The claim that it is possible to obtain knowledge by sense perception has been a target for skepticism since the dawn of philosophy. In contrast, there have been no sustained skeptical challenges to our claim to be able to obtain knowledge by introspection. What accounts for the difference? Is it impossible to extend the arguments that have been formulated by skeptics so as to obtain new arguments that apply to introspective beliefs? If so, why? If one were to try to construct a skeptical argument concerning introspective beliefs, at exactly what points would one encounter problems?

It will be useful to begin by considering a line of thought that is frequently used to justify skepticism about sense perception. Let PB be the set of propositions that represent the perceptual beliefs of a certain normal subject, S. Further, let SH be the hypothesis that is obtained by conjoining the following five propositions: S is part of an elaborate psychology experiment that is being conducted in a laboratory on a remote planet; S is a brain in a vat; S is connected to a computer that monitors all of S's thoughts; all of the sense experiences S has had up to now have been caused by events inside the computer; and in the future the computer will provide experiences like the ones that S has had in the past (that is, experiences that confirm the members of PB and that lead S to adopt new beliefs that are consistent with the members of PB).