The Structure of the Skeptical Argument

Much has been written about epistemological skepticism in the last ten or so years, but there remain some unanswered questions concerning the structure of what has become the canonical Cartesian skeptical argument. In this paper, I would like to take a closer look at this structure in order to determine just which epistemic principles are required by the argument. A standard way of presenting the argument is as follows. Let P be some arbitrary proposition about the external world, such as the proposition that I am now sitting. Let SK be some logically possible proposition which is incompatible with P-a skeptical counterpossibility to P-such as the proposition that I am a brain in a vat with sense experience qualitatively indistinguishable from my actual experience. Let us call the following argument A: