Some Notes on Jaegwon Kim, Philosophy of Mind, Chapter 4
The Identity Theory

Philosophy of Mind
Curtis Brown

What Is "the Identity Theory"?

The general idea is that (types of) mental states are identical with types of neural states.

In this context, "is identical with" means "is the same thing as," or in this case "is the same state as" (not merely "is exactly similar to")

Mind-Brain Correlations

The beginning point for discussion of the identity theory is the existence of systematic and extensive relationships between mental phenomena and neural phenomena.

There is a lot we don't know about these relationships, but there are also many things we do know.

Here is a quick list just to give some idea how extensive the relationships here are. (My examples, mostly not Kim's)

1. Different types of brain damage lead to different types of mental deficits

• loss of the ability to form new memories (Korsakoff's syndrome, as in the movie Memento)
• blindsight: no visual experience in part of the visual field, but still have capacity to gain visual information
• genuine blindness with the illusion of being able to see
• inability to speak; relatively normal cognition (e.g. Broca's aphasia)
• sophisticated grammar, severely impaired cognition (e.g. "chatterbox syndrome")
• split-brain phenomena

2. Direct stimulation of the brain leads to mental phenomena

• Wilder Penfield (1975): elicited movements that the patient did not feel were his or her own
• J.M.R. Delgado (1969): elicited movements (turning the head) that patients did think were voluntary and gave different explanations for on different occasions (looking for slippers, heard a noise, etc.)
• Brasil-Neto et al. (1992): affected which index finger would be tapped
• memories and vivid experiences can also be produced by direct stimulation
• and of course a strong enough blow to the head can render someone unconscious

3. Drugs can influence mood, emotion, cognition, etc.

• alcohol, caffeine, marijuana, antidepressants, antipsychotics, Ritalin, etc. etc.

Many of these phenomena seem difficult to explain on the hypothesis that the brain merely provides input to (and receives output from) the mind.
 

Kinds of Explanation of Correlations

So we have correlations between M (mental phenomena) and B (brain phenomena). Why?

1. Causal interactionism (Descartes): Ms cause Bs and vice versa
2. pre-established harmony (Leibniz): no causation between M and B, but God makes mind and brain in such a way that they will remain synchronized, as the owner of a clock shop might set and wind twenty accurate clocks which would then remain synchronized
3. occasionalism (Malebranche): no causation between M and B, but God regularly simultaneously causes related events to occur in both mind and brain
4. double-aspect theory (Spinoza): there's really only one thing here, but it has two very different aspects
5. epiphenomenalism (T.H. Huxley): one-way causation. Cases of apparent mental causation (either of physical phenomena or of other mental phenomena) are illusory; both events have a common physical cause. Compare: it may seem as though the phases of the moon cause changes in the tides, but in fact both have a common cause.
6. emergentism: there's no explanation, it's just a brute fact. (More on this in chapter 10.)
7. the identity theory: M and B are not separate phenomena; in fact, they are the same thing. (Compare: water = H2O, heat = molecular motion. You could ask what the explanation is of the fact that things get hotter whenever their molecules speed up, and vice versa, but this would be misguided: there aren't actually two separate things here.)

Arguments for the Identity Theory

1. Argument from simplicity. Suppose we have two rival theories about the relation between mind and body. One is the identity theory, the view that mental states simply are neural states. The other is a dualist theory. The dualist theory involves more kinds of states than the identity theory, and it will also need to invoke more laws of nature, since it seem that to explain the correlations between neural and mental phenomena will require some sort of natural laws linking the two. In general, if we have two theories that explain the same phenomena, we should prefer the simpler one ("Ockham's razor": shave away the unnecessary parts of a theory).

Of course, the "other things being equal" is important here: Ockham's razor only applies if both theories do explain the same phenomena. If there are phenomena that the more complex theory explains and the simpler one doesn't, then other things aren't equal.

2. Explanatory arguments. The principle of inference to the best explanation is often regarded as an important methodological principle in both scientific and everyday reasoning. If we have rival hypotheses which attempt to explain the same phenomena, we should prefer the one that best explains the phenomena.

We could regard the argument from simplicity as an application of this kind of explanatory argument, since (other things being equal) a simpler hypothesis is thought to be a better explanation than a more complex one.

Kim considers two versions of explanatory arguments. According to the first, the identity theory provides the best explanation of psychoneural correlations. Kim regards this as problematic because in a sense, if the identity theory is true, then there aren't any psychoneural correlations of the kind envisioned (it doesn't seem to make sense to say that something is correlated with itself).

Second version: psychoneural identities are part of the best explanation of other phenomena, for instance the fact that pain causes distress. Kim suggests the following sample explanation:

neurophysiological laws
Cfs causes neural state N
Pain = Cfs
Distress = neural state N
Therefore, pain causes distress

Kim objects that the identities are not part of the explanation here. The only explanatory work is done in the first two lines; the conclusion is simply a "rewrite" of line 2.

I'm not sure I completely understand Kim's objection here. Perhaps it's true that the identity claims themselves are not literally part of the explanation of why pain causes distress. However, they do enable us to see that the explanation of the fact that Cfs causes N in terms of neurophysiological laws already is an explanation of how pain causes distress. So the question is still which explanation is the best: a dualist explanation (which we haven't got, and if we did would be more complicated than the neurophysiological one), or the neurophysiological one, which if the identity theory is correct constitutes an explanation of why pain causes distress. It still seems that we can compare these two and choose the better of the two?

3. Suppose mental state type M is correlated with (or perhaps identical with) physical state type P1: they always occur simultaneously. (Perhaps M is the state of intending to raise my arm, and P1 is a brain state of some kind.) Now consider tokens of these types on a particular occasion: m is my present intention to raise my arm, and p1 is the corresponding brain state.

Suppose that in this case my arm does go up (call that event p2). The question is: what caused p2?

It seems likely that we will be able to explain p2 in terms of p1: my brain state before my arm went up caused my arm to go up. But if so, what causal role does this leave for m, my intention to raise my arm? If m is a different event from p1, then it seems we need to either say that m does not cause p1 (leaving us with epiphenomenalism), or that p1 has two different causes (making it "overdetermined"). Neither option seems satisfactory.

However, if m = p1, the problem seems to go away. My intention causes my arm to go up; my brain state causes my arm to go up; but this doesn't mean that there are two different causes, because my intention is my brain state!

Criticisms of the Identity Theory

Quick and dirty versions.

1. Epistemological arguments. (Similar to the epistemological arguments for dualism considered earlier.)

version 1: you can know about mental states without knowing anything about brain states. So by Leibniz' law they can't be the same. This version is vulnerable to the same criticisms we discussed earlier (epistemological properties are relational in a hidden way, so Leibniz's Law doesn't apply to them).

version 2: the identity theory says that we discover empirically that pain = C-fiber excitation (hypothetical example only, not meant as serious neuroscience). But you can discover empirically that X = Y only if you have independent ways of identifying X and Y. But then even if X = Y, the features we use to identify them must be different, leaving us wiith something irreducible.

version 3: I have special access to my mental states in a way I don't have special access to my brain states. So they must be different.

2. Modal argument.

1. If X = Y, then it is a necessary truth that X = Y.
2. I can conceive that M is not identical with P (for instance by imagining a possible world in which one exists and the other doesn't)
3. Therefore, it is not a necessary truth that M = P (from 2, assuming conceivability implies possibility)
4. Therefore, M is not identical with P (from 1 and 3, by contraposition)

3. Multiple Realization argument

1. if mental events are identical with neural events, then nothing that doesn't have a nervous system like ours could have mental events
2. it's at least possible that other beings have mental events with different physical realizations than ours
3. so mental events can't be type-identical with neural events

Note that argument 3 is compatible with token-token identity, just not with type-type identity.