"Here is one hand": the proof of the external world

Moore's proof; "here is one hand, and here is another"; common sense against the sceptic

G. E. Moore 1873 · 1958Good cannot be defined by anything else: equating 'good' with 'pleasant' is a mistake.

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What it claims

Things outside me exist: the external world exists.

Why

I am more certain that I have two hands here than of any skeptical premise that would deny it.

The same idea through history

Samuel Johnson1763According to Boswell, who reports the scene, Samuel Johnson answers Berkeley's thesis by kicking a large stone: "I refute it thus." This is already common sense against the sceptic, before Reid and Moore. The story comes from Boswell, not from a text by Johnson.

Thomas Reid1764Reid already defends "common sense": some beliefs, such as the existence of the bodies we perceive, are more secure than any philosopher's reasoning that puts them in doubt.

Immanuel Kant1787Kant writes that it is "a scandal to philosophy" to have to accept the existence of things outside us "on faith", for lack of a proof. Moore opens his 1939 paper with this sentence, and believes he can give the missing proof.

G. E. Moore1925In "A Defence of Common Sense", Moore lists truths that he says he knows with certainty (there is a living body which is his, the Earth has existed for a long time...) and argues that philosophers who deny them contradict themselves in using them.

G. E. Moore1939Moore turns this common sense into a proof: he shows his two hands, and concludes that things outside him exist.

In short

""Here is one hand": the proof of the external world" is an argument by G. E. Moore (G. E. Moore, Proof of an External World, 1939, 1925) on the question "Is the world real?" (answer: the world is real). Its claim: Things outside me exist: the external world exists. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 2 premises. AskPhilosophy presents 2 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

G. E. Moore, Proof of an External World, 1939

Valid

The sceptic says: you cannot be sure that a world exists outside your mind, perhaps you are dreaming. In 1939 the English philosopher G. E. Moore replied in a way that made people smile and think: he raised a hand, said "here is one hand", raised the other, said "and here is another", and concluded that two hands exist, hence that there exist at least some things outside him. His underlying idea is that we are more sure of having hands than of any premise in a sceptical reasoning: when a reasoning leads to denying something so certain, it is the reasoning that must give way. The argument is presented here in its strongest form; premise P3 is the one Moore assumes without writing it down.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.

Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a case where I say "I know there are two hands here", where hands imply an external world, but where there are no hands (I am dreaming): the conclusion fails.

With P3 the argument is valid: I know there are hands, knowing requires that it is true, and if there are hands then the external world exists. Without P3 it no longer is: one could believe one knows, or know in a weak sense, and still be wrong. The whole debate therefore concerns P1: does Moore have the right to say he knows, without first proving that he is not dreaming? Moore answers yes: one can know without being able to prove.

Analogy. A pupil tells the teacher: "You cannot prove that 2 + 2 is 4, so you do not know it." The teacher replies that he is more sure that 2 + 2 is 4 than he is of any rule in your reasoning. Moore says the same of his hands.

Objections

Each objection is a conversation: click to see who replies and where the debate stands.

The sceptic (an objection Moore foresees himself) 1939 · attacks premise 1

You do not know that here is a hand. To know it you would first have to prove that you are not dreaming, as Descartes pointed out. And you cannot prove that. So your proof assumes what it wants to establish: it begs the question.

Moore replies

Moore Moore's reply (1939)

Moore grants that he cannot prove that he is not dreaming. He has, he says, conclusive reasons for asserting that he is not dreaming and conclusive evidence that he is awake, but that is very different from being able to prove it: he could not list all his evidence.

The debate goes on (our summary) This is the heart of the disagreement. Moore replies that one is more sure of having hands than of the truth of the sceptical premise that one must be able to rule out the vat in order to know. The sceptic replies that this "certainty" is exactly what is in question. The brain-in-a-vat debate is taken up in another argument on the site, being written.

The brain in a vat (modern version, Putnam 1981) 1981

Imagine a brain kept in a vat and wired to a computer that sends it exactly the same experiences as yours. Its "conclusive reasons" would be identical to yours, and yet it would be wrong. Reasons that one would also have in the situation where one is mistaken cannot be enough for knowledge.

See the rest of the debate (1)

Moore Moore, on the difference between knowing and proving (1939)

Moore says that one can know things that one cannot prove. Those who are dissatisfied with his proof because he did not prove its premises have, in his view, no good reason to be.

Ludwig Wittgenstein 1969 · attacks premise 1

In On Certainty, written in 1949-1951 and published in 1969, Wittgenstein starts from Moore's sentence: if we grant that he knows he has a hand, we grant him everything else. But he doubts that "I know" makes sense here. We say "I know" when a doubt is possible and can be removed by looking more closely. Yet nobody seriously doubts that he has hands: this is not a knowledge resting on proofs, it is what our whole game of proofs and doubts rests on.

no reply

No reply possible: objection raised in 1969, after the author's death (1958).

The debate goes on (our summary) Wittgenstein does not say Moore is wrong about the facts: he does not doubt the hands. He says Moore's proof proves nothing, because it does not answer the sceptic on his own ground: it calmly assumes what the sceptic questions. Moore's certainty exists, but it is not knowledge in the ordinary sense. Scholars still discuss how best to read this unfinished text.