G.E.Moore's 'ADefense of Common Sense' was first published in 1925 andhis 'Proof of an External World' fourteen years later.1 Apparently Wittgenstein had a long-standing interest in these papers and during the last eighteen months of his life, stimulated2 by discussions with Norman Malcolm while his house guest in Ithaca in 1949, he composed the four short sets of rough notes we now have as On Certainty. My question here is whether these notes gesture at a principled and stable response to the issue at which Moore's papers had been directed - the issue of scepticism, and particularly scepticism about our knowledge of the material world. My eventual and hesitant answer will be: yes - though the development here must be sketchy and incomplete. It will be focused upon one specific though very general form of sceptical argument - certainly as disturbing as any - which we may begin by eliciting, ironically enough, from consideration of something that was supposed to help: Moore's curious 'Proof' itself.