G.E. Moore was a tremendously influential figure, although no-one reads him himself any more. He persuaded Bertrand Russell that idealism was nonsense, and so initiated the rejection of metaphysics that characterized British twentieth-century philosophy, at its height in the works of Russell, Wittgenstein, J.L. Austin, and Gilbert Ryle.

He was modest, charming, and much admired by a wide circle. Born in London on 4 November 1873, as a young man he was part of the elite Cambridge group known as the Apostles. It was Russell who persuaded him to switch from classics to philosophy. His first defence of common sense was an article entitled 'The Nature of Judgment' in Mind in 1899. He was editor of Mind from 1921 to 1947, and professor of Mental Philosophy and Logic at Cambridge from 1925 to 1939. He was awarded the OM in 1951 and died in 1958.

His wife's name was Dorothy. Tom Stoppard's play Jumpers is about a lecturer in mental philosophy and logic whose name is George Moore, and whose wife's name is Dorothy, and who labours under the misfortune that many of his students confuse him with the other George Moore, who is of course dead. (The real Dorothy Moore loved the play.)

Moore's logic is not a mere assertion of certainty. Although he does not adduce a long chain of proof that here is a hand, the salient point is that, no matter what a purported proof consisted in, it could not be more certain than our common-sense certainty that this is my hand.

This argument was not invented by Moore: it was equally clearly stated by Epicurus in his 'canonic': "If you fight against all your sensations, you will have no standard to which to refer, and thus no means of judging even those judgements which you pronounce false." (Number 23 of the aphorisms collected by Diogenes Laertius; see also Lucretius, Book IV, lines 469-521.)

Moore's argument, though much stronger than a sceptical counter-argument, is not entirely immune to criticism. Wittgenstein, in On Certainty, analyses the conditions under which one can say "I know...". There must be some way in which you can be mistaken. In the case of the hand it is not clear what it would mean to be wrong about it, or to test or confirm it. (Circumstances can be imagined where it does make sense but they are different from those Moore was in: e.g. waking up in hospital after a bad accident you can check that you have hands.)

Wittgenstein is not doubting the existence of the hand (he is quite as certain as Moore is), but he is saying that it is not appropriate to say further that we have knowledge of it, because the process of knowledge is a process of finding out, of discriminating between possibilities, etc. "Here is my hand" makes sense; "I know that this is my hand" doesn't. For Wittgenstein such things are not discovered, they are not open to the daily practices of refutation and confirmation; rather they are the groundwork or bedrock upon which knowledge is built. Nothing could possibly count as evidence to the contrary.

Philosophasters of later years got this terribly wrong by seeming to think that if the existence of my hand was unprovable, then it was somehow open to doubt. The former is true, the latter utterly false. It simply leads back to idealism, which is what the great G.E. Moore comprehensively demolished in the first place.