"If someone believes that he has flown from America to England in the last few days, then, I believe, he cannot be making a mistake.
And just the same if someone says that he is at this moment sitting at a table and writing.
But even if in such cases I can’t be mistaken, isn’t it possible that I am drugged?”
If I am and if the drug has taken away my consciousness,
then I am not now really talking and thinking. I cannot seriously suppose that I am at this moment dreaming.
Someone who, dreaming, says “I am dreaming", even if he speaks audibly in doing so, is no more right than if he said in his dream “it is raining", while it was in fact raining. Even if his dream were connected with the noise of the rain."
Was just browsing through the wikipedia article http://en.wikipedia.org/wiki/Ludwig_Wittgenstein.