Mark asked:

I'm trying to understand the basic relationship between mind and body, using the terminology of
Descartes and twentieth century philosophy. What are the arguments for and against the identity of
body and mind?

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If you want the argument against the identity of body and mind, you can't do better than get it from the
horse's mouth:

1. Because I know that all the things I conceive clearly and distinctly can be produced by God
   precisely as I conceive them, it is sufficient for me to be able to conceive clearly and distinctly one
   thing without another, to be certain that the one is distinct or different from the other, because they
   can be placed in existence separately, at least by the omnipotence of God; and it does not matter by
   what power this separation is made, for me to be obliged to judge them to be different.

2. And therefore, from the mere fact that I know with certainty that I exist, and that I do not observe
   that any other thing belongs necessarily to my nature or essence except that I am a thinking thing, I
   rightly conclude that my essence consists in this alone, that I am a thinking thing, or a substance
   whose whole essence or nature consists in thinking.

3. Because, on the one hand, I have a clear and distinct idea of myself in so far as I am only a
   thinking and unextended thing, and because, on the other hand I have a distinct idea of the body in
   so far as it is only an extended thing but which does not think, it is certain that I, that is to say my
   mind, by which I am what I am, is entirely and truly distinct from by body, and may exist without it.

Descartes 'Sixth Meditation', Discourse on Method and the MeditationsF.E. Sutcliffe (tr.) Penguin, p.
156.

I've snipped out a little, and added numbering, to make Descartes' argument a little clearer. A lot of
students get thrown by talk of 'separations' that God can, or cannot perform. Though Descartes gives
the strong impression that his Meditationswould be worthless if you rejected his proofs of the
existence of God, it is possible to run points 1. through 3. without invoking the actual existence of the
Deity.

So here's my version:

1. If A can exist without B, then A cannot be one and the same thing as B. This applies to the
   particular case where A my mind, or some part of my mind, and B is a body, or some part of a human
   body. For example, if the thoughts I am thinking now can exist in the absence of brain processes,
   then my thoughts cannot be brain processes.

2. I know with certainty the thought that I am thinking now, 'I wonder what I shall have for lunch.' But
   that certainty does not extend to my knowledge of facts about the physical world. I could be thinking
   that very same thought, even though there was no physical world and all my life was an experience
   produced by an evil demon. Being physical cannot, therefore, be part of the essence of my thoughts
   and feelings.

3. It follows that my thoughts and feelings can exist in the absence any corresponding physical
   processes. Therefore, my thoughts and feelings cannot be identical with any physical processes,
   neither can the subject of those thoughts and feelings, myself, be identical with any physical thing.

In the 50's and 60's - in the work of the 'Australian materialists' Smart and Armstrong - the idea arose
that Descartes could be refuted simply by insisting on the scientific factthat the brain is responsible
for conscious processes. The fact that my thought, 'I wonder what I shall have for lunch' arises from a
brain process is sufficient to establish the identity of that thought with a brain process. Since the work
of Saul Kripke in the 70's (see his influential article, reprinted as Naming and NecessityBlackwell/
Harvard 1980) that idea has gone out of fashion. If two things are identical, Kripke argued, they must
be identical in all possible worlds.

Is that a vindication of Descartes? I would argue that we have to accept the validity of Descartes'
argumentbut attack the validity of its main premiss, the idea my knowledge of my own thoughts and
feelings is independent of any knowledge I may possess about the physical world. The idea, for
example, that I know that I exist, even if the entire universe is merely my own dream. The rejection of
Descartes' conception of self-knowledge is the upshot of Wittgenstein's argument against a private
language in the Philosophical Investigations.

Geoffrey Klempner