my favorite moment in the history of philosophy is g.e. moore's "proof of the external world" or "proof that there are things external to the mind": here is one hand, and here is another. it's decisive, baby.
on the other hand, one of my least favorite moments in the history of philosophy is wittgenstein's treatment of moore's proof: on certainty. now it was assembled after his death into a simulacrum of the investigations. so it's rougher than it might appear to be. but it's wittgenstein at his worst; i'd say he never takes a clear position on moore's proof, though he obviously thinks something is wrong with it. but he never says clearly what, and the basic idea that 'here is a hand' - spoken by moore as he delivers a lecture and waves his hands around - is a 'grammatical' or 'logical' statement, is question-begging and also sort of obviously false. but a typical passage is something like #218: "Can I believe for one moment that I have been ever been in the stratosphere? No. So do I know the contrary, like Moore?" well (a) yes, and (b) you're the frigging philosopher; you tell me.
wittgenstein definitely has his moments, and i don't evaluate the whole authorship as negatively as i once did. and yet i do feel that one of the basic goals of that authorship is to convey the impression of the genius of its author. now actually answering a question like that - 'do i know?' - is too primitive for a genius, and neither direct answer is amazing enough to make you gasp or grope for the meaning. thus the hemming and hawing, the backandforthing, the 'this thing is much more difficult than we ever thought,' etc. when you get down to it, the heart of the sort-of objections is just the oldest nostrum: to have knowledge, you have to have a justification. the whole thing doesn't amount to a hill of beans. the contrast with moore's own essential egolessness and comparatively straightforward and genuinely radical positions is instructive.