G.E. Moore Refuted Skepticism by Holding Up His Hands

In 1939, G.E. Moore delivered an annual philosophical lecture to the British Academy and used it to refute skepticism about the external world. He raised his right hand. "Here is one hand," he said. He raised his left. "And here is another." Two hands therefore exist; therefore there are external objects. He treated this as a complete proof.

The maneuver is not a joke, though it sounds like one. Moore's claim is that he is more confident he has hands than he is confident in any premise of any skeptical argument that would conclude he doesn't. The skeptic runs: if you can't rule out being a brain in a vat, you don't know you have hands; you can't rule out being a brain in a vat; therefore you don't know you have hands. Moore takes the same conditional and runs it backward: you do know you have hands; therefore the antecedent is false. Same logical form, opposite direction. Philosophers call it the Moorean shift.

Nothing in pure logic decides which way to run a valid conditional. You pick the direction by which premise you trust more. Moore was betting that the perceptual fact beat the philosopher's premise that perception cannot rule out radical deception. Most people, he thought, would make the same bet outside the seminar room.

Wittgenstein found this maddening and clarifying. He spent the last six weeks of his life on the notes published as On Certainty, much of it arguing with Moore. His complaint: Moore had taken a sentence that grounds our practice of doubting and treated it as a piece of knowledge, stripping it of its role. "I know I have a hand" isn't a discovery; it's bedrock the discoveries rest on.

The two philosophers were doing very different things with the same gesture. Both views are still on the syllabus.