I were asking the question: What reason have we for believing in the existence of other persons? in this sense, it would follow that if each of us has a reason for believing in his own existence, these reasons, taken together, would be a reason for believing in the existence of all of us. But I am not asking the question in this sense: it is plain that this is not its natural sense. What I do mean to ask is: Does each single one of us know any proposition, which is a reason for believing that others exist? I am using "we," that is to say, in the sense of "each of us." But again I do mean each of us: I am not merely asking whether some one man knows a proposition which is a reason for believing that other men exist. It would be possible that some one man, or some few men, should know such a proposition, and yet the rest know no such proposition. But I am not asking whether this is the case. I am asking whether among propositions of the kind which (as we commonly suppose) all or almost all men know, there is any which is a reason for supposing that other men exist. And in asking this question I am not begging the question by supposing that all men do exist. My question might, I think, be put quite accurately as follows. There are certain kinds of belief which, as we commonly suppose, all or almost all men share. I describe this kind of belief as "our" beliefs, simply as an easy way of pointing out which kind of belief I mean, but without assuming that all men do share them. And I then ask: Supposing a single man to have beliefs of this kind, which among them would be a good reason for supposing that other men existed having like beliefs?
This, then, is the first restriction which I put upon the meaning of my question. And it is, I think, a restriction which, in their natural meaning, the words suggest. When we ask: What reason have we for believing that other people exist? we naturally understand that question to be equivalent to: What reason has each of us for that belief? And this question again is naturally equivalent to the question: Which among the propositions that a single man believes, but which are of the kind which (rightly or wrongly) we assume all men to believe, are such that they would not be true unless some other person than that man existed? But there is another restriction which, I think, the words of my question also naturally suggest. If we were to ask anyone the question: How do you know that you did see that statement in the Times? and he were to answer "Because I did see it in the Times and in the Standard too," we should not think that he had given us a reason for the belief that he saw it in the Times. We should not think his answer a reason, because it asserts the very thing for which we require a reason. And similarly when I ask: How do we know that any thing or person exists, other than ourselves and what we directly perceive? What reason have we for believing this? I must naturally be understood to mean: What proposition, other than one which itself asserts or presupposes the existence of something beyond ourselves and our own perceptions, is a reason for supposing that such a thing exists? And this restriction obviously excludes an immense number of propositions of a kind which all of us do believe. We all of us believe an immense number of different propositions about the existence of things which we do not directly perceive, and many of these propositions are, in my sense, good reasons for believing in the existence of still other things. The belief in the existence of a statement in the Times, when we have not seen that statement, may, as I implied, be a good reason for believing that someone is dead. But no such proposition can be a good answer to my question, because it asserts the very kind of thing for which I require a reason: it asserts the existence of something other than myself and what I directly perceive. When I am asking: What reason have I for believing in the existence of anything but myself, my own perceptions, and what I do directly perceive? you would naturally understand me to mean: What reason, other than the existence of such a thing, have I for this belief?
Each of us, then, we commonly assume, believes some true propositions, which do not themselves assert the existence of anything other than himself, his own perceptions, or what he directly perceives. Each of us, for instance, believes that he himself has and has had certain particular perceptions: and these propositions are propositions of the kind I mean—propositions which do not themselves assert the existence of anything other than himself, his own perceptions, and what he directly perceives: they are, I think, by no means the only propositions of this kind, which most of us believe: but they are propositions of this kind. But, as I say, I am not assuming that each of us—each of several different people—does believe propositions of this kind. All that I assume is that at least one man does believe some such propositions. And then I ask: Which among those true propositions, which one man believes, are such that they would probably not be true, unless some other man existed and had certain particular perceptions? Which among them are such that it follows (in the wide sense, which I have explained) from their truth, that it is more likely than not that some other man has perceptions? This is the meaning of my question, so far as I have hitherto explained it: and I hope this meaning is quite clear. It is in this sense that I am asking: What reason have we for believing that other people exist? How do we know that they exist? This, indeed, is not all that I mean by that question: there is one other point—the most important one—which remains to be explained. But this is part of what I mean to ask; and before I go on to explain what else I mean, I wish first to stop and enquire what is the answer to this part of my question. What is the answer to the question: Which among the true propositions, of a kind which (as we commonly assume) each of us believes, and which do not themselves assert the existence of anything other than that person himself, his own perceptions, or what he directly perceives, are such that they would probably not be true unless some other person existed, who had perceptions in some respects similar to his own?
Now to this question the answer is very obvious. It is very obvious that in this sense we have reasons for believing in the existence of other persons, and also what some of those reasons are. But I wish to make it quite plain that this is so: that in this sense one man has a reason for believing that another has certain perceptions. All that I am asking you to grant, is, you see, that some of you would not be having just those perceptions which you now have, unless I, as I read this paper, were perceiving more or less black marks on a more or less white ground; or that I on the other hand, should not be having just those perceptions which I now have, unless some other persons than myself were hearing the sounds of my voice. And I am not asking you even to grant that this is certain—only that it is positively probable—more likely than not. Surely it is very obvious that this proposition is true. But I wish to make it quite clear what would be the consequences of denying that any such propositions are true—propositions which assert that the existence of certain perceptions in one man are a reason for believing in the existence of certain perceptions in another man—which assert that one man would probably not have had just those perceptions which he did have, unless some other man had had certain particular perceptions. It is plain, I think, that, unless some such propositions are true, we have no more reason for supposing that Alexander the Great ever saw an elephant, than for supposing that Sindbad the Sailor saw a Roc; we have no more reason for supposing that anybody saw Julius Caesar murdered in the Senate House at Rome, than for supposing that somebody saw him carried up to Heaven in a fiery chariot. It is plain, I think, that if we have any reason at all for supposing that in all probability Alexander the Great did see an elephant, and that in all probability no such person as Sindbad the Sailor ever saw a Roc, part of that reason consists in the assumption that some other person would probably not have had just those perceptions which he did have, unless Alexander the Great had seen an elephant, and unless Sindbad the Sailor had not seen a Roc. And most philosophers, I think, are willing to admit that we have some reason, in some sense or other, for such propositions as these. They are willing to admit not only that some persons probably did see Julius Caesar murdered in the Senate House; but also that some persons, other than those who saw it, had and have some reason for supposing that some one else probably saw it. Some sceptical philosophers might, indeed, deny both propositions; and to refute their views, I admit, other arguments are needed than any which I shall bring forward in this paper. But most philosophers will, I think, admit not only that facts, for which there is, as we say, good historical evidence, are probably true; but also that what we call good historical evidence really is in some sense a good reason for thinking them true. Accordingly I am going to assume that many propositions of the following kind are true. Propositions, namely, which assert that one man would probably not have certain perceptions which he does have, unless some other man had certain particular perceptions. That some of you, for instance, would probably