This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
In 1939, G.E. Moore stood before an audience and offered what he called a proof that an external world exists. He held up one hand, said here is one hand, held up the other, said and here is another, and concluded that since two hands are two external objects, external objects exist. The Stanford Encyclopedia's account of Moore's career treats this performance, absurd as it can sound out of context, as one of the most argued over moments in twentieth century philosophy, precisely because Moore himself did not think it settled what people assumed it settled. The entry is explicit that Moore's avowed aim was always to prove the existence of an external world, not to prove his knowledge of the existence of one, a distinction Moore himself later underscored by separating the claim that there are no material things from the quite different claim that nobody knows for certain that there are any. The proof, on this reading, is a proof of existence, valid, with premises Moore took himself plainly to know, and yet the entry notes that whether it actually defeats the skeptic, someone who doubts precisely whether Moore knows he has a hand in the relevant sense, remains disputed rather than settled by the demonstration itself.
That gap between what the proof shows and what people wanted it to show runs through the rest of Moore's career. His defense of common sense, laid out in papers where he insisted on truisms such as that the earth existed for many years before his own body was born, works the same way. It does not argue the skeptic out of skepticism so much as assert, with total confidence, propositions that Moore judged more certain than any philosophical argument that could be marshaled against them. Critics have long asked whether this is philosophy at all, or simply a refusal to engage, and the entry treats that question as still open rather than answered in Moore's favor.
The same structure, a claim that looks decisive and turns out to open more questions than it closes, appears earlier in Moore's career in Principia Ethica, where he argued that goodness cannot be defined in terms of any natural property, a mistake he named the naturalistic fallacy. His method for testing any proposed definition, such as good means what we desire to desire, was to ask whether it remains an open question whether the thing so defined is actually good. If the question still feels open once the definition is granted, Moore argued, the definition has failed. This open question argument shaped a century of debate in metaethics, but it too proves less on inspection than it seems to at first, since critics have pointed out that many successful definitions in other domains also seem to leave an open question feel to competent speakers, which threatens to make Moore's test prove too much, ruling out true definitions along with false ones. Even Moore's most famous discovery of an oddity in ordinary language, the observation that a sentence like it's not raining but I believe that it is sounds absurd even though it could be true, generated its own puzzle, since the entry notes the absurdity is one of assertion rather than of the content asserted, a distinction later philosophers spent decades trying to pin down precisely. Moore's legacy, across ethics, epistemology and language, is less a set of results than a set of demonstrations that keep being reopened by the very people who admire them.