one major figure and style of objection this way. So, Putnam’s worries about octopi and Fred’s pain at 6:00 v.s. Fred’s pain at 6:15 count as actual variation while Fodor’s worries count as nomological, Kripke’s modal argument is metaphysical, and Chalmer’s zombie argument is logical. All of these arguments are united by trying to show that there is or can be variation.

Polger has a lot of interesting things to say in response to each of these objections.  Against actual variation he argues that even if we grant, as we might not, that we find the very same psychological properties across species on Earth (that is to say, even if an octopus can feel the very same kind of sensation that I do when I experience pain) there is still very little reason to think that psychological properties are multiply realizable in a way that is threatening to the identity theory. Sure there may be differences between species but that is no reason to rule out similarities a priori! Some people cite neural plasticity as a possible source of trouble. To this Polger replies, “evidence from plasticity is compatible with the neurobiological variations being variants within a more general kind that is also neurobiological.” Lacking any reason to believe in actual variation we also have no reason to believe in nomological variation, what about metaphysical variation? Here Polger endorses type-b physicalism and argues that Kripke’s argument is question begging. If the mind-brain identities are true then they are necessarily true. This leads Polger to the last kind of variation which he calls logical variation. It is here that we find Polger’s discussion of Chalmer’s zombie argument. His main complaint is that the argument rests on an assumption about the nature of reduction that the type-b physicalist will reject.

In the final section of the paper Polger introduces two further claims which he thinks should be endorsed by contemporary identity theorists.

9. Variability. Sensation processes are multiply constituted.

10. Strong Physicalism. Physicalism is necessarily true; all worlds are physicalist worlds.

In defense of accepting 9 Polger argues as follows,

accepting that there is…variability in the world is a far cry from accepting that it is the kind of variability that would be problematic for identity theories. Identity theories claim that sensations are brain processes, but they do not take any stand on the nature of brain processes. In particular, the identity theorist need not suppose that the world is organized into homogeneous columns of organization so that there is a one-to-one relation between sensations and microphysical processes. The identity theorist identifies sensations with brain processes, not with molecular or subatomic processes that occur inside brains.

I have always been sympathetic to this kind of argument and have seen some of my own work as generally supporting it. But what about 10? Why ought we accept that? The basic reason is to avoid the following reductio of the identity theory;

C1. Sensations are identical to brain processes in all possible worlds. (identity theory)

C2. Physicalism is contingent; there are some non-physicalist worlds containing non-physical sensations. (contingent physicalism)

C3. There are some worlds in which sensations are not identical to brain processes. (from C2)

C4. The identity theory is false.

Polger’s answer to this argument is to give up C2 thereby blocking C3. This may seem dramatic and I take 10, together with 7*, to entail that there are strong necessities in Dave Chalmers’ sense, “but”, says Polger, “so it goes. Just as there are necessary a posteriori truths, there are necessary a posteriori falsehoods.” 

But it is just at this point that the difference between the kind of identity theory that Polger has and one that is in 2-D. Once we start thinking in 2 dimensional semantics we can see an equivocation in the redictio. C1 should be modified as C1*

C1* The secondary intension of ‘Sensations are brain processes’ is necessary; the primary intension of ‘sensations are brain processes’ is contingent (identity theory in 2-D)

Once we do that we do not have the worry about the reductio. Adopting C1* is tantamount to a compromise between 7 and 7*. In effect we agree that there is an a priori knowable description or reference fixer and an a posteriori identified physical state. Given that we know independently that identities like this are 2-necessary in Dave Chalmers’ sense we can conclude that those identities are necessary in spite of possible worlds where the a priori knowable description picks out a non-physical property. 


  1. On zombies, you said:
    . It is here that we find Polger’s discussion of Chalmer’s zombie argument. His main complaint is that the argument rests on an assumption about the nature of reduction that the type-b physicalist will reject.

    This sounds interesting, but I don’t really follow. You seem to summarizing the following from the paper:
    In particular, the a posteriori identity theorist quite reasonably rejects the problematic notion of “logical” conditions and the positivist demand for a special kind of “reduction” of which there are no non-trivial examples and that has long since been rejected by philosophers of science. The burden is on the objector to show independently that the identity theorist, or any physicalist, should require such reductions. And no such positive argument has been given.

    I don’t really understand this, and its connection to Chalmers. First, doesn’t Chalmers eschew talk of reduction for talk of supervenience? Chalmers’ claim is that the logical possibility of zombies shows materialism is false. I don’t see the naive view of reduction in that argument, or anything about reduction at all. I’d like to, though: perhaps a philosopher in the house can explain what’s going on.

  2. Joshua Stern

    If the identity theory is true, then there is a necessary one-to-one relation between sensations and brain processes.


    My love of chocolate and your love of chocolate are never going to be identical because your brain is not my brain.

    Anyone positing this is just looking for an excuse to toss not just identity but reduction.

    Have a nice day.

  3. Joshua Stern

    There is no strict identity between being a Snickers bar and a particular set of peanuts.

    But I cannot see much of a “therefore” that follows in a way that discredits identity, Snickers, or peanuts.

    So, as far as it goes, Smart is right, and there is an identity (of sorts) between being a Snickers bar, and certain peanut processes.

    Of course, rather a lot rides on that “(of sorts)” business. But opponents of reduction theories and causal theories and such always love to toss the whole identity concept, without really offering anything in its place, and their suggested directions tend to be dispositional, irreductive, neoessentialist, etc. There is an alternative, but it takes one into a discussion of what it means to belong to natural kinds or categories … the dreaded discussion of universals versus nominalism.

  4. Joshua Stern

    Do you suggest that Polger has overlooked this, so that different tokens of sensations do not have to uniquely reduce to the identical token of brain state?

    That would work for me, but it means taking a strong position about identity, that it needs be looked at as tokens. Again, that works for me, but probably not for all.

  5. Richard Brown

    It looks like this issue has been resolved but just to be clear…the idea is that given the physical facts <em>in toto</em> we should be able to deduce the qualitative facts a priori, at least in principle. In this passage Polger is emphasizing the fact that Smart accepted this and this is the reason that he claims that the mind-brain identities are contingent. If we give up that idea, claims Polger, we no longer need to say that the identities are contingent just because the cannot be known a priori.

  6. Richard Brown


    I am not sure why you say this since Polger accepts it and he is an identity theorist…maybe, as the ensuing discussion suggests, you are confusing this for a statement of token identity? Polger, at least, is clear that this is a statement of type identity. So, for every sensation type S1, S2, …, Sn and every brain state type B1, B2, …, Bn there will be a necessary 1-1 correspondence such that, say, S1=B1, S2=B2, …, Sn=Bn and this entails that all tokens of, say, S1 also be tokens of B1, etc…

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