  In two of his last writings, ‘Four Forms of Skepticism’ and ‘Certainty’, Moore, perhaps dissatisfied with these earlier arguments and with the misunderstanding of his ‘proof’, returned to the issue and set himself the challenge of refuting Cartesian skepticism. Notoriously, by the end of ‘Certainty’ Moore acknowledges defeat: having agreed that if he does not know that he is not dreaming, then he does not know such things as that he is standing up and talking, he accepts (with reservations) that he cannot know for certain that he is not dreaming. Most commentators agree that Moore lost his way here. But it is not clear where, since Moore makes no obvious mistake. Nonetheless the viability of a ‘common sense’ response to skepticism remains an important feature of later discussions of the topic. Moore was clearly right when, for example, he remarked that despite Russell's frequent skeptical professions, Russell was nonetheless perfectly sure, without a shadow of doubt, on thousands of occasions, that he was sitting down. But what is difficult to achieve here is a formulation of the skeptical dialectic which both shows the importance of Moore's ‘common sense’ affirmations of certainty and yet avoids a dogmatic insistence that knowledge does not need to be vindicated in the face of skeptical argument. I myself think that Wittgenstein's writings On Certainty, which were much influenced by Moore, best indicate how this is to be achieved, but this is not the place to demonstrate this achievement.