In this paper I argue that Cartesian skepticism about the external world is self-defeating. The Cartesian skeptic holds that we are not justified in believing claims about the external world on the grounds that we cannot rule out the possibility of our being in a radical skeptical scenario. My argument against this position builds upon a critique of Wilson in Analysis, 72, 668–673. Wilson argues that the Cartesian’s skeptical reasoning commits him to mental state skepticism and that this undermines his (...) ability to claim that he is truly skeptical about the external world. I show that Wilson’s argument does not succeed. She is correct that the Cartesian is committed to mental state skepticism. However, she is mistaken in thinking that there is anything incoherent about combining the Cartesian position that one ought to doubt external world claims with doubt about whether one is in fact such a skeptic. I propose an alternative explanation as to why mental state skepticism proves to be the Cartesian’s undoing. I suggest that mental state skepticism leads to epistemic skepticism and that this undermines the Cartesian’s position that belief about the external world is unjustified. (shrink)