 But the disjunction of these positions can seem, not just unappealing, but inevitable; for the proposition that I am sitting entails that I am not a brain in a vat, and it’s hard to see—presuming that I know that this entailment holds—how I could know the former without thereby being in a position to know the latter. The contextualist about “knows” aims to provide the answer: the extension of “knows” depends on features of the context of utterance. Perhaps—to take one among many possible contextualist views—a pair of a subject and a proposition p will be in the extension of “knows” relative to a context only if that subject is able to rule out every possibility which is both (i) inconsistent with p and (ii) salient in C. The idea is that “I know that I am sitting down” can be true in a normal setting, simply because the possibility that I am a brain in a vat is not normally salient; but typically “I know that I am not a brain in a vat” will be false, since discussion of skeptical scenarios makes them salient, and (if the skeptical scenario is well-designed) I will lack the evidence needed to rule them out. See for discussion, among many other places, the entry on  epistemic contextualism,  Cohen (1986), DeRose (1992), and David Lewis (1996).