Identity theory or, in full, mind-brain identity theory, is the philosophical thesis that the mind can be identified with the brain. This is often stated in the form “mental states are identical to brain states.”
I find the thesis puzzling. Perhaps that’s partly because it doesn’t make sense to me. That’s because I have trouble making sense of “mental state” and of “brain state”. I find it far from clear what either of those means. So I am puzzled that people want to say that there is an identity between two confusing things.
Presumably, part of the motivation for identity theory, is to have an answer for proponents of dualism. But we surely don’t need identity theory for that. It should suffice to claim that activity normally attributed to the mind is the result of what is happening in the brain.
A music analogy
Here’s a somewhat analogous situation. We often discuss music in terms of the pitch (or note) being played, or the combination of notes in a chord. We might perhaps talk of that as the harmonic state of the music. If the music is being played on a piano, we could also talk of the physical state of the piano.
As far as I know, there is no call for a music identity theory, that harmonic states are identical to physical piano states (for piano music). Such a music identity theory would be false. There is a relation between physical states and harmonic states. The mathematics of Fourier analysis gives us that relation. But it is a far different relation from a correspondence between physical states and harmonic states, such as would be given by a music identity theory.
Analogously, we should consider the possibility that the relation between mind talk and brain talk could be more complex than the kind of correspondence that a mind-brain identity theory suggests.
Brain states
Mind-brain identity theory claims an identity between mental states and brain states. But what’s a brain state? Here, I shall explain why it is far from clear what we might mean by “brain state.”
I’ll start by looking at computer states. We do talk of computers in terms of states, at least in some of our theoretical analysis. So we have a reasonably clear idea of what we mean by “computer state.”
If a small fleck of dust lands on my computer, that’s a change in the physical state. Yet nobody would say that it is a change in the computer state. If the power lines to the computer change from 120 Volts to 119.5 volts, there will be a small but measurable physical change to the electrical state of most of the components of the computer. Yet, since this will not affect computation, most people would not consider that to be a change of computer state.
Clearly, when we talk of computer state, we are not talking about its physical state. Rather, we have a good theoretical understanding of the computer and how it operates. On the basis of that theoretical understanding, we have developed a systematic way of dividing into computational states.
For the brain, we as yet lack an adequate theoretical understanding. So we do not currently have a systematic way of dividing into brain states.
The problem goes further than that. Suppose that you could come up with a systematic way of dividing my brain activity into states. That would not be very useful for something line a mind-brain identity theory, if it only applied to my brain. And that’s where our next difficulty lies. Your brain is different from mine. A systematic way of dividing my brain into states would likely not work at all for your brain. Maybe we will eventually have a theory of brain operations which allows a systematic division into states that can apply to all brains. We do not currently have that. And I’m inclined to think it unlikely that there could be such a theory.
This is why I do not find the notion of “brain state” sufficiently meaningful to support a mind-brain identity theory.
The Heretical Philosopher 