" But whether I am so must admit of being established objectively. Suppose now I say "I'm incapable of being wrong about this: that is a book" while I point to an object. What would a mistake here be like? And have I any clear idea of it? "I know" often means: I have the proper grounds for my statement. So if the other person is acquainted with the language-game, he would admit that I know. The other, if he is acquainted with the language-game, must be able to imagine how one may know something of the kind. The statement "I know that here is a hand" may then be continued: "for it's my hand that I'm looking at." Then a reasonable man will not doubt that I know. - Nor will the idealist; rather he will say that he was not dealing with the practical doubt which is being dismissed, but there is a further doubt behind that one. - That this is an illusion has to be shown in a different way. "Doubting the existence of the external world" does not mean for example doubting the existence of a planet, which later observations proved to exist. - Or does Moore want to say that knowing that here is his hand is different in kind from knowing the existence of the planet Saturn? Otherwise it would be possible to point out the discovery of the planet Saturn to the doubters and say that its existence has been proved, and hence the existence of the external world as well. Moore's view really comes down to this: the concept 'know' is analogous to the concepts 'believe', 'surmise', 'doubt', 'be convinced' in that the statement "I know..." can't be a mistake. And if that is so, then there can be an inference from such an utterance to the truth of an assertion. And here the form "I thought I knew" is being overlooked. - But if this latter is inadmissible, then a mistake in the assertion must be logically impossible too. And anyone who is acquainted with the language-game must realize this - an assurance from a reliable man that he knows cannot contribute anything. It would surely be remarkable if we had to believe the reliable person who says "I can't be wrong"; or who says "I am not wrong". If I don't know whether someone has two hands (say, whether they have been amputated or not) I shall believe