One need not know whether one is being deceived by an evil demon – that is, whether the skeptical hypotheses obtain – in order to know that one is sitting by a fire holding a piece of paper in one’s hand (or to know almost all the things we commonly take ourselves to know), so long as one is prepared to accept the possibility that the skeptical hypotheses may obtain.
Nozick, in his “Knowledge and Skepticism” demonstrates that knowledge is indeed possible. His four-point definition of knowledge is quite operable if a skeptical hypothesis (in his terms, “SK”) does not hold. Nozick states the conditions that, for or one to know that P, 1. P must be true; 2. one must believe that P; 3. if P were not true, one would not believe that P; and 4. if P were true, one would believe that P. (Nozick 23-26) To continue with Descartes’ example of sitting by the fire holding a piece of paper, one knows that one is doing so, according to Nozick, if: 1. one is actually sitting by the fire, holding a piece of paper; 2. one believes that to be so; 3. if it were not so, one wouldn’t believe it; and 4. if it were so, one would believe it, as one does.
Even if a skeptical hypothesis SK does not hold, however, according to Nozick one still cannot have sure knowledge of its holding or not. One would know that SK does not obtain if 1. SK is not the case; 2. one does not believe SK to be the case; 3. if SK were the case, one would not believe that SK were not the case; and 4. if SK were not the case, one would not believe it were. (Nozick 32) The problem, of course, arises in condition 3: if the demon were, in fact, deceiving us, we would believe he were not. “The sceptic has carefully chosen his situations SK so that if they held we (still) would believe they did not. … it follows that we do not know that SK doesn’t hold.” (Nozick 32)
Knowing that one is sitting by the fire with the paper in one’s hand with Cartesian indubitability is thus impossible, since one would not know that a skeptical hypothesis prevailed even if it did. Nozick himself admits, “The sceptic asserts we do not know his possibilities don’t obtain, and he is right. Attempts to avoid scepticism by claiming we do know these things are bound to fail.” (Nozick 33) No satisfactory answer can be given to the skeptic; under the demon’s sway, we would never know it until (and unless) we were released from his grip on our perceptions.
This poses no great problem to one, however, unless one obsesses over the possibility that a skeptical hypothesis actually obtains. While one cannot know that such possibilities are not the case, neither can one know that they are the case, and one has no more reason to believe that they are prevailing than that they are not. Nozick concludes by noting that “Only by fixating on the sceptical possibilities SK can [the skeptic] maintain his sceptical virtue; otherwise, unsurprisingly, he is forced to confess to sins of credulity.” (Nozick 42) The best one can do is be prepared to reevaluate the responsible regulation of one’s intellectual life should one learn a skeptical hypothesis obtains.