Certainty

Suppose I say: 'I know for certain that I am standing up; it is absolutely certain that I am; there is not the smallest chance that I am not.' Many philosophers would say: 'You are wrong: you do not know that you are standing up; it is not absolutely certain that you are; there is some chance, though perhaps only a very small one, that you are not.' And one argument which has been used as an argument in favour of saying this, is an argument in the course of which the philosopher who used it would assert: 'You do not know for certain that you are not dreaming; it is not absolutely certain that you are not; there is some chance, though perhaps only a very small one, that you are.' And from this, that I do not know for certain that I am not dreaming, it is supposed to follow that I do not know for certain that I am standing up. It is argued: If it is not certain that you are not dreaming, then it is not certain that you are standing up. And that if I don't know that I'm not dreaming, I also don't know that I'm not sitting down, I don't feel at all inclined to dispute. From the hypothesis that I am dreaming, it would, I think, certainly follow that I don't know that I am standing up; though I have never seen the matter argued, and though it is not at all clear to me how it is to be proved that it would follow. But, on the other hand, from the hypothesis that I am dreaming, it certainly would not follow that I am not standing up; for it is certainly logically possible that a man should be fast asleep and dreaming, while he is standing up and not lying down. It is therefore logically possible that I should both be standing up and also at the same time dreaming that I am; just as the story, about a well-known Duke of Devonshire, that he once dreamt that he was speaking in the House of Lords and, when he woke up, found that he was speaking in the House of Lords, is certainly logically possible. And if, as is commonly assumed, when I am dreaming that I am standing up it may also be correct to say that I am thinking that I am standing up, then it follows that the hypothesis that I am now dreaming is quite consistent with the hypothesis that I am both thinking that I am standing up and also actually standing up. And hence, if as seems to me to be certainly the case and as this argument assumes, from the hypothesis that I am now dreaming it would follow that I don't know that I am standing up, there follows a point which is of great importance with regard to our use of the word 'knowledge,' and therefore also of the word 'certainty' — a point which has been made quite conclusively more than once by Russell, namely that from the conjunction of the two facts that a man thinks that a given proposition p is true, and that p is in fact true, it does not follow that the man in question knows that p is true: in order that I may be justified in saying that I know that I am standing up, something more is required than the mere conjunction of the two facts that I both think I am and actually am — as Russell has expressed it, true belief is not 463identical with knowledge; and I think we may further add that even from the conjunction of the two facts that I feel certain that I am and that I actually am it would not follow that I know that I am, nor therefore that it is certain that I am. As regards the argument drawn from the fact that a man who dreams that he is standing up and happens at the moment actually to be standing up will nevertheless not know that he is standing up, it should indeed be noted that from the fact that a man is dreaming that he is standing up, it certainly does not follow that he thinks he is standing up; since it does sometimes happen in a dream that we think that it is a dream, and a man who thought this certainly might, although he was dreaming that he was standing up, yet think that he was not, although he could not know that he was not. It is not therefore the case, as might be hastily assumed, that, if I dream that I am standing up at a time when I am in fact lying down, I am necessarily deceived: I should be deceived only if I thought I was standing when I wasn't; and I may dream that I am, without thinking that I am. It certainly does, however, often happen that we do dream that so-and-so is the case, without at the time thinking that we are only dreaming; and in such cases, I think we may perhaps be said to think that what we dream is the case is the case, and to be deceived if it is not the case; and therefore also, in such cases, if what we dream to be the case happens also to be the case, we may be said to be thinking truly that it is the case, although we certainly do not know that it is.