Descartes suggests that the existence of
the mind or soul is more certain than that
of the body for the following reason:

When I ask myself if I have a mind or
soul, I prove it, since questioning is a
form of thinking, and a mind or soul is
just a thing that thinks.

However, when I ask myself if there's a
real world "out there" apart from my mind,
no such logical argument applies. I could
think there's a world out there when there
isn't with no logical contradiction.

Proof of the external world requires an
elaborate proof on Descartes part,
proving God exists and that he is good
and is no deceiver (and that any false
beliefs we have are the result of of our
misuse of our power of judgement). All
the soul requires to be proved is "I think,
therefore, I am."

Prof. Borrowdale