Theresa asked:

Descartes provides the following argument for Cartesian Dualism:

(a) I can doubt that my body exists.

(b) I cannot doubt that I exist.

(c) If(a) and (b) then I am not my body.

(d) So I am not my body.

(e) IfI am not my body, then Cartesian Dualism is true.

(f) So Cartesian Dualism is true.

How would you explain why Descartes accepts each step of this argument?

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(a) According to Descartes, I can doubt that my body exists for exactly the same reasons as I can
doubt the existence of anything that I know about through the senses. My senses can be deceived. It
is impossible to know whether I am perceiving correctly or hallucinating or dreaming. I could even be
the victim of a systematic Matrix-like delusion about reality, brought about by an evil demon.

(b) I cannot doubt that I exist because, according to the cogito argument, in the very act of doubting I
presuppose my own existence. Doubting is a kind of thinking: I am thinking, therefore I am.

(c) My body and my "I" must be separate substances, therefore, because it is not possible both to
doubt and not to doubt the existence of one and the same thing. It would be a contradiction
comparable to both knowing and not knowing that David Beckham is the captain of the English
soccer team.

(d) So, if my body and my "I" are separate substances, "I" cannot be my body. My body is a physical
thing; my "I" must be a non-physical.

(e) Therefore, the doctrine of dualism, that asserts the reality of the physical body and the
non-physical mind or soul, is true.

We can question Descartes' argument at various points. Is it, for example, true that we can doubt the
existence of our own body in exactly the same way as we can doubt the existence of other objects?
Do we not have an awareness of our body, an inner awareness, that is closer to a mental experience
than a perceptual experience? Against that point, however, we could point to phantom limb
experiences, whereby amputees are still "aware" of limbs that have been amputated. We would,
perhaps, have to class some types of mental experience as vulnerable, leaving only the more
cognitive mental processes (essentially, thinking) as demon-proof, at least in so far as if there is
thought at all then there must be a subject doing the thinking, an "I". Even this, however, is thought to
be an unjustified assumption by some commentators.

Another problem with Descartes' argument is that it just isn't true that knowing for certain that "I" exist
and not knowing for certain that my body exists means that they cannot be one and the same thing.
We can know things under different aspects. For example, Victoria may know for certain that her
husband David is the captain of the English soccer team, but not know for certain that David is the
father of her sons, but that does not rule out the possibility that the captain of the English soccer team
is the father of Victoria's sons.

In the same way, my ignorance of the brain compared with my knowledge of my own mental states
(my thoughts) does not rule out the possibility that my mental states simply are brain states — in
other words, aspects of one and the same thing. Therefore, Descartes' argument fails to establish
dualism.

Graham Nutbrown