Case Study: The Mind-Brain Identity Theory

The mind-brain identity theory was notably defended by J.J.C. Smart (1959, 1984). Smart’s chief argument for the theory appealed to Ockham’s Razor. More recently, Smart’s theory and his argument have both been defended by Christopher Hill (1991, 35–40). We will consider the theory itself, and then evaluate Smart’s argument from Ockham’s Razor.

The mind-brain identity theory is a physicalist theory. It claims that the mind is identical to the physical brain. More fully, the theory is that every mental property (or type) is identical to some physical property.

Having a pain, or thinking that Vienna is a capital city, or wanting a drink from the fridge are each mental properties, and different people, or the same person at different times, can have the same mental property.

The identity theory claims that a person’s having a certain mental prop-erty is identical to that person’s having a certain physical propprop-erty — a physical property of their central nervous system. (In fact, Smart never originally intended to claim that all mental properties are identical with physical properties, although that is the theory that many philosophers have attributed to him.) Although the theory that mental properties are identical to physical properties of the brain is a philosophical theory, the theory further claims that it is for science to discover just which physical properties are identical to which mental properties. The theory is pre-sented as a contingent truth about mental properties, as opposed to a necessary truth about them. It is also taken to be a theory that needs to be supported by empirical evidence. The identity theory is opposed to dualism, the theory that mental and physical properties are fundamental and distinct properties.

The argument from Ockham’s Razor for the identity theory runs as follows. There is inductive evidence for the claim that instances of each mental property are correlated one-to-one with instances of some physi-cal property. So, for instance, there is inductive evidence that any person has severe pain (a certain mental property) when and only when that

person has a certain physical property of the brain. On this much, dual-ists and physicaldual-ists can agree: instances of each mental property are correlated one-to-one with instances of some physical property. For the dualist this correlation will be a brute fact. But whereas the dualist posits both mental properties and physical properties as fundamental proper-ties, the identity theorist posits only physical properties as fundamental properties. Recall that one formulation of Ockham’s Razor says that if hypothesis H1 is simpler than hypothesis H2, then, other things being equal, H1 is a better hypothesis than H2, and we should accept H1 rather than H2. In particular, other things being equal, we should prefer the hypothesis that posits fewer coincidences — fewer brute facts — than its rival (Maudlin 2007, 179–80). Therefore, the identity theory is a better theory than dualism, and we should accept the former theory.

Schematically, the argument runs:

(P1) Correlation: for every mental property M, there is some physical property P such that instances of M are corre-lated 1-1 with instances of P.

(P2) Simplicity: other things being equal, taking M to be identical to P is simpler than taking M to be non-identical to P.

(C) \ Identity theory: So M is identical to P.

Premise (P1) has been questioned on the ground that creatures of dif-ferent species can have the same mental property although they differ in their brains’ physical properties.¹ Perhaps for humans to have pain is for C-fibres in their brains to fire, for dolphins to feel pain is for D-fibres in their brains to fire, and for octopi to feel pain is for their brains to have yet another physical property again. Mental properties, so the claim goes, are multiply realized by different physical properties.

In response to this, the identity theorist may claim that the identi-ties between mental and physical properidenti-ties are “restricted.” It is not the property of pain that is identical with C-fibres firing. It is the property of pain-in-humans that is identical with C-fibres firing. It is the property of pain-in-dolphins that is identical with D-fibres firing. And so on. In each case a (restricted) mental property is identified with some physical property (Jackson and Braddon-Mitchell 1996, 99–100).

Even if dualists grant that there is a one-to-one correlation between instances of every mental property and instances of some physical prop-erty, typically they will challenge the application of Ockham’s Razor to

1 See, for example, Mucciolo (1974).

this correlation. They will raise the issue of how a mental property could be identical with a physical property. They will claim that there is an

“explanatory gap” between whatever physical properties a creature has and whatever mental properties it has, and that this gap excludes the identification of mental properties with physical properties (Chalmers 1996, 169). Dualists have two ways of making the case for this explanatory gap. First, many mental properties have intentionality: they purport to represent how the world is. Although some physical things represent, such as signposts and words on a page, their representational powers are derived from our use of them. The representational powers of mental states, by contrast, are intrinsic. To think of Alberta, or to wish for a sunny day, is to represent certain objects or situations. These states do not have these representational powers because we use them in certain ways, much as we use a signpost or a diary. Dualists conclude that this differ-ence in kind between representational mental properties and physical properties rules out their being identical. Second, many mental proper-ties have an experiential “raw feel”: there is something it is like to have an itch, or to feel scared, or to have an orange after-image. These mental properties have distinctive experiential qualities (“qualia”). No physical property has such an experiential quality. A simple thought-experiment tells us as much: we can imagine a race of beings who are physically indistinguishable from us and who behave as we do, but who have no inner mental life and no experiences. In that sense, they are “zombies.”

(This thought experiment was outlined in chapter 3, §2.) Dualists con-clude that this difference in kind between experiential mental properties and physical properties rules out their being identical.

It may be that the identity theory can solve these and any other prob-lems facing it. We will not pursue the issue further here. (Rey [1997, chap-ter 2] discusses what he calls such “temptations to dualism.”) The key point is that until there is general agreement that the theory can solve these problems, it would be unwarranted to use Ockham’s Razor to argue for the identity theory. Smart would agree (1966, 381):

… if the view that experiences just are brain processes can be defended against a priori objections it should be preferred, as against dualism, as a more simple, elegant, and economical hypothesis.

The above response by dualists to Smart’s argument from Ockham’s Razor is representative of the response that many philosophers have when objections from Ockham’s Razor are made against their theories.

The response is to deny that other things are equal between the rival

hypotheses. Moreover, if the only reason for selecting hypothesis H1 as opposed to H2 is that H1 is simpler than H2, the defender of H1 would be conceding that in all other respects H2 is a good hypothesis. That would be a substantive concession, and perhaps one that the defender of H1

should review. For example, the identity theory of mind is a physicalist theory of mind. That theory then coheres better with a physicalist theory of (say) life than does dualism and that seems to be a further good feature of the identity theory.

(Q2) asked whether simplicity is a good feature of a philosophical or scientific hypothesis. The present section has not given a direct answer to that question, but two relevant points have emerged. First, faced with the charge that a hypothesis is not simple, a defender of the hypothesis can be expected to claim that other things are not equal and that the charge is unwarranted. Second, where a hypothesis is a simple one, we should expect it to have other good features not shared by its rivals.