Proof of an External World: does 'here is one hand' constitute a valid proof of the existence of material objects?

G.E. Moore, answered from the texts and cited to the page.

The position in brief

The proof has the form: Premise 1: Here is one hand. Premise 2: Here is another. Conclusion: Therefore, at least two material objects exist; therefore, an external world exists. The proof is rigorous in the senses that matter for proof: the premises are different from the conclusion; the premises are more certain than the conclusion of any skeptical argument; the inference is valid.12

The objection "but how do you KNOW you have hands?" misunderstands what the proof claims. The proof does not claim to address that question; it claims that the question of whether material objects exist is settled by the holding-up of the hands plus the inference. If I know I have hands (and I do — this is one of the truisms of the 1925 Defence), then I know material objects exist.

If someone denies that I know I have hands, they are denying something more certain than any premise that would lead to the denial. The proof is therefore a proof in the strict sense; what it does not do is provide a non-question-begging response to the philosophical skeptic about external-world knowledge in general — but that is a separate question.34

The anti-skeptical work the proof actually does is structural: it shows that the existence of an external world can be demonstrated within the framework of common-sense certainty, without needing to engage the skeptic's general challenge on its own terms.5

Sources

  1. Cannot I prove any of these things? It seems to me that, so far from its being true, as Kant declares to be his opinion, that there is only one possible proof of the existence of things outside of us, namely the one which he has given, I can now give a large number of different proofs, each of which is a perfectly rigorous proof; and that at many other times I have been in a position to give many others.
    VII Proof of an External World, p. 146
  2. Cannot I prove any of these things? It seems to me that, so far from its being true, as Kant declares to be his opinion, that there is only one possible proof of the existence of things outside of us, namely the one which he has given, I can now give a large number of different proofs, each of which is a perfectly rigorous proof; and that at many other times I have been in a position to give many others.
    Chapter 9: Proof of an External World, p. 166
  3. But now 1 am perfectly well aware that, in spite of all that 1 have said, many philosophers will still feel that 1 have not given any satisfactory proof of the point in question. And 1 want briefly, in conclusion, to say something as to why this dissatisfaction with my proofs should be felt. One reason why, is, 1 think, this.
    Chapter 9: Proof of an External World, pp. 168–169
  4. But now I am perfectly well aware that, in spite of all that I have said, many philosophers will still feel that I have not given any satisfactory proof of the point in question. And I want briefly, in conclusion, to say something as to why this dissatisfaction with my proofs should be felt. One reason why, is, I think, this.
    VII Proof of an External World, pp. 148–149
  5. Other Questions There still rem^n seven essays about which I have so far said nothing at all in this "Reply," namely those by Mr. Wisdom, Miss Ambrose, Mr, Lazerowitz and Mr. Malcolm; those by Mr. McKeon and Mr. McGill; and, lastly, Miss Stebbing's. And I have also said nothing about the main subjects of the essays of Mr. Murphy and Mr. Mace.
    A Reply to My Critics, pp. 133–134

Where the argument is made

  • 'Proof of an External World' (Proceedings of the British Academy 1939, in Philosophical Papers 1959 and Selected Writings)
  • 'A Defence of Common Sense' (1925) for the related anti-skeptical strategy
  • 'Certainty' (in Philosophical Papers)
  • 'Four Forms of Scepticism' (in Philosophical Papers)

How the argument was received

The 1939 Proof is one of the most-discussed texts in twentieth-century epistemology. Stroud's The Significance of Philosophical Skepticism (1984) is the major contemporary critique. The contemporary debate divides between dogmatists (Pryor) who hold the proof works because perceptual experience gives immediate justification for the premises, conservativists (Wright) who hold the proof requires antecedent entitlement to the relevant background commitments, and skeptics who hold the proof fails to engage the skeptic's actual concern. Coliva's Moore and Wittgenstein (2010) is the major contemporary systematic study. The position remains contested.