The following argument is logically valid:
1. Whatever is corporeal is dubitable.
2. My own existence is indubitable.
3. Therefore, I am not corporeal.
(1) may be taken at face value and granted, since we need only posit some possible world in which the corporeal things one observes are illusory. (2), of course, is succinctly expressed by Descartes' maxim, "I think, therefore I am" (cogito ergo sum). I cannot doubt my own existence without first existing! However, if something is true of X, but not of Y, then X and Y cannot be identical. Therefore, it necessarily follows that the mind, or self, is distinct from the body. This is not to say that the mind isn't dependent on the body, or that there is no correlation between mind and body, but only that the two really are distinct substances.
If the above line of reasoning is accepted, then the most commonly accepted naturalistic theory of the mind (materialism) must be rejected. Further, the metaphysical Naturalist will have to rethink his or her worldview.