at them from all sides, tried to make sure it wasn't 'all done by mirrors', etc., we should not be sure whether we ought to call this doubting. We might describe his way of behaving as like the behaviour of doubt, but this game would be not be ours.
 On the other hand a language-game does change with time.
 If someone said to me that he doubted whether he had a body I should take him to be a half-wit. But I shouldn't know what it would mean to try to convince him that he had one. And if I had said something, and that had removed his doubt, I should not know how or why.
 I do not know how the sentence "I have a body" is to be used.
That doesn't unconditionally apply to the proposition that I have always been on or near the surface of the earth.
 Someone who doubted whether the earth had existed for 100 years might have a scientific, or on the other hand philosophical, doubt.
 I would like to reserve the expression "I know" for the cases in which it is used in normal linguistic exchange.
 I cannot at present imagine a reasonable doubt as to the existence of the earth during the last 100 years.
 I can imagine a man who had grown up in quite special circumstances and been taught that the earth came into being 50 years ago, and therefore believed this. We might instruct him: the earth has long... etc. - We should be trying to give him our picture of the world.
This would happen through a kind of persuasion.
 The schoolboy believes his teachers and his schoolbooks.
 I could imagine Moore being captured by a wild tribe, and their expressing the suspicion that he has come from somewhere between the earth and the moon. Moore tells them that he knows etc. but he can't give them the grounds for his certainty, because they have fantastic ideas of human ability to fly and know nothing about physics. This would be an occasion for making that statement.
 But what does it say, beyond "I have never been to such and such a place, and have compelling grounds to believe that"?
 And here one would still have to say what are compelling grounds.
 "I don't merely have the visual impression of a tree: I know that it is a tree".
 "I know that this is a hand." - And what is a hand? - "Well, this, for example".
 Am I more certain that I