Edit: This post has been turned into a paper discussing Kripke’s argument against identity theory.
So the story goes, identity theorists claim that mental states are identical with brain states. Identity theorists usually espouse one of two versions of this claim. Either 1) types of mental states are identical with types of brain states, or 2) tokens of mental states are identical with tokens of brain states. In either case, the particular feeling I get when I stub my toe is identical with some particular thing going on in my brain, sometimes said to be C-fiber firings.
The concept of a rigid designator was developed by Saul Kripke in a series of lectures later called “Naming and Necessity.” A rigid designator is a designator that picks out the same thing in all possible worlds. Within the lectures Kripke applied the idea to the identity statements proposed by identity theorists. To be clear, Kripke meant for his argument to apply to claims made by both kinds of identity theorists. Token identity theorists feel their claims are unaffected by Kripke’s argument, but as I recount the argument we will see that both kinds are refuted.
The argument goes like this:
1) For any identity statement of the form R1 = R2 where R stands for a rigid designator, if the statement is true is will be necessarily true.
2) The statement, “The particular feeling I get when I stub my toe is identical with some particular thing going on in my brain” is an identity statement of that form. Note the use of “particular.”
3) The statement is contingent. We can imagine that brain state occurring without being accompanied by the particular feeling I get. We can also imagine the feeling without the brain state.
4) Therefore, by a simple use of Modus Tollens, the identity statement given in line 2 is not true.
The real force of the argument is found in line 1. R1 = R2 will have to be necessarily true, that is to say, true in all possible worlds, if it is true at all, because R1 and R2 designate the same thing in all possible worlds. If it is true in one world it will have to be true in all worlds. If this is at all difficult to understand, it will be because the logic is so tight, it seems like a tautology.
Naturally identity theorists will not simply give up. They usually attack the third premise, by saying we actually cannot imagine the sensation without the brain state. Some people mistakenly think they can imagine heat not being identical to molecular movement. But, that is just an illusion.
Token identity theorists sometimes try to dodge Kripke’s argument. John Searle attempts to show that they are not spared in Mind: A Brief Introdiction. On page 69, He writes, “If you grant me that there really are two features to this experience, the feeling of pain and the firing of C-fibers, then it will look like Kripke’s argument will go through.” Before Searle writes any more, his argument has already failed; he has assumed that which he is trying to prove. The original claim was that the feeling of pain was identical to some firing of C-fibers. They claimed that there are not “two features to this experience” but one, under two names. Searle should have been trying to show why Kripke’s argument still applies to token identity statements, instead he asked us to assume with him that the statement wasn’t one of identity.
Kripke himself was successful in maintaining his attack on token identity statements. When one thinks they can imagine heat without molecular movement they have only imagined something by which we identify heat (a contingent property of heat) existing without heat itself. They imagine the sensation of heat without any molecular movement. But in the case with a pain and a brain state, that by which we identify pain is pain. We do not pick out the pain by some contingent property. That pain has a certain sensation is not a contingent property. That certain sensation, or the property whereby we pick it out, is that it is a pain. Being a pain, or the sensation of pain, is an essential property of a pain.
Another way of identifying a pain is by its causal relations. Such is claimed by a subset of identity theorists, called functionalists. That is, they say a brain state is a pain because it was caused by certain stimuli and causes certain behavior. So when someone says they can imagine the usual cause of a pain without a pain, the functionalist cannot say that it is an illusion and that they are really just imagining a contingent property of the pain without the pain itself. By their own definition, a pain just is its causal relations. The causal relations are an essential property.
Kripke’s argument simply establishes that mental states are not identical to brain states. It still is possible that they be correlated, maybe even concomitant phenomena. Materialists do not like this because they want to explain the mind with only reference to physical facts. This argument validates the common sense intuition that pains and brain processes are just two different kinds of things.