I just read Moores 'Proof of an external world' and I feel like I almost get it. To me the structure seems to be this:

  1. He clarifies what 'the existence of things outside of us' is (anything that exists independent of any persons experience).
  2. Per definition, knowing of the existence of an object (one hand) implies it exists outside of us. The assertion 'here is one hand' after all does not mean that 'I am dreaming of this hand right now', but when someone says this they actually do mean there is a real hand that is part of the things outside of us as defined above.
  3. He claims that he knows this for his hand (I guess his epistemic intuition, which contrasts the sceptics intuition).
  4. Then he says a complete proof is given as follows: I know 'here is one hand' and I know 'here is another'. Therefore two hands exist, i.e. things outside of us exist.

I can prove now, for instance, that two human hands exist. How? By holding up my two hands, and saying, as I make a certain gesture with the right hand, 'Here is one hand', and adding, as I make a certain gesture with the left, 'and here is another'. And if, by doing this, I have proved ipso facto the existence of external things, you will all see that I can also do it now in numbers of other ways: there is no need to multiply examples.

I do not understand why he needs step 4. For me the whole argument seems to be concluded at step 3. My guess is that it's more like a joke to make the argument a little bit longer (otherwise it would just be 'here is one hand, therefore things outside of us exist').

Question: What does Moore need the second hand for in his famous argument for the existence of the external world?

  • Yes, Moore's "proof" works also for mutilated... Mar 28 at 7:33
  • 1
    That is not Moore's phrasing. He rather states what is to be proved (following Kant) as "existence of things outside of us", with "things" in the plural. And continues:"if I can prove that there exist now two sheets of paper, or two human hands, or two shoes, or two socks, etc... if, at any time, I can prove any one of them, I shall have proved the existence of things outside of us." As for "proof of the external world", it is used only once in scare quotes, referring to what others "want", which is "not quite easy to say".
    – Conifold
    Mar 28 at 10:15
  • @Conifold Thank you for linking it, I had read it thoroughly before posting and am trying to wrap my head around it. In my post I was trying to explain how I understand it and deviated somewhat from Moore's language I guess (I replaced what he just calls 'existence of things outside of us' with 'the existence of the external world' as he calls the lecture 'proof of an external world', so I was assuming these to be equivalent - after reverting this, do you still think I am contradicting Moore now? If so, where do I go wrong?
    – Guenterino
    Mar 28 at 17:59
  • To prove existence of "things" in the plural, one thing is not enough. That is why Moore needs the second hand.
    – Conifold
    Mar 28 at 18:03
  • @Conifold Thank you - that clarifies things. That means this part of the whole argument is more satirical in nature, the actual argument is what comes before and after, i.e. steps 1. to 3. in my explanation above, and his later assertion that he can know things he cannot prove (like that he holds up a hand).
    – Guenterino
    Mar 28 at 18:06


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