If the sceptic is right, the I do not know I have hands.
I know I have hands
Therefore, the sceptic is wrong.
The argument was first put forward by G.E Moor in 'Proof Of An External World'. Although the argument seems simple at first, its strength makes itself apparent in the attempts to offer rebbutals. Although Moore has not succeeded in proving that we have knowledge of an external world, he has shown that believing such a thing over the sceptics alternate position is less questionable. First I will try to outline both the sceptical view and Moore's argument and then turn to some criticisms of Moore's argument, I will conclude with an appeal to a non-naive view of common sense.
The sceptical argument is that we cannot have knowledge of an external world, if we cannot have knowledge of any of the particular aspects of the the external world. For instance I cannot know that I have hands, because there is always a plausible alternative which negates the truth value of the the fact that I have hands, and therefore the sceptic is right in asserting that we cannot know anything about the external world. The thought experiment often used, is that we are a brain in a vat hooked up to machine, which makes us believe all sorts of false informationa including the fact that we have hands.
Moore's response is to just negate the truth value of the second premis of the sceptics argument “we don't know we have hands” and in the process denying that the thought experiment has any real force. Instead he offers what he calls a rigourous proof, of his position; he shows by holding up one hand that he has a hand and then holding up another and saying there is another, and deduces from this that he has hands. The reason Moore thinks this is an effective argument is because the two premises are true, he doesn't just believe them to be true, and the argument is valid, therefore the argument is sound.
The first criticism usually offered up is that Moore has not demonstrated that he has knowledge of the external world, he has just assumed that he knows he has two hands, and then derived a proof, that he knows he has hands. And therefore has offered an argument that assumes the very thing he is trying to prove. Moore's argument is subtle here in that, he is trying to show that it is less questionable to believe that we have hands, than it is to believe that we don't, and therefore we should accept the former over the latter. The idea, I think, is that not only does moore think it is a better explanation, it also does not bring in superfluous information that no one can demonstrate, and the fact that he can demonstrate his argument at any time, means that it should be accepted over the thought that he might not have hands,...