"Faced with a skeptic about reason, or about everything except reason, many philosophers would be willing to start a conversation, out of politeness, curiosity, competitiveness, or the desire to save a soul. But their inability to achieve a dialectical triumph over such a resourceful opponent does not oblige them to become skeptics about reason, or everything except reason, themselves. There is no bad faith in continuing to claim (and have) knowledge of the contested truths. For the anti-skeptic is not obliged to treat dialectic as the measure of all things. Indeed, the claim that dialectic is the measure of all things faces self-defeat, for it cannot triumph dialectically over its denial; even if it appeared to be getting the better of the argument, would not taking that to establish its truth beg the question?"
-- Timothy Williamson, The Philosophy of Philosophy, p.240.