What does Moore need the second hand for?

Question

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?

Answer 1

Moore’s argument:

1. Here is a hand.
2. Here is another hand.
3. Therefore, there now exists two hands.

3 proves that 'there are things outside of us'. This implies an external world exists.

According to Moore there are three conditions necessary for a proof to be considered rigorous (p9):

1. The premises must be known.
2. The conclusion must be different than the premise(s).
3. If the premises are true, then the conclusion must be true.

For the conclusion to be different than the premise(s), the second hand is necessary. Otherwise the premise would be identical to the conclusion. Now, in Moore's words: In asserting the premiss I was asserting much more than I was asserting in asserting the conclusion.

So there you have it: Moore hands are needed...