EPISTEMOLOGY writings find him exploring new and disquieting ideas on the nature of the ultimate justification of knowledge and belief.2 Wittgenstein on Certainty Descartes’s scepticism has had a more enduring effect than his rationalism: philosophers have been more impressed by the difficulties raised in his First and Third Meditations than by the replies to those difficulties in the Fourth and Sixth Meditations Husserl’s transcendental idealism is only the last of a long series of unsuccessful attempts to respond to Cartesian scepticism about the external world while accepting the Cartesian picture of the internal world Wittgenstein’s private language argument, which showed that there was no way of identifying items of consciousness without reference to the public world, cut the ground beneath the whole notion of Cartesian consciousness But it was only in the last years of his life, in the epistemological writings published posthumously as On Certainty, that Wittgenstein addressed Cartesian scepticism head-on In response to sceptical doubt of the kind presented in the First Meditation, Wittgenstein makes two initial points First, doubt needs grounds (OC 323, 458) Second, a genuine doubt must make a difference in someone’s behaviour: someone is not really doubting whether he has a pair of hands if he uses his hands as we all (OC 428) In reply, Descartes could agree with the first point; that is why he invented the evil genius, to provide a ground for suspicion of our intuitions The second point he would answer with a distinction: the doubt he is recommending is a theoretical, methodological doubt, not a practical one Wittgenstein’s next criticism is much more substantial A doubt, he claims, presupposes the mastery of a language-game In order to express the doubt that p one must understand what is meant by saying p Radical Cartesian doubt destroys itself because it is bound to call in question the meaning of the words used to express it (OC 369, 456) If the evil genius is The similarity between the two is pointed out by Dagfinn Føllesdal in his paper ‘Ultimate Justification in Husserl and Wittgenstein’, in M E Reicher and J C Marek (eds.), Experience and Analysis (Vienna: OăBT & HPT, 2005), to which I am indebted for the quotation in the above paragraph 165