The Nature and Reality of the Objects of Perception

There are two beliefs in which almost all philosophers, and almost all ordinary people are agreed. Almost everyone believes that he himself and what he directly perceives do not constitute the whole of reality: he believes that something other than himself and what he directly perceives exists or is real. I do not mean to say that almost everyone believes that what he directly perceives is real: I only mean that he does believe that, whether what he directly perceives is real or not, something other than it and other than himself certainly is so. And not only does each of us thus agree in believing that something other than himself and what he directly perceives is real: almost everyone also believes that among the real things, other than himself and what he directly perceives, are other persons who have thoughts and perceptions in some respects similar to his own. That most people believe this I think I need scarcely try to show. But since a good many philosophers may appear to have held views contradictory of this one, I will briefly point out my reason for asserting that most philosophers, even among those (if any) who have believed the contradictory of this, have yet held this as well. Almost all philosophers tell us something about the nature of human knowledge and human perception. They tell us that we perceive so and so; that the nature or origin of our perceptions is such and such; or (as I have just been telling you) that men in general have such and such beliefs. It might, indeed, be said that we are not to interpret such language too strictly: that, though a philosopher talks about human knowledge and our perceptions, he only means to talk about his own. But in many cases a philosopher will leave no doubt upon this point, by expressly assuming that there are other perceptions, which differ in some respects from his own: such, for instance, is the case when (as is so common nowadays) a philosopher introduces psycho-genetic considerations into his arguments—considerations concerning the nature of the perceptions of men who existed before and at a much lower stage of culture than himself. Any philosopher, who uses such arguments, obviously assumes that perceptions other than his own have existed or been real. And even those philosophers who think themselves justified in the conclusion that neither their own perceptions nor any perceptions like theirs are ultimately real, would, I think admit, that phenomenally, at least, they are real, and are certainly more real than some other things.

Almost everyone, then, does believe that some perceptions other than his own, and which he himself does not directly perceive, are real; and believing this, he believes that something other than himself and what he directly perceives is real. But how do we know that anything exists except our own perceptions, and what we directly perceive? How do we know that there are any other people, who have perceptions in some respects similar to our own?

I believe that these two questions express very exactly the nature of the problem which it is my chief object, in this paper, to discuss. When I say these words to you, they will at once suggest to your minds the very question, to which I desire to find an answer; they will convey to you the very same meaning which I have before my mind, when I use the words. You will understand at once what question it is that I mean to ask. But, for all that, the words which I have used are highly ambiguous. If you begin to ask yourselves what I do mean by them, you will find that there are several quite different things which I might mean. And there is, I think, great danger of confusing these different meanings with one another. I think that philosophers, when they have asked this question in one sense, have often answered it in quite a different sense; and yet have supposed that the answer which they have given is an answer to the very same question which they originally asked. It is precisely because there is this ambiguity—this danger of confusion, in the words which I have used, that I have chosen to use them. I wish to point out as clearly as I can, not only what I do mean by them, but also some things which I do not mean; and I wish to make it clear that the questions which I do not mean to ask, are different questions from that which I do mean to ask.

I will take the second of my two questions, since there is in the other an additional ambiguity to which I do not now wish to call attention. My second question was: How do we know that there exist any other people who have perceptions in some respects similar to our own? What does this question mean?

Now I think you may have noticed that when

you make a statement to another person, and he answers “How do you know that that is so?” he very often means to suggest that you do not know it. And yet, though he means to suggest that you do not know it, he may not for a moment wish to suggest that you do not believe it, nor even that you have not that degree or kind of conviction, which goes beyond mere belief, and which may be taken to be essential to anything which can properly be called knowledge. He does not mean to suggest for a moment that you are saying something which you do not believe to be true, or even that you are not thoroughly convinced of its truth. What he does mean to suggest is that what you asserted was not true, even though you may not only have believed it but felt sure that it was true. He suggests that you don’t know it, in the sense that what you believe or feel sure of is not true.

Now I point this out, not because I myself mean to suggest that we don’t know the existence of other persons, but merely in order to show that the word “know” is sometimes used in a sense in which it is not merely equivalent to “believe” or “feel sure of.” When the question “How do you know that?” is asked, the questioner does not merely mean to ask “how do you come to believe that, or to be convinced of it?” He sometimes, and I think generally, means to ask a question with regard to the truth, and not with regard to the existence of your belief. And similarly when I ask the question “How do we know that other people exist?” I do not mean to ask “How do we come to believe in or be convinced of their existence?” I do not intend to discuss this question at all. I shall not ask what suggests to us our belief in the existence of other persons or of an external world; I shall not ask whether we arrive at it by inference or by “instinct” or in any other manner, which ever has been or may be suggested: I shall discuss no question of any kind whatever with regard to its origin, or cause, or the way in which it arises. These psychological questions are not what I propose to discuss. When I ask the question “How do we know that other people exist?” I do not mean: “How does our belief in their existence arise?”

But if I do not mean this what do I mean? I have said that I mean to ask a question with regard to the truth of that belief; and the particular question which I mean to ask might be expressed in the words: What reason have we for our belief in the existence of other persons? But these are words which themselves need some explanation, and I will try to give it.

In the first place, then, when I talk of “a reason,” I mean only a good reason and not a bad one. A bad reason is, no doubt, a reason, in one sense of the word; but I mean to use the word “reason” exclusively in the sense in which it is equivalent to “good reason.” But what, then, is meant by a good reason for a belief? I think I can express sufficiently accurately what I mean by it in this connection, as follows:—A good reason for a belief is a proposition which is true, and which would not be true unless the belief were also true. We should, I think, commonly say that when a man knows such a proposition, he has a good reason for his belief; and, when he knows no such proposition, we should say that he has no reason for it. When he knows such a proposition, we should say he knows something which is a reason for thinking his belief to be true—something from which it could be validly inferred. And if, in answer to the question “How do you know so and so?” he were to state such a proposition, we should, I think, feel that he had answered the question which we meant to ask. Suppose, for instance, in answer to the question “How do you know that?” he were to say “I saw it in the Times.” Then, if we believed that he had seen it in the Times, and also believed that it would not have been in the Times, unless it had been true, we should admit that he had answered our question.

We should no longer doubt that he did know what he asserted, we should no longer doubt that his belief was true. But if, on the other hand, we believed that he had not seen it in the Times—if, for instance, we had reason to believe that what he saw was not the statement which he made, but some other statement which he mistook for it; or if we believed that the kind of statement in question was one with regard to which there was no presumption that, being in the Times, it would be true: in either of these cases we should, I think, feel that he had not answered our question. We should still doubt whether what he had said was true. We should still doubt whether he knew what he asserted; and since a man cannot tell you how he knows a thing unless he does know that thing, we should think that, though he might have told us truly how he came to believe it, he had certainly not told us how he knew it. But though we should thus hold that he had not told us how he knew what he had asserted, and that he had given us no reason for believing it to be true; we must yet admit that he had given us a reason in a sense—a bad reason, a reason which was no reason because it had no tendency to show that what he believed was true; and we might also be perfectly convinced that he had given us the reason why he believed it—the proposition by believing which he was induced also to believe his original assertion.

I mean, then, by my question, “How do we know that other people exist?” what, I believe, is ordinarily meant, namely, “What reason have we for believing that they exist?” and by this again I mean, what I also believe is ordinarily meant, namely, “What proposition do we believe, which is both true itself and is also such that it would not be true, unless other people existed?” And I hope it is plain that this question, thus explained, is quite a different question from the psychological question, which I said I did not mean to ask—from the question, “How does our belief in the existence of other people arise?” My illustration, I hope, has made this plain. For I have pointed out that we may quite well hold that a man has told us how a belief of his arises, and even what was the reason which made him adopt that belief, and yet may have failed to give us any good reason for his belief—any proposition which is both true itself, and also such that the truth of his belief follows from it. And, indeed, it is plain that if any one ever believes what is false, he is believing something for which there is no good reason, in the sense which I have explained, and for which, therefore, he cannot possibly have a good reason; and yet it plainly does not follow that his belief did not arise in anyway whatever, nor even that he had no reason for it—no bad reason. It is plain that false beliefs do arise in some way or other—they have origins and causes: and many people who hold them have bad reasons for holding them—their belief does arise (by inference or otherwise) from their belief in some other proposition, which is not itself true, or else is not a good reason for holding that, which they infer from it, or which, in some other way, it induces them to believe. I submit, therefore, that the question, “What good reason have we for believing in the existence of other people?” is different from the question, “How does that belief arise?” But when I say this, I must not be misunderstood; I must not be understood to affirm that the answer to both questions may not, in a sense, be the same. I fully admit that the very same fact, which suggests to us the belief in the existence of other people, may also be a good reason for believing that they do exist. All that I maintain is that the question whether it is a good reason for that belief is a different question from the question whether it suggests that belief: if we assert that a certain fact both suggests our belief in the existence of other persons and is also a good reason for holding that belief, we are asserting two different things and not one only. And hence, when I assert, as I shall assert, that we have a good reason for our belief in the existence of other persons, I must not be understood also to assert either that we infer the existence of other persons from this good reason, or that our belief in that good reason suggests our belief in the existence of other persons in any other way. It is plain, I think, that a man may believe two true propositions, of which the one would not be true, unless the other were true too, without, in any sense whatever, having arrived at his belief in the one from his belief in the other; and it is plain, at all events, that the question whether his belief in the one did arise from his belief in the other, is a different question from the question whether the truth of the one belief follows from the truth of the other.

I hope, then, that I have made it a little clearer what I mean by the question: “What reason have we for believing in the existence of other people?” and that what I mean by it is at all events different from what is meant by the question: “How does our belief in the existence of other people arise?” But I am sorry to say that I have not yet reached the end of my explanations as to what my meaning is. I am afraid that the subject may seem very tedious. I can assure you that I have found it excessively tedious to try to make my meaning clear to myself. I have constantly found that I was confusing one question with another, and that, where I had thought I had a good reason for some assertion, I had in reality no good reason. But I may perhaps remind you that this question, “How do we know so and so?” “What reason have we for believing it?” is one of which philosophy is full; and one to which the most various answers have been given. Philosophy largely consists in giving reasons; and the question what are good reasons for a particular conclusion and what are bad, is one upon which philosophers have disagreed as much as on any other question. For one and the same conclusion different philosophers have given not only different, but incompatible, reasons; and conversely different philosophers have maintained that one and the same fact is a reason for incompatible conclusions. We are apt, I think, sometimes to pay too little attention to this fact. When we have taken, perhaps, no little pains to assure ourselves that our own reasoning is correct, and especially when we know that a great many other philosophers agree with us, we are apt to assume that the arguments of those philosophers, who have come to a contradictory conclusion, are scarcely worthy of serious consideration. And yet, I think, there is scarcely a single reasoned conclusion in philosophy, as to which we shall not find that some other philosopher, who has, so far as we know, bestowed equal pains on his reasoning, and with equal ability, has reached a conclusion incompatible with ours. We may be satisfied that we are right, and we may, in fact, be so; but it is certain that both cannot be right: either our opponent or we must have mistaken bad reasons for good. And this being so, however satisfied we may be that it is not we who have done so, I think we should at least draw the conclusion that it is by no means easy to avoid mistaking bad reasons for good; and that no process, however laborious, which is in the least likely to help us in avoiding this should be evaded. But it is at least possible that one source of error lies in mistaking one kind of reason for another—in supposing that, because there is, in one sense, a reason for a given conclusion, there is also a reason in another, or that because there is, in one sense, no reason for a given conclusion, there is, therefore, no reason at all. I believe myself that this is a very frequent source of error: but it is at least a possible one. And where, as disagreements show, there certainly is error on one side or the other, and reason, too, to suppose that the error is not easy to detect, I think we should spare no pains in investigating any source, from which it is even possible that the error may arise. For these reasons I think I am perhaps doing right in trying to explain as clearly as possible not only what reasons we have for believing in an external world, but also in what sense I take them to be reasons.

I proceed, then with my explanation. And there is one thing, which, I think my illustration has shown that I do not mean. I have defined a reason for a belief as a true proposition, which would not be true unless the belief itself—what is believed— were also true; and I have used, as synonymous with this form of words, the expressions: A reason for a belief is a true proposition from which the truth of the belief follows from which it could be validly inferred. Now these expressions might suggest the idea that I mean to restrict the word “reason,” to what, in the strictest sense, might be called a logical reason—to propositions from which the belief in question follows, according to the rules of inference accepted by Formal Logic. But I am not using the words “follow,” “validly inferred,” in this narrow sense; I do not mean to restrict the words “reason for a belief” to propositions from which the laws of Formal Logic state that the belief could be deduced. The illustration which I gave is inconsistent with this restricted meaning. I said that the fact that a statement appeared in the Times might be a good reason for believing that that statement was true. And I am using the word “reason” in the wide and popular sense, in which it really might be. If, for instance, the Times stated that the King was dead, we should think that was a good reason for believing that the King was dead; we should think that the Times would not have made such a statement as that unless the King really were dead. We should, indeed, not think that the statement in the Times rendered it absolutely certain that the King was dead. But it is extremely unlikely that the Times would make a statement of this kind unless it were true; and, in that sense, the fact of the statement appearing in the Times would render it highly probable--much more likely than not—that the King was dead. And I wish it to be understood that I am using the words “reason for a belief” in this extremely wide sense. When I look for a good reason for our belief in the existence of other people, I shall not reject any proposition merely on the ground that it only renders their existence probable—only shows it to be more likely than not that they exist. Provided that the proposition in question does render it positively probable that they exist, then, if it also conforms to the conditions which I am about to mention, I shall call it a “good reason.”

But it is not every proposition which renders it probable that other people exist, which I shall consider to be a good answer to my question. I have just explained that my meaning is wide in one direction—in admitting some propositions which render a belief merely probable; but I have now to explain that it is restricted in two other directions. I do mean to exclude certain propositions which do render that belief probable. When I ask: What reason have we for believing in the existence of other people? a certain ambiguity is introduced by the use of the plural “we.” If each of several different persons has a reason for believing that he himself exists, then it is not merely probable, but certain, according to the rules of Formal Logic, that, in a sense, they “have a reason for believing” that several people exist; each has a reason for believing that he himself exists; and, therefore, all of them, taken together, have reasons for supposing that several persons exist. If, therefore, 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 Cæsar 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 Cæsar 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 not be having precisely the perceptions which you are having, unless I were having the perception of more or less black marks on a more or less white ground. And, in this sense, I say, we certainly have reasons for supposing that other people have perceptions similar, in some respects, to those which we sometimes have.

But when I said I was going to ask the question:

What reason have we for supposing that other people exist? you will certainly not have thought that I merely meant to ask the question which I have just answered. My words will have suggested to you something much more important than merely this. When, for instance, I said that to the question “How do you know that?” the answer “I saw it in the Times” would be a satisfactory answer, you may have felt, as I felt, that it would not in all circumstances be regarded as such. The person who asked the question might, in some cases, fairly reply: “That is no answer: how do you know that, because you saw a thing in the Times, it is therefore true?” In other words he might ask for a reason for supposing that the occurrence of a particular statement in the Times was a reason for supposing that statement true. And this is a question to which we all believe that there may be an answer. We believe that, with some kinds of statements which the Times makes—some kinds of statements with regard to Fiscal Policy for example—the fact that the Times makes them is no reason for supposing them to be true: whereas with regard to other kinds of statements, which it makes, such a statement, for instance, as that the King was dead, the fact that it makes them is a reason for supposing them true. We believe that there are some kinds of statements, which it is very unlikely the Times would make, unless they were true; and others which it is not at all unlikely that the Times might make, although they were not true. And we believe that a reason might be given for distinguishing, in this way, between the two different kinds of statement: for thinking that, in some cases (on points, for instance, which, as we should say, are not simple questions of fact) the Times is fallible, whereas in other cases, it is, though not absolutely infallible, very unlikely to state what is not true.

Now it is precisely in this further sense that I wish to consider: what reason have we for believing that certain particular things, other than ourselves, our own perceptions, and what we directly perceive, are real? I have asserted that I do have certain perceptions, which it is very unlikely I should have, unless some other person had certain particular perceptions; that, for instance, it is very unlikely I should be having precisely those perceptions which I am now having unless someone else were hearing the sound of my voice. And I now wish to ask: What reason have I for supposing that this is unlikely? What reason has any of us for supposing that any such proposition is true? And I mean by “having a reason” precisely what I formerly meant. I mean: What other proposition do I know, which would not be true, unless my perception were connected with someone else’s perception, in the manner in which I asserted them to be connected? Here again I am asking for a good reason; and am not asking a psychological question with regard to origin. Here again I am not asking for a reason, in the strict sense of Formal Logic; I am merely asking for a proposition which would probably not be true, unless what I asserted were true. Here again I am asking for some proposition of a kind which each of us believes; I am asking: What reason has each of us for believing that some of his perceptions are connected with particular perceptions of other people in the manner I asserted? —for believing that he would not have certain perceptions that he does have, unless some other person had certain particular perceptions? And here again I am asking for a reason—I am asking for some proposition other than one which itself asserts: When one man has a perception of such and such a particular kind, it is probable that another man has a perception or thought of this or that other kind.

But what kind of reason can be given for believing a proposition of this sort? For believing a proposition which asserts that, since one particular thing exists, it is probable that another particular thing also exists? One thing I think is plain, namely that we can have no good reason for believing such a proposition, unless we have good reason for believing some generalisation. It is commonly believed, for instance, that certain so-called flint arrow-heads, which have been discovered, were probably made by prehistoric men; and I think it is plain that we have no reason for believing this unless we have reason to suppose that objects which resemble these in certain particular respects are generally made by men—are more often made by men than by any other agency. Unless certain particular characteristics which those arrow-heads have were characteristics which belonged at least more frequently to articles of human manufacture than to any articles not made by men, it would surely be just as likely as not that these arrowheads were not made by men—that they were, in fact not arrow-heads. That is to say, unless we have reason to assert a generalisation—the generalisation that objects of a certain kind are generally made by men, we have no reason to suppose that these particular objects, which are of the kind in question, were made by men. And the same, so far as I can see, is true universally. If we ever have any reason for asserting that, since one particular thing exists, another probably exists or existed or will exist also part of our reason, at least, must consist in reasons for asserting some generalisation —for asserting that the existence of things of a particular kind is, more often than not, accompanied or preceded or followed by the existence of things of another particular kind. It is, I think, sometimes assumed that an alternative to this theory may be found in the theory that the existence of one kind of thing “intrinsically points to,” or is “intrinsically a sign or symbol of” the existence of another thing. It is suggested that when a thing which thus points to the existence of another thing exists, then it is at least probable that the thing “pointed to” exists also. But this theory, I think, offers no real alternative. For, in the first place, when we say that the existence of one thing A is a “sign of or “points to” the existence of another thing B, we very commonly actually mean to say that when a thing like A exists, a thing like B generally exists too. We may, no doubt, mean something else too; but this we do mean. We say, for instance, that certain particular words, which we hear or read, are a “sign” that somebody has thought of the particular things which we call the meaning of those words. But we should certainly hesitate to admit that the hearing or reading of certain words could be called a “sign” of the existence of certain thoughts, unless it were true that when those words are heard or read, the thoughts in question generally have existed. If when those words were heard or read, the thoughts had generally not existed, we should say that, in one sense of the word at all events, the hearing of the words was not a sign of the existence of the thoughts. In this sense, therefore, to say that the existence of A “points to” or “is a sign of” the existence of B is actually to say that when A exists, B generally exists also. But, no doubt, the words “points to” “is a sign of” may be used in some other sense: they may, for instance, mean only that the existence of A suggests in some way the belief that B exists. And in such a case we certainly might know that the existence of A pointed to the existence of B, without knowing that when A existed B generally existed also. Let us suppose, then, that in some such sense A does “point to” the existence of B; can this fact give us a reason for supposing it even probable that B existed. Certainly it can, provided it is true that when A does point to the existence of B, B generally exists. But surely it can do so, only on this condition. If when A points to the existence of B, B, nevertheless, does not generally exist, then surely the fact that A points to the existence of B can constitute no probability that B does not exist: on the contrary it will then be probable that, even though A “points to’’ the existence of B, B does not exist. We have, in fact, only substituted the generalisation that As pointing to B is generally accompanied by the existence of B, for the generalisation that A’s existence is generally accompanied by the existence of B. If we are to have any reason for asserting that, when A points to or is a sign of the existence of B, B probably exists, we must still have a reason for some generalisation—for a generalisation which asserts that when one thing points to the existence of another, that other generally exists.

It is plain, then, I think, that if we are to find a reason for the assertion that some particular perception of mine would probably not exist, unless someone else were having or had had a perception of a kind which I can name, we must find a reason for some generalisation. And it is also plain, I think, that in many cases of this kind the generalisation must consist in an assertion that when one man has a certain kind of perception, some other man generally has had some other perception or belief. We assume, for instance, that when we hear or read certain words, somebody besides ourselves has thought the thoughts, which constitute the meaning of those words; and it is plain, I think, that we have no reason for this assumption except one which is also a reason for the assumption that when certain words are heard or read, somebody generally has had certain thoughts. And my enquiry, therefore, at least includes the enquiry: What reasons have we for such generalisations as these? for generalisations which assert a connection between the existence of a certain kind of perception in one man, and that of a certain kind of perception or belief in another man?

And to this question, I think, but one answer can be given. If we have any reason for such generalisations at all, some reason must be given, in one way or another, by observation—by observation, understood in the wide sense in which it includes “experiment.” No philosopher, I think, has ever failed to assume that observation does give a reason for some generalisations—for some propositions which assert that when one kind of thing exists, another generally exists or has existed in a certain relation to it. Even those who, like Hume, imply that observation cannot give a reason for anything, yet constantly appeal to observation in support of generalisations of their own. And even those who hold that observation can give no reason for any generalisation about the relation of one man’s perceptions to another’s, yet hold that it can give a reason for generalisations about the relation of some to others among a man’s own perceptions. It is, indeed, by no means agreed how observation can give a reason for any generalisation. Nobody knows what reason we have, if we have any, for supposing that it can. But that it can, everyone, I think, assumes. I think, therefore, most philosophers will agree, that if we can find any reason at all for generalisations of the kind in which I am interested, a reason for some of them at all events must be found in observation. And what I propose to ask is: What reason can be found in observation for even a single proposition of the kind I have described? for a proposition which asserts that when one man has one kind of perception, another man generally has or has had another.

But, when it is said that observation gives us a reason for generalisations, two things may be meant neither of which I mean. In the first place, we popularly use “observation” in a sense in which we can be said to observe the perceptions, feelings and thoughts of other people: in which, therefore, we can be said to observe the very things with regard to which I am asking what reason we have for believing in their existence. But it is universally Not now in 1921. ² agreed that there is a sense in which no man can observe the perceptions, feelings or thoughts of any other man. And it is to this strict sense that I propose to confine the word. I shall use it in a sense, in which we can certainly be said to observe nothing but ourselves, our own perceptions, thoughts and feelings, and what we directly perceive. And in the second place, it may be said that observations made by another person may give me a reason for believing some generalisation. And it is certainly the case that for many of the generalisations in which we all believe, if we have a reason in observation at all, it is not in our own observation that we have it: part of our reason, at all events, lies in things which other people have observed but which we ourselves have not observed. But in asking this particular question, I am not asking for reasons of this sort. The very question that I am asking is: What reason has any one of us for supposing that any other person whatever has ever made any observations? And just as, in the first meaning which I gave to this question, it meant: What thing, that any single man observes is such that it would probably not have existed, unless some other man had made a particular observation? So now I am asking: Which among the things, which one single man observes, are such that they would probably not have existed, unless it were true that some of them generally stood in certain relations to observations of some other person? I am asking: Which among my own observations give me a reason for supposing that some of them are of a kind which are generally preceded or accompanied by observations of other people? Which, for instance, among my own observations give a good reason for the generalisation that when I hear certain words, somebody else has generally had certain particular thoughts, or that whenever anyone hears certain words, somebody else has generally had the thoughts which constitute what we call the meaning of those words? I am asking: Which among the vast series of observations, which any one individual makes during his lifetime, give a good reason for any generalisation whatever of this kind—a generalisation which asserts that some of them are generally preceded by certain thoughts, perceptions or feelings in other persons? I quite admit that there are some generalisations of this kind for which the observations of some particular men will not give a reason. All that I ask is: Is there even one generalisation of this kind, for which the kind of observations, which (as we commonly assume) each man, or nearly every man does make, do give a reason? Among observations of the kind which (as we commonly assume) are common to you and to me, do yours, by themselves, give any reason for even one such generalisation? And do mine, by themselves, give any reason for even one such generalisation? And if they do, which, among these observations, is it which do so?

My question is, then: What reason do my own observations give me, for supposing that any perception whatever, which I have, would probably not occur, unless some other person had a certain kind of perception? What reason do my own observations give me for supposing, for instance, that I should not be perceiving what I do now perceive, unless someone were hearing the sound of my voice? What reason do your own observations give you for supposing that you would not be perceiving just what you are perceiving, unless I were perceiving more or less black marks on a more or less white ground? The question does, I think, appear to be a reasonable one; and most philosophers, I think, have assumed that there is an answer to it. Yet it may be said that there is no answer to it: that my own observations give me no reason whatever for any single proposition of this kind. There are certain philosophers (even apart from thorough sceptics, with whom, as I have said, I am not now arguing) who have denied that they do. There are certain philosophers who hold that nothing which any single one of us observes or can observe, gives the slightest reason for supposing that any of his own perceptions are generally connected with certain perceptions in other people. There are philosophers who hold that the only generalisations for which our own observations do give any warrant are generalisations concerning the manner in which our own perceptions, thoughts and feelings do and probably will succeed one another; and who conclude that, this being so, we have no reason whatever for believing in the existence of any other people. And these philosophers are, I think, right in drawing this conclusion from this premiss. It does not, indeed, follow from their premiss that we have not a reason in the sense which I first explained, and in which, I insisted, it must be admitted that we have a reason. It does not follow that some of our perceptions are not such as would probably not exist, unless some other person had certain perceptions. But, as I have urged, when we say that we have a reason for asserting the existence of something not perceived, we commonly mean something more than this. We mean not only that, since what we perceive does exist, the unperceived thing probably exists too; we mean also that we have some reason for asserting this connection between the perceived and the unperceived. And holding, as we do, that no reason can be given for asserting such a connection, except observation, we should say that, if observation gives no reason for asserting it, we have no reason for asserting it; and having no reason for asserting this connection between the perceived and the unperceived, we should say that we have none either for asserting the even probable existence of the unperceived. This, I think, is what we commonly mean by saying that we have no reason to believe in the existence of a particular thing which we do not perceive. And hence, I think, those philosophers who hold that our own observations give us no reason whatever for any generalisation whatever concerning the connection of any of them with those of other people, are quite right in concluding that we have no reason to assert that any other person ever did have any particular thought or perception whatever. I think that the words of this conclusion, understood in their natural meaning, express precisely what the premiss asserts. We need not, indeed, conclude, as many of these philosophers are inclined to do, that, because we have no reason for believing in the existence of other people, it is therefore highly doubtful whether they do exist. The philosophers who advocate this opinion commonly refute themselves by assigning the existence of other people as part of their reason for believing that it is very doubtful whether any other people exist. That for which we have no reason may, nevertheless, be certainly true. And, indeed, one of the philosophers who hold most clearly and expressly that we do know not only the existence of other people but also that of material objects, is also one of those who deny most emphatically that our own observations can give any reason for believing either in the one or in the other. I refer to Thomas Reid. Reid, indeed, allows himself to use not only the word “observe,” but even the word “perceive,” in that wide sense in which it might be said that we observe or perceive the thoughts and feelings of others: and I think that the fact that he uses the words in this sense, has misled him into thinking that his view is more plausible and more in accordance with Common Sense than it really is: by using the words in this sense he is able to plead that “observation” really does give a reason for some of those generalisations, for which Common Sense holds that “observation” (in a narrower sense) does give a reason. But with regard to what we observe or perceive, in the strict sense to which I am confining those words, he asserts quite explicitly that it gives us no reason either for believing in the existence of material objects or for believing in the existence of other minds. Berkeley, he says, has proved incontrovertibly that it gives us no reason for the one, and Hume that it gives us no reason for the other.

Now these philosophers may be right in holding this. It may, perhaps, be true that, in this sense, my own observations give me no reason whatever for believing that any other person ever has or will perceive anything like or unlike what I perceive. But I think it is desirable we should realise how paradoxical are the consequences which must be admitted, if this is true. It must then be admitted that the very large part of our knowledge, which we suppose to have some basis in experience, is by no means based upon experience, in the sense, and to the extent, which we suppose. We do for instance, commonly suppose that there is some basis in experience for the assertion that some people, whom we call Germans, use one set of words to express much the same meaning which we express by using a different set of words. But, if this view be correct, we must admit that no person’s experience gives him any reason whatever for supposing that, when he hears certain words, any one else has ever heard or thought of the same words, or meant anything by them. The view admits, indeed, that I do know that when I hear certain words, somebody else has generally had thoughts more or less similar to those which I suppose him to have had: but it denies that my own observations could ever give me the least reason for supposing that this is so. It admits that my own observations may give me reason for supposing that if anyone has ever had perceptions like mine in some respects, he will also have had other perceptions like others of mine: but it denies that they give me any reason for supposing that any one else has had a perception like one of mine. It admits that my own observations may give me reason for supposing that certain perceptions and thoughts in one person (if they exist) will be followed or preceded by certain other perceptions and thoughts in that person: but it denies that they give me any reason whatever for any similar generalisation concerning the connection of a certain kind of perception in one person with a certain kind of perception in another. It admits that I should not have certain perceptions, which I do have, unless someone else had had certain other perceptions; but it denies that my own observations can give me any reason for saying so— for saying that I should not have had this perception, unless someone else had had that. No observations of mine, it holds, can ever render it probable that such a generalisation is true; no observation of mine can ever confirm or verify such a generalisation. If we are to say that any such generalisation whatever is based upon observation, we can only mean, what Reid means, that it is based on a series of assumptions. When I observe this particular thing, I assume that that particular thing, which I do not observe, exists; when I observe another particular thing, I again assume that a second particular thing, which I do not observe, exists; when I observe a third particular thing, I again assume that a third particular thing, which I do not observe, exists. These assumed facts—the assumed fact that one observation of mine is accompanied by the existence of one particular kind of thing, and that another observation of mine is accompanied by the existence of a different particular kind of thing, will then give me a reason for different generalisations concerning the connection of different perceptions of mine with different external objects—objects which I do not perceive. But (it is maintained) nothing but a mass of such assumptions will give me a reason for any such generalisation.

Now I think it must be admitted that there is something paradoxical in such a view. I think it may be admitted that, in holding it, the philosopher of Common Sense departs from Common Sense at least as far in one direction as his opponents had done in another. But I think that there is some excuse for those who hold it: I think that, in one respect, they are more in the right than those who do not hold it—than those who hold that my own observations do give me a reason for believing in the existence of other people. For those who hold that my observations do give me a reason, have, I believe, universally supposed that the reason lies in a part of my observations, in which no such reason is to be found. This is why I have chosen to ask the question: What reason do my observations give me for believing that any other person has any particular perceptions or beliefs? I wish to consider which among the things which I observe will give such a reason. For this is a question to which no answer, that I have ever seen, appears to me to be correct. Those who have asked it have, so far as I know, answered it either by denying that my observations give me any reason or by pointing to a part of my observations, which, as it seems to me, really do give none. Those who deny are, it seems to me, right in holding that the reason given by those who affirm is no reason. And their correct opinion on this point will, I think, partly serve to explain their denial. They have supposed that if our observations give us any reason at all for asserting the existence of other people, that reason must lie where it has been supposed to lie by those who hold that they do give a reason. And then, finding that this assigned reason is no reason, they have assumed that there is no other.

I am proposing then to ask: Which among the observations, which I make, and which (as we commonly suppose) are similar in kind to those which all or almost all men make, will give a reason for supposing that the existence of any of them is generally connected with the existence of certain kinds of perception or belief in other people? And in order to answer this question, it is obvious we must first consider two others. We must consider, in the first place: Of what nature must observations be, if they are to give a reason for any generalisation asserting that the existence of one kind of thing is generally connected with that of another? And we must consider in the second place: What kinds of things do we observe?

Now to the first of these questions I am not going to attempt to give a complete answer. The question concerning the rules of Inductive Logic, which is the question at issue, is an immensely difficult and intricate question. And I am not going to attempt to say, what kind of observations are sufficient to justify a generalisation. But it is comparatively easy to point out that a certain kind of observations are necessary to justify a generalisation: and this is all that I propose to do. I wish to point out certain conditions which observations must satisfy, if they are to justify a generalisation; without in any way implying that all observations which do satisfy these conditions, will justify a generalisation. The conditions, I shall mention, are ones which are certainly not sufficient to justify a generalisation; but they are, I think, conditions, without which no generalisation can be justified. If a particular kind of observations do not satisfy these conditions, we can say with certainty that those observations give us no reason for believing in the existence of other people; though, with regard to observations which do satisfy them, we shall only be able to say that they may give a reason.

What conditions, then, must observations satisfy, if they are to justify a generalisation? Let us suppose that the generalisation to be justified is one which asserts that the existence of a kind of object, which we will call A, is generally preceded, accompanied, or followed by the existence of a kind of object, which we call B. A, for instance, might be the hearing of a certain word by one person, and B the thought of that which we call the meaning of the word, in another person; and the generalisation to be justified might be that when one person hears a word, not spoken by himself, someone else has generally thought of the meaning of that word. What must I have observed, if the generalisation that the existence of A is generally preceded by the existence of B, ’is to be justified by my observations? One first point, I think, is plain. I must have observed both some object, which is in some respects like A, and which I will call α, and also some object in some respects like B which I will call β: I must have observed both α and β, and also I must have observed β preceding α. This, at least, I must have observed. But I do not pretend to say how like α and β must be to A and B; nor do I pretend to say how often I must have observed β preceding α, although it is generally held that I must have observed this more than once. These are questions, which would have to be discussed if we were trying to discover what observations were sufficient to justify the generalisation that the existence of A is generally preceded by that of B. But I am only trying to lay down the minimum which is necessary to justify this generalisation; and therefore I am content to say that we must have observed something more or less like B preceding something more or less like A, at least once.

But there is yet another minimum condition. If my observation of β preceding α is to justify the generalisation that the existence of A is generally preceded by the existence of B, it is plain, I think, that both the β and the α, which I observed, must have existed or been real; and that also the existence of β must really have preceded that of α. It is plain that if, when I observed α and β, α existed but β did not, this observation could give me no reason to suppose that on another occasion when A existed, B would exist. Or again, if, when I observed β preceding α, both β and α existed, but the existence of β did not really precede that of α, but, on the contrary, followed it, this observation could certainly give me no reason to suppose that, in general, the existence of A was preceded by the existence of B. Indeed this condition that what is observed must have been real might be said to be included in the very meaning of the word “observation.” We should, in this connection, say that we had not observed β preceding α, unless β and a were both real, and β had really preceded α. If I say “I have observed that, on one occasion, my hearing of the word ‘ moon ’ was followed by my imagining a luminous silvery disc,” I commonly mean to include in my statement the assertion that I did, on that occasion, really hear the word “moon,” and really did have a visual image of a luminous disc, and that my perception was really followed by my imagination. If it were proved to me that this had not really happened, I should admit that I had not really observed it. But though this condition that, if observation is to give reason for a generalisation, what is observed must be real, may thus be said to be implied in the very word “observation,” it was necessary for me to mention the condition explicitly. It was necessary, because, as I shall presently show, we do and must also use the word “observation” in a sense in which the assertion “I observe A” by no means includes the assertion “A exists”—in a sense in which it may be true that though I did observe A, yet A did not exist.

But there is also, I think, a third necessary condition which is very apt to be overlooked. It may, perhaps, be allowed that observation gives some reason for the proposition that hens’ eggs are generally laid by hens. I do not mean to say that any one man’s observation can give a reason for this proposition: I do not assume either that it can or that it cannot. Nor do I mean to make any assumption as to what must be meant by the words “hens” and “eggs,” if this proposition is to be true. I am quite willing to allow for the moment that if it is true at all, we must understand by “hens” and “eggs,” objects very unlike that which we directly observe, when we see a hen in a yard, or an egg on the breakfast-table. I am willing to allow the possibility that, as some Idealists would say, the proposition “Hens lay eggs” is false, unless we mean by it: A certain kind of collection of spirits or monads sometimes has a certain intelligible relation to another kind of collection of spirits or monads. I am willing to allow the possibility that, as Reid and some scientists would say, the proposition “Hens lay eggs” is false, if we mean by it anything more than that: Certain configurations of invisible material particles sometimes have a certain spatio-temporal relation to another kind of configuration of invisible material particles. Or again I am willing to allow, with certain other philosophers, that we must, if it is to be true, interpret this proposition as meaning that certain kinds of sensations have to certain other kinds a relation which may be expressed by saying that the one kind of sensations “lay” the other kind. Or again, as other philosophers say, the proposition “Hens lay eggs ” may possibly mean: Certain sensations of mine would, under certain conditions, have to certain other sensations of mine a relation which may be expressed by saying that the one set would “lay” the other set. But whatever the proposition “Hens’ eggs are generally laid by hens” may mean, most philosophers would, I think, allow that, in some sense or other, this proposition was true. And they would also I think allow that we have some reason for it; and that part of this reason at all events lies in observation: they would allow that we should have no reason for it unless certain things had been observed, which have been observed, Few, I think, would say that the existence of an egg “intrinsically points” to that of a hen, in such a sense that, even if we had had no experience of any kind concerning the manner in which objects like eggs are connected with animals like hens, the mere inspection of an egg would justify the assertion: A hen has probably existed.

I assume, then, that objects having all the characteristics which hens’ eggs have (whatever these may be) are generally laid by hens (whatever hens may be); and I assume that, if we have any reason for this generalisation at all, observation gives us some reason for it. But now, let us suppose that the only observations we had made were those which we should commonly describe by saying that we had seen a hen laying an egg. I do not say that any number of such observations, by themselves, would be sufficient to justify our generalisation: I think it is plain that they would not. But let us suppose, for the moment, that we had observed nothing else which bore upon the connection between hens and eggs; and that, if therefore our generalisation was justified by any observations at all, it was justified by these. We are supposing, then, that the observations which we describe as “seeing hens lay eggs” give some reason for the generalisation that eggs of that kind are generally laid by hens. And if these observations give reason for this, obviously in a sense they give reason for the generalisation that the existence of such an egg is generally preceded by that of a hen; and hence also, they give us reason to suppose that if such an egg exists, a hen has probably existed also—that unless a hen had existed, the egg would not have existed. But the point to which I wish to call attention is that it is only in a limited sense that they do give reason for this. They only give us reason to suppose that, for each egg, there has existed a hen, which was at some time near the place where the egg in question then was, and which existed at a time near to that at which the egg began to exist. The only kind of hens, whose existence they do give us reason to suppose, are hens, of which each was at some time in spatial and temporal proximity (or, if Idealists prefer, in the relations which are the “intelligible counterparts” of these) to an egg. They give us no information at all about the existence of hens (if there are any) which never came within a thousand miles of an egg, or which were dead a thousand years before any egg existed. That is to say, they do give us reason to suppose that, if a particular egg exists, there has probably existed a hen which was at some time near that egg; but they give us no reason to suppose that, if a particular egg exists, there must have existed a hen which never came near that egg. They do give us reason to suppose that, for each egg, there has probably existed a hen which at some time stood to the egg in question in that relation which we have observed to hold between an egg and a hen, when we observed the hen laying an egg. But they give us no reason to infer from the existence of an egg any other kind of hen: any hen which never stood to the egg in the relation in which we have observed that some hens do stand to eggs.

What I wish to suggest is that this condition is a universal condition for sound inductions. If the observation of β preceding α can ever give us any reason at all for supposing that the existence of A is generally preceded by that of B, it can at most only give us reason to suppose that the existence of an A is generally preceded by that of a B which stands to our A in the same relation in which β has been observed to stand to α. It cannot give the least reason for supposing that the existence of an A must have been preceded by that of a B, which did not stand to A in the observed relation, but in some quite different one. If we are to have any reason to infer from the existence of an A the existence of such a B, the reason must lie in some different observations. That this is so, in the case of hens’ eggs and hens, is, I think, obvious: and, if the rule is not universal, some reason should at least be given for supposing that it does apply in one case and not in another.

Having thus attempted to point out some conditions which seem to be necessary, though not sufficient, where observation is to give any reason for a generalisation, I may now proceed to my second preliminary question: What kinds of things do we observe?

In order to illustrate how much and how little I mean by “observation” or “direct perception,” I will take as an instance a very common visual perception. Most of us are familiar with the experience which we should describe by saying that we had seen a red book and a blue book side by side upon a shelf. What exactly can we be said to observe or directly perceive when we have such an experience? We certainly observe one colour, which we call blue, and a different colour, which we call red; each of these we observe as having a particular size and shape; and we observe also these two coloured patches as having to one another the spatial relation which we express by saying they are side by side. All this we certainly see or directly perceive now, whatever may have been the process by which we have come to perceive so much. But when we say, as in ordinary talk we should, that the objects we perceive are books, we certainly mean to ascribe to them properties, which, in a sense which we all understand, are not actually seen by us, at the moment when we are merely looking at two books on a shelf two yards off. And all such properties I mean to exclude as not being then observed or directly perceived by us. When I speak of what we observe, when we see two books on a shelf, I mean to limit the expression to that which is actually seen. And, thus understood, the expression does include colours, and the size and shape of colours, and spatial relations in three dimensions between these patches of colour, but it includes nothing else.

But I am also using observation in a sense in which we can be said actually to observe a movement. We commonly say that we can sometimes see a red billiard ball moving towards a white one on a green table. And, here again, I do not mean to include in what is directly perceived or observed, all that we mean by saying that the two objects perceived are billiard-balls. But I do mean to include what (we should say) we actually see. We actually see a more or less round red patch moving towards a more or less round white patch; we see the stretch of green between them diminishing in size. And this perception is not merely the same as a series of perceptions—first a perception of a red patch with a green stretch of one size between it and the white; then a perception of a red patch with a green stretch of a different size between it and the white; and so on. In order to perceive a movement we must have a different perception from any one of these or from the sum of them. We must actually see the green stretch diminishing in size.

Now it is undoubtedly difficult, in some instances, to decide precisely what is perceived in this sense and what is not. But I hope I have said enough to show that I am using “perceive” and “observe” in a sense in which, on a given occasion, it is easy to decide that some things certainly are perceived, and other things, as certainly, are not perceived. I am using it in a sense in which we do perceive such a complex object as a white patch moving towards a red one on a green field; but I am not using it in any sense in which we could be said to “perceive” or “observe” that what we saw moving was a billiard-ball. And in the same way I think we can distinguish roughly between what, on any given occasion, we perceive, as we say, “by any one of the other senses,” and what we do not perceive by it. We can say with certainty that, on any given occasion, there are certain kinds of “content” which we are actually hearing, and others which we are not actually hearing; though with regard to some again it is difficult to say whether we are actually hearing them or not. And similarly we can distinguish with certainty in some instances, between what we are on a given occasion, actually smelling or feeling, and what we are not actually smelling or feeling.

But now, besides these kinds of “things,” “objects,” or “contents,” which we perceive, as we say, “by the senses,” there is also another kind which we can be said to observe. Not only can I observe a red and blue book side by side; I can also observe myself observing them. I can perceive a red patch moving towards a white, and I can also perceive my perception of this movement. And what I wish to make as plain as I can is that my perception of the movement of a coloured patch can at least be distinguished from that movement itself. I wish to make it plain that to observe a coloured patch moving is to observe one thing; and to observe myself observing a coloured patch moving is another. When I observe my own perception of a movement, I observe something more than when I merely observe the movement, and something very different from the movement. I may perceive a red and a blue book side by side on a shelf; and at another time I may perceive a red ball moving towards a white. The red and blue patch, of one shape, at rest side by side, are different from the red, of another shape, moving towards the white; and yet, when I say that both are “perceived,” I mean by “perceived” one and the same thing. And since, thus, two different things may both be perceived, there must also be some difference between each of them and what is meant by saying that it is perceived. Indeed, in precisely the same way in which I may observe a spatial relation between a red patch and a blue (when I observe them “side by side”) I do, when I observe my own perception of them, observe a spatial relation between it and them. I observe a distance between my perception and the red and blue books which I perceive, comparable in magnitude with the breadth or height of the blue book, just as these are comparable in magnitude with one another. And when I say I observe a distance between my perception of a red book and that red book itself, I do not mean that I observe a distance between my eyes, or any other part of what I call my body, and the red patch in question. I am talking not of my eyes, but of my actual perception. I observe my perception of a book to be near the book and further from the table, in exactly the same sense in which I observe the book to be near the shelf on which it stands, and further from the table. And just as, if the distance between a red patch and a white is to be perceived, the red patch must be different from the white, so, if I perceive a certain distance between my perception and the red patch, my perception must be different from the red patch which I perceive.

I assume, then, that we observe, on the one hand, coloured patches of certain shapes and sizes, and their spatial relations to one another, together with all the other kinds of “contents,” which we should usually be said to perceive “through the senses.” And, on the other hand, we also sometimes observe our own perceptions of such “contents” and our thoughts. And these two kinds of “content” are different from one another: my perception of a red patch with gold letters on it, is not itself a red patch with gold letters on it; and hence, when I observe my perception of this patch, I observe something different from that which I observe when I merely perceive the patch. Either of these two kinds of “content” —either colours, moving or at rest, sounds, smells, and all the rest—or, on the other hand, my perceptions of these—either of these two kinds, or both, might conceivably, since both are observed, give grounds for a generalisation concerning what exists. But, as I have said, if observations are to give any ground for such a generalisation, it must be assumed that what is observed exists or is real. And since, as I have insisted, when I observe my perception of a red patch with gold letters on it, I observe something different from what I observed when I merely observed a red patch with gold letters on it, it follows that to assume the existence of my perception of this red and gold is not the same thing as to assume the existence of the red and gold itself.

But what, it may be asked, do I mean by this property of “ existence” or “reality,” which may, it would seem, belong to every content, which I observe, or may again belong to none, or which may belong to some and not to others? What is this property which may belong to my perception of a movement, and yet not belong to the movement perceived, or which may again belong to the movement perceived and not to my perception of it, or which may again belong to both or to neither?

It is necessary, I think, to ask this question at this point, because there are some philosophers who hold that, in the case of some kinds of “content,” at all events, to say that they “exist” is to say that they are “perceived.” Some hold that to say “A exists” is to say neither more nor less than “A is perceived” —that the two expressions are perfect synonyms; and others again would say that by “A exists or is real” we may mean more than that “A is perceived,” but that we must at least mean this. Now, I have hitherto used the word “existence” pretty freely, and I think that, when I used it, I used it in its ordinary sense. I think it will generally have suggested to you precisely what I meant to convey, and I think that, in some cases at all events, it will not even have occurred to you to doubt whether you did understand what I meant by it. But, if these philosophers are right, then, if you have understood what I meant by it, I have all along been using it in a sense, which renders the end of my last paragraph perfect nonsense. If these philosophers are right, then, when I assert that what is perceived may yet not exist, I am really asserting that what is perceived may yet not be perceived—I am contradicting myself. I am, of course, quite unaware that I am doing so. But these philosophers would say either you are contradicting yourself, or you are not using the word exists” in its ordinary sense. And either of these alternatives would be fatal to my purpose. If I am not using the word in its ordinary sense, then I shall not be understood by anyone; and, if I am contradicting myself, then what I say will not be worth understanding.

Now, with one class of these philosophers—the class to which, I think, Berkeley belongs—I think I can put myself right comparatively easily. The philosophers I mean are those who say that it is only in the case of one particular class of “contents” (the kind of “content” which Berkeley calls “ideas”) that to say “the ‘ content ’ A exists” is to say “A is perceived,” and who admit that in the case of other contents—myself and my perceptions and thoughts, for example—to say that these exist or are real, is to say of them something different from this. These philosophers admit, that is to say, that the word “exists” has two different senses: and that in only one of these senses is it synonymous with the words “is perceived.” When (they hold) I say of such a content as a red patch with gold letters on it that it “exists” I do mean that it is perceived; but when I say of my perception of such a patch that it exists, I do not mean that my perception is perceived but something different from this. Now, it would be nothing strange that one and the same word should be used in two different senses; many words are used in many different senses. But it would, I think, be something very strange indeed, if in the case of a word which we constantly apply to all sorts of different objects, we should uniformly apply it to one large class of object in the one sense and the one sense only and the other large class in the other sense and the other sense only. Usually, in the case of such ambiguous words, it happens that, in different contexts, we apply it to one and the same object in both senses. We sometimes wish to say of a given object that it has the one property, and sometimes we wish to say of the same object that it has the other property; and hence we apply the same word to the same object, at one time in one sense, and at another in the other. I think, therefore, that, even if there were these two different senses of the word “existence,” it would be very unlikely that we should not commonly, in some contexts, apply it in the sense, in which (as is alleged) it does apply to perceptions, to “contents” which are not perceptions. Indeed, I think, it is quite plain that we constantly do ask, with regard to what is not a perception, whether it exists, in precisely the same sense, in which we ask, with regard to a perception, whether it exists. We ask in precisely the same sense: Was the Roc a real bird, or merely an imaginary one? and, did Sindbad’s perception of the Roc really exist, or is it a fiction that he perceived a Roc? I think, therefore, that the sense in which these philosophers admit that we do apply the word “existence” to perceptions, is one in which we also commonly apply it to “contents” other than perceptions. But, even if this is not the case, I can set myself right with them by a simple explanation. I need merely explain that the sense in which I am proposing to enquire whether a red patch exists, is precisely the sense in which they admit that my perception of a red patch does exist. And in this sense, it is plain that to suppose that a thing may exist, which is not perceived, or that it may not exist, although it is perceived, is at least not self-contradictory.

But there may be other philosophers who will say that, in the case of a perception also, to say that it exists or is real is to say that it is perceived—either that alone or something more as well. And to these philosophers I would first point out that they are admitting that the proposition “This perception is real” is significant. There is some sense or other in which we may say: “Alexander’s perception of an elephant was real or did exist, but Sindbad’s perception of a Roc was not real—never did exist”: the latter proposition is, in some sense or other, not self-contradictory. And then I would ask of them: When they say, that to call a perception “real” is to assert that it is perceived, do they mean by this that to call it real is to assert that it is really perceived, or not? If they say “No,” then they are asserting that to call a perception “real” is merely to say that it was perceived in the sense in which Sindbad did perceive a Roc: they are asserting that to call it “real” is not to say, in any sense, that it was really perceived: they are asserting that to call a perception “real” is to say that it was perceived, in some sense quite other than that in which we ordinarily use the word: for we certainly commonly mean, when we say “A was perceived,” that a perception of A was “real”: we should commonly say that Sindbad did not perceive a Roc—meaning that no such perception ever did exist. I do not think they do mean this; and, in any case, if they do, I think it is plain that they are wrong. When we say that a perception is “real,” we certainly do not mean merely that it is the object of another perception, which may itself be quite unreal—purely imaginary. I assume, therefore, that when they say: To call a perception “real” is to say that it is perceived; they mean, what we should naturally understand, namely, that: To call it “real” is to say that it is really perceived—to say that it is the object of another perception, which is also real in the same sense. And, if they mean this, then what they say is certainly untrue. Their definition of reality is circular. It cannot be the case that the only sense in which a perception may be said to be real, is one in which to call it so is to assert that not it alone, but another perception is real also. It cannot be the case that the assertion “A is real” is identical with the assertion “A and B are both real,” where A and B are different, and “real” is used in the same sense as applied to both. If it is to be true that the assertion “A is real” ever, in any sense, includes the assertion ‘‘ A is really perceived,” there must be another sense of the word “real,” in which to assert “A is real” is to assert less than “A is really perceived” —the sense, namely, in which we here assert that the perception of A is real.

We find, therefore, that the other class of philosophers were at least right in this: they were right in allowing that the sense in which we commonly say that our perceptions exist is one in which “exist” does not include, even as a part of its meaning, “is perceived.” We find that there is a common sense of the word “existence,” in which to say “A exists” must mean less than “A is really perceived”; since, otherwise, the only possible definition of the word “existence” would be a circular definition. And I may point out that two other definitions, which have been sometimes suggested by philosophers as giving what we commonly mean by “reality” or “existence” are vitiated by the same fault—they also are circular. Some philosophers have sometimes suggested that when we call a thing “real,” we mean that it is “systematically connected” in some way with other things. But, when we look into their meaning, we find that what they mean is (what, indeed, is alone plausible)— systematically connected with other real things. And it may possibly be the case that we sometimes use the word “real” in this sense: but, at least, it must be certainly the case, that, if we do, we also use it in another and simpler sense—the sense in which it is employed in the proposed definition. And other philosophers have suggested that what we mean by “real” is—“connected in some way with a purpose—helping or hindering, or the object of a purpose.” But if we look into their meaning, we find they mean—connected with a real purpose. And hence, even if we do sometimes mean by “real,” “connected with a real purpose,” it is plain we also sometimes mean by “real” something simpler than this—that namely, which is meant by “real” in the proposed definition.

It is certain, therefore, that we do commonly use the word “existence” in a sense, in which to say “A exists” is not to say “A is perceived,” or “A is systematically connected with other real things,” or “A is purposive.” There is a simpler sense than any of these—the sense in which we say that our own perceptions do exist, and that Sindbad’s perceptions did not exist. But when I say this, I am by no means denying that what exists, in this simple sense, may not always also exist in all the others; and that what exists in any of them may not also always exist in this. It is quite possible that what exists is always also perceived, and that what is perceived always also exists. All that I am saying is that, even if this is so, this proposition is significant—is not merely a proposition about the meaning of a word. It is not self-contradictory to suppose that some things which exist are not perceived, and that some things which are perceived do not exist.

But, it may be asked: What is this common simple sense of the word “exists”? For my own part, it seems to me to be so simple that it cannot be expressed in other words, except those which are recognised as its synonyms. I think we are all perfectly familiar with its meaning: it is the meaning which you understood me to have throughout this paper, until I began this discussion. I think we can perceive at once what is meant by asserting that my perception of black marks on a white ground is “real,” and that no such perception as Sindbad’s of a Roc was ever “ real”: we are perfectly familiar with the property which the one perception is affirmed to possess, and the other to be without. And I think, as I have said, that this property is a simple one. But, whatever it is, this, which we ordinarily mean, is what I mean by “existence” or “reality.” And this property, we have seen, is certainly neither identical with nor inclusive of that complex one which we mean by the words “is perceived.”

I may now, then, at last approach the main question of my paper. Which among the “contents” which I observe will give me reason to suppose that my observation of some of them is generally preceded or accompanied or followed by the existence of certain particular perceptions, thoughts or feelings in another person? I have explained that the “contents” which I actually observe may be divided into two classes: on the one hand, those which, as we commonly say, we perceive “through the senses”; and, on the other hand, my perceptions of these last, my thoughts, and my feelings. I have explained that if any of these observed contents are to give reason for a generalisation about what exists, they must exist. And I have explained that with regard to both classes of “contents” I am using the word “exist” in precisely the same sense—a sense, in which it is certainly not self-contradictory to suppose that what is perceived, does not exist, and that what is not perceived, does exist; and, in which, therefore, the assumption that a red patch with gold letters on it exists, is a different assumption from the assumption that my perception of a red patch with gold letters on it exists; and the assumption that my perception of a red patch with gold letters on it exists, is a different assumption from the assumption that a red patch with gold letters on it exists.

What, then, that we observe, can give us any reason for believing that anyone else has certain particular perceptions, thoughts or feelings? It has, I think, been very commonly assumed that the observation of my own perceptions, thoughts, and feelings, can, by itself, give me such a reason. And I propose, therefore, to examine this assumption. If, as I hope to show, it is false; it will then follow, that if our own observation gives us any reason whatever, for believing in the existence of other persons, we must assume the existence, not only of our own perceptions, thoughts and feelings, but also of some, at least, among that other class of data, which I may now, for the sake of brevity, call “sense-contents”; we must assume that some of them exist, in precisely the same sense in which we assume that our perceptions, thoughts, and feelings exist.

The theory which I propose to examine is, then, the following. My observation of my own thoughts, feelings, and perceptions may, It asserts, give me some reason to suppose that another person has thoughts, feelings, and perceptions similar to some of mine. Let us assume, accordingly, that my own thoughts, feelings, and perceptions do exist; but that none of the “sense-contents,” which I also observe, do so. Where among my perceptions am I to look for any which might conceivably give me a reason for supposing the existence of other perceptions similar to my own? It is obvious where I must look. I have perceptions which I call perceptions of other people’s bodies; and these are certainly similar in many respects to other perceptions of my own body. But I also observe that certain kinds of perceptions of my own body are preceded by certain other perceptions, thoughts, or feelings of mine. I may, for instance, observe that when I perceive my hand suddenly catch hold of my foot in a particular way, this perception was preceded by a particular kind of feeling of pain. I may, perhaps, observe this often enough to justify the generalisation that the perception of that particular motion of my body is generally preceded by that particular feeling of pain. And in this way I may perhaps have reason for quite a number of generalisations which assert that particular kinds of perceptions of my own body are generally preceded by other particular kinds of perceptions, thoughts, or feelings of my own.

But I may also, no doubt, have the perception, which I call the perception of another person’s hand catching hold of his foot, in a manner similar to that in which I have perceived my own hand catch hold of my own foot. And my perception of another person’s hand catching hold of his foot may undoubtedly be similar in many respects to my perception of my own hand catching hold of my own foot. But I shall not observe the same kind of feeling of pain preceding my perception of his hand catching hold of his foot, which I have observed preceding my perception of my hand catching hold of my foot. Will my generalisation, then, give me any reason to suppose that nevertheless my perception of his hand catching hold of his foot is preceded by a similar feeling of pain, not in me but in him? We undoubtedly do assume that when I perceive another person’s body making movements similar to those which I have observed my own body making, this perception has generally been preceded by some feeling or perception of his similar to that which I have observed to precede my perception of similar movements in my own body. We do assume this; and it is precisely the kind of generalisation, which, I have insisted, must be admitted to be true. But my present question is: Will such observations as I have described give any reason for thinking any such generalisation true? I think it is plain that they will not give the slightest reason for thinking so. In the first place, all the perceptions which I call perceptions of another person’s body differ very considerably from any of those which I call perceptions of my own. But I am willing to waive this objection. I am not offering any theory as to what degree of likeness is sufficient to justify a generalisation: and therefore I will allow that the degree of likeness may be sufficient. But there remains an objection which is, I think, quite fatal to the proposed inference. This objection is that the inference in question plainly does not satisfy the third condition which I suggested above as necessary, wherever any generalisation is to be justified by observation. I am willing to allow that my observations of the fact that my perception of a certain movement in my own body is preceded by a certain feeling of pain, will justify the generalisation that my perception of any such movement, whether in my own body or in that of another person, is generally preceded by a similar feeling of pain. And I allow, therefore, that when I perceive a certain movement in another’s body, it is probable that the feeling of pain exists, though I do not perceive it. But, if it is probable that such a feeling of pain exists, such a feeling must stand in the same relation to my perception of the movement in another person’s body, in which a similar feeling of pain has been observed by me to stand to my perception of such a movement in my own body. That is to say the only kind of feeling of pain, which my observations do justify me in inferring, if (as I admit they may) they justify me in inferring any at all, is a feeling of pain of my own. They cannot possibly justify the belief in the existence of any such feeling except one which stands to my perception in the same relation in which my feelings do stand to my perceptions—one, that is to say, which is my own. I have no more reason to believe that the feeling of pain which probably precedes my perception of a movement in another person’s body can be the feeling of another person, than, in my former example, I had reason to suppose that the hen, whose existence probably preceded that of a given egg, could be a hen, which had never been near the egg in question. The two cases are exactly analogous. I observe a feeling of pain of my own preceding a perception of my own. I observe the two, that is to say, as standing to one another, in those relations (whatever they may be) in which any perception of mine stands to any other thought, perception or feeling of mine, and which are, at all events, different from any relation in which a perception or feeling of another person can stand to one of mine. I never perceive the feeling and the perception as standing in any other relation. In any case, therefore, where I do observe something like the perception, but do not observe the feeling, I can only be justified (if justified in inferring any feeling at all), in inferring an unperceived feeling of my own.

For this reason I think that no observations of my own perceptions, feelings or thoughts can give me the slightest reason for supposing a connection between any of them and any feeling, perception, or thought in another person. The argument is perfectly general, since all my perceptions, feelings and thoughts do have to one another those relations in virtue of which I call them mine; and which, when I talk of a perception, feeling or thought as being another persons, I mean to say that it has not got to any of mine. I can, therefore, merely from observation of this class of data never obtain the slightest reason for belief in the existence of a feeling, perception, or thought which does not stand in these relations to one of mine—which is, that is to say, the feeling, perception or thought, of another person. But how different is the case, if we adopt the hypothesis, which I wish to recommend—if we assume the existence of that other class of data which I have called “sense-contents!” On this hypothesis, that which I perceive, when I perceive a movement of my own body, is real; that which I perceive when I perceive a movement of another’s body is real also. I can now observe not merely the relation between my perception of a movement of my body and my own feelings, but also a relation between a real movement of my body and my own feelings. And there is no reason why I should not be justified in inferring that another person’s feelings stand in the same relation to the real movements of his body, in which I observe my own feelings to stand to similar real movements of mine.

But there is another argument which may still be urged by those who hold that my own perceptions, thoughts, and feelings, by themselves, may be sufficient to justify a belief in the existence of other persons. It may be said: “Our observation of our own perceptions may be sufficient to verify or confirm the hypothesis that other persons exist. This hypothesis is one which “works.” The assumption that other persons have particular thoughts, feelings and perceptions enables us to predict that they will have others and that our own perceptions will be modified accordingly: it enables us to predict future perceptions of our own; and we find that these predictions are constantly verified. We observe that we do have the perceptions, which the hypothesis leads us to expect we should have. In short, our perceptions occur just as they would do, if the hypothesis were true; our perceptions behave as if other persons had the perceptions, thoughts and feelings which we suppose them to have. Surely, then, they confirm the truth of the hypothesis—they give some reason to think it probably true? ’

All this, which I have supposed an opponent to urge, I admit to be true. I admit that the fact that an hypothesis works may give some reason to suppose it true. I admit that my perceptions occur just as they would do, if other people had the perceptions which I suppose them to have. I admit that that assumption enables me to make predictions as to future perceptions of my own, and that I observe these predictions to come true. I admit all this. But I admit it only in a sense in which it in no way conflicts with the position which I am maintaining. The words, which I have put into the mouth of a supposed opponent, may, in fact, mean three different things, which it is worth while to distinguish. In two of those meanings, which I shall admit to be true and which are what make them seem plausible, they do not deny what I assert. Only in the third sense are they an objection to my position: and in that sense they are false.

One of the meanings which I admit to be true is as follows:—I have not only admitted but insisted that some of my perceptions are just such as would occur if another person had certain particular feelings: I have insisted that I should not have just those perceptions which I do have, unless some other person had certain feelings and perceptions which I suppose him to have. And I admit further that the fact that I have one of the perceptions in question—for instance, that of another person’s hand catching hold of his foot—this fact, together with the true assumption that I should not have this perception, unless some other person felt pain, will justify the assertion that another person has felt pain. In this sense, I admit, the fact that I perceive what I do perceive will give me reason to suppose that another person has felt pain. And, on the other hand, I also admit that the fact that I have this perception, together with the true assumption that when I have it another person has felt pain, may help to justify the assumption that the perception in question is one which I should not have had unless another person had felt pain—it helps to justify the generalisation that certain of my perceptions are just what would occur, if another person had felt pain. In general terms, that is to say, I admit that the occurrence of B, together with the assumption that B is just the sort of thing which would occur if A existed, will justify the assertion that A exists in that particular instance. And I also admit that the occurrence of B, together with the assumption that A exists in that particular instance, may help to justify the assumption that B is just the sort of thing which would exist, if A existed. In other words: When it is said that the observation of B’s existence confirms or verifies the assumption that A exists, either of two things may be meant. It may be meant that, assuming B to be the sort of thing which would exist if A existed, the observation of B confirms the assumption that A exists in this particular instance. Or, on the other hand, it may be meant that, assuming A to exist in this particular instance, the observation of B may confirm the generalisation, that B is just the sort of thing which would exist, if A existed. Either the one or the other of these two things is, I think, what is generally assumed, when it is assumed that what we do observe confirms or verifies the assumption that there exists some particular thing which we don’t observe. And I am admitting that both these assumptions are true.

But neither of them conflicts in any way with the position I am maintaining. What I am maintaining is that no observation of my own perceptions, by itself, can confirm the generalisation that any one of them is just what would occur if another person had a particular feeling. I admit this generalisation to be true; and I admit that my observation of my own perceptions and feelings may give me reason to suppose that if another person has certain perceptions or feelings he will also have certain others. What I deny is that they give me the slightest reason to suppose that the existence of any such feeling or perception in another has any connection with the existence of any perception of my own— to suppose that any perception of my own is the sort of thing which would occur if another person had a particular feeling. What therefore, my opponent must affirm is that the observation of a perception of my own without the assumption (which Reid makes) that in that particular instance any feeling or perception of another person, of any kind whatever, has preceded it, may give me reason to suppose that that perception of my own is of a kind which is generally preceded by a particular kind of feeling in another person. And this, I think, is plainly false.

But there is yet a third thing which may be meant, and which I am willing to admit may be true. It may be said: “I believe many generalisations of the following kind. I believe that when I have a perception A, some other person has generally had a feeling X; I believe that the existence of the feeling X is generally followed, in the same person, by that of the feeling Y; and I believe also that when another person has the feeling Y, I generally have the perception B. I believe all this.” And it must, I think, be admitted that we do believe generalisations of this kind, and generalisations in which there are not merely two steps between A and B, but a great number of steps. “But then,” it may be said, “my belief in this generalisation causes me, when I observe my perception A, to expect that I shall have the perception B; and such expectations, I observe, are constantly realised.” And this also, I think, must be admitted to be true. “But, finally,” it may be said, “beliefs which produce expectations which are constantly realised are generally true. And hence the fact that these beliefs of mine about the connection of feelings in other persons with perceptions of my own do lead to expectations which are realised, gives me reason to suppose that these generalisations are true and hence that other persons do have particular kinds of feelings.” And I am willing to admit that this also is true. I am willing to admit that true predictions can, as a rule, only be produced by true beliefs. The generalisation that this is so, is, indeed, one which can only be justified by the observations of beliefs, which are, in some way, independently proved to be true; and hence, if it is to be justified, without assuming the existence of anything other than my own perceptions, thoughts, and feelings, it can only be justified by my observation that beliefs with regard to the manner in which these succeed one another generally lead to true predictions. Whether the observation of such beliefs alone could give sufficient reason for it, is, I think, doubtful; but I am willing to admit that it may be so. One thing, however, is, I think, quite plain: namely, that this generalisation “Beliefs which lead to true predictions are generally true” cannot be true, unless some other of the “contents” which I observe, beside my own perceptions, thoughts, and feelings, do exist. That is to say, in giving a reason for supposing the existence of other people, this generalisation also gives a reason for the very theory which I am advocating, namely, that some of those data which I have called “sense-contents” do exist. It does this, because it is quite certain that beliefs in generalisations about the existence of sense-contents can (and do) constantly lead to true predictions. The belief that when I have observed a fire of a certain size in my grate, something similar to what I have observed will continue to exist for a certain time, can, and constantly does, lead to the true prediction that, when I come back to my room in half an hour’s time, I shall observe a fire of a certain size still burning. We make predictions on such grounds, I think, every day and all day long. And hence unless such beliefs as that what I observe when I see a fire burning does exist, are true, we certainly have no reason to suppose that beliefs which lead to true predictions are generally true. And hence on this hypothesis also it remains true: that, unless some of the contents which I observe other than my own perceptions, thoughts, and feelings, do exist, I cannot have the slightest reason for supposing that the existence of certain perceptions of my own is generally connected with that of certain perceptions, thoughts, or feelings in any other person.

I conclude therefore that, unless some of the observed data which I have called sense-contents do exist, my own observations cannot give me the slightest reason for believing that anybody else has ever had any particular perception, thought, or feeling. And, having arrived so far towards an answer to my first question: How do we know that any other persons exist? I may now point out that precisely the same answer must be given to my second question: How do we know that any particular kind of thing exists, other than ourselves, our perceptions, thoughts, and feelings, and what we directly perceive? There is a view concerning what exists, which deserves, I think, much more respect than it generally receives from philosophers nowadays. The view I mean is the view that material objects, such as they are conceived by physical science, do really exist. It is held by some persons (and Reid is among them) that we do know of the existence, not only of other persons, but also of the movements of matter in space. It is held that we do know, with considerable precision, what kinds of movements of matter generally precede my perception, when I have a particular perception. It is held, for instance, that when I perceive a red and blue book side by side on a shelf, at a certain distance from me, there have existed, between two material objects, which may be called books, and another kind of material object, which may be called my eyes, certain wave-like motions of a material medium; that there have existed two different sets of waves, of which the one is connected with my perception of red and the other with my perception of blue; and that the relative heights and breadths of the two different sets of waves, and the relative velocity of their movements are very exactly known. It is held that some men have a vast amount of very precise information about the existence of objects of this kind; and I think the view that this is so deserves a great deal of respect. But what I wish now to point out is that no one’s observation of his own perceptions, thoughts and feelings, can, by itself, give him the slightest reason for believing in the existence of any such material objects. All the arguments by which I have tried to show that this kind of observation alone can give me no reason to believe in the existence of any kind of perception or feeling in another person, apply, with at least equal force, to show that it can give me no reason to believe in the existence of any kind of material object. On the other hand, if we are to admit the principle that “Beliefs which lead to true predictions, are generally true,” this principle will give us at least as much reason to believe in the existence of certain kinds of material objects as to believe in the existence of other persons; since one of the most remarkable facts about beliefs in the existence of such objects is that they do so often lead to true predictions. But it must be remembered that we can have no reason for believing this principle itself, unless our own perceptions, thoughts and feelings are not the only kind of observed “content” which really does exist: we can have no reason for it, unless some such things as what I perceive, when I see a red and blue book side by side, do really exist.

It would seem, therefore, that if my own observations do give me any reason whatever for believing in the existence either of any perception in any other person or of any material object, it must be true that not only my own perceptions, thoughts and feelings, but also some of the other kinds of things which I directly perceive—colours, sounds, smells, etc.—do really exist: it must be true that some objects of this kind exist or are real in precisely the same simple sense in which my perceptions of them exist or are real. Is there then any reason to think that this is not true? Is there any reason to think, for instance, that none of the colours which I perceive as occupying areas of certain shapes and sizes really exist in the areas which they appear to occupy? This is a question which I wished to discuss at length, because I think that it is one in which there are real difficulties. But I have given so much space to other questions, that I can only deal with it very briefly here.

Some philosophers are very fond of asserting that a colour cannot exist except when it is perceived; and it might possibly be thought that when I suggest that colours do really exist, I am suggesting that they do exist when they are not perceived. I wish, therefore, briefly to point out that the question whether anything does exist, when it is not perceived, is one which I have not argued and shall not attempt to argue in this paper. I have, indeed, tried to show that since “exists” does not mean “is perceived,” it is, at least, conceivable that things should exist, when they are not perceived. But I have admitted that it is quite possible none do so: it may be the case that whenever a thing exists, it is also at the same time perceived, for anything that I have said or shall say to the contrary. I think, indeed, that, if such things as colours do exist, my observation of their behaviour will justify me in concluding that they also exist when I myself am, at least, not aware of perceiving them: but since I have not attempted to determine what kinds of observation are sufficient to justify a generalisation, I do not pretend to say whether this is so or not: and still less do I pretend to say whether, if they exist when I do not perceive them, we are justified in supposing that someone else must be perceiving them. The question whether anything exists, when it is not perceived, and, if so, what things, seems to me to be one which can only be settled by observation; and thus, I conceive, observation might justify us in concluding that certain kinds of things— pains, for example, do not exist, when they are not perceived and that other kinds of things—colours, for example, do exist, when they are not perceived. The only way, in which, so far as I am aware, the theory I am advocating does conflict with ordinary Idealistic conclusions, is that it does suggest that things, which are not “spiritual,” do sometimes exist, as really and as truly, as things which are.

The theory, therefore, that nothing exists, except when it is perceived, is no objection (even if it be true) to the supposition that colours do exist. What objections are there to this supposition? All serious objections to it are, I think, of one type. They all rest upon the assumption that, if a certain kind of thing exists at a certain time in a certain place, certain other kinds of things cannot exist at the same time in the same place. They are all, that is to say, of the same type as Berkeley’s argument: that, though the same body of water may appear to be simultaneously both hot and cold (if one of the hands we plunge into it is warm and the other cold), yet the heat and the cold cannot both really be in the same body at the same time. And it is worth noticing that anyone who uses this argument must admit that he understands what is meant by “really existing in a given place,” and that he means by it something other than “being perceived as in a given place.” For the argument itself admits that both the heat and the cold are really perceived as being in the, same place, and that there is no difficulty in supposing that they are so; whereas it urges that there is a difficulty in supposing that they both really exist in it.

Now there is one obvious defect in this type of argument, if designed to prove that no sensible quality exists at any place where it is perceived as being—a defect, which Berkeley himself admits in his “Principles,” though he omits to notice it where he repeats the argument in his “Hylas.” Even if we assume that the heat and the cold cannot both exist in the same place (and I admit that, in this case, the contrary assumption does seem repugnant to Common Sense), it does not follow that neither exists there. That is to say this type of argument, even if we grant its initial assumption, will only entitle us to conclude that some sensible qualities which we perceive as being in a certain place at a certain time, do not exist in that place at that time. And this conclusion, I am inclined to think, is true. In the case, for instance, of the so-called “images” which we perceive in a looking-glass, we may very readily admit that the colours and shapes which we perceive do not exist at the places where they appear to be—namely at various distances behind the glass. But yet, so far as I can see, we have no reason whatever for supposing that they do not, except the assumption that our observations give us reason to believe that other sensible qualities do exist in those positions behind the glass; and the assumption that where these other sensible qualities do exist, those which we see in the glass do not exist. I should, therefore, admit that some sensible qualities which we perceive as being in certain places, do not exist in those places, while still retaining my belief that others do. And perhaps this explanation is the one which should also be adopted in the case of sensible qualities which appear to be at a great distance from us. When, for instance, (as we say), “we see the moon,” what we perceive (if the moon be full) is a round bright silver disc, of a small size, at a place very distant from us. Does that silver disc exist at that place? With what suppositions does the assumption that it does conflict? Only, so far as I can see, with the supposition that the place in question is really occupied by a body such as science has taught us to suppose that the moon really is—a spherical body immensely larger than objects, in comparison with which the silver disc which we perceive is small; or else with the supposition that the place in question is really occupied by some part of our atmosphere, or some part of the medium which science supposes to exist between our atmosphere and the moon; or else with the supposition that the place in question is really occupied by what we might see, if the moon were nearer to us by many thousands of miles. Unless we suppose that some other object is in the place, in which the silver disc appears to be, and that this object is of a kind which cannot occupy the same place which is occupied by a silver disc, we have no reason to suppose that the silver disc does not really exist in the place where it appears to be. And, in this case, we perhaps have reason for both suppositions and should therefore conclude that the silver disc, which we perceive, does not exist in any real place.

Part, therefore, of these objections to our theory may, I think, be met by admitting that some of the sensible qualities which we perceive do not exist at the places where they appear to exist, though others do. But there is, I think, another class of cases, in which we may be justified in denying that two things which (it is asserted) cannot occupy the same space, really cannot. I will take an instance which is, I think, typical. When we look at a drop of blood with the naked eye, we perceive a small red spot, uniformly red all over. But when (as we say) we look at the same object under a microscope of a certain power, I am informed that we see a much larger spot, of similar shape, indeed, but not uniformly red—having, in fact, small red spots at different positions in a yellowish field. And if we were again to look at the same object through a microscope of much higher power still, we might perceive yet a third different arrangement of colours. Is there any fatal objection to supposing that all three appearances—the uniform red spot, the yellowish field with reddish spots in it, and the third, whatever that may be—do all really occupy the same real spatial area? I cannot see that there is. We are familiar with the idea that a given spatial area may contain parts which are invisible to us. And hence, I think it is quite conceivable that parts of a given area may be really occupied by one colour, while the whole is really occupied by another. And this, I think, is what we actually do believe in many cases. At all events, we certainly believe that the area which appears to be occupied by one colour really is the same area as that which appears to be occupied by another. And, unless we assume that the area, in both cases, really is the same, we can certainly have no reason to deny that each colour does really occupy the area which it appears to occupy.

For these reasons I think that the difficulties in the way of supposing that some of the sensible qualities which we perceive as being in certain places, really exist in the places in which we perceive them to be, are not insuperable. I have indeed not done justice to these difficulties; but then, neither have I done justice to what is to be said on the other side. At all events, I think it is plain that we have no reason to assert, in any case whatever, that a perceived colour does not really exist in the place where it is perceived as being, unless we assume that that very same place really is occupied by something else—either by some different sensible qualities or by material objects such as physical science supposes to exist. But what reason can we give for such an assumption? I have tried to show that our own observations can give us none, unless we assume that some of the sensible qualities, which we observe as occupying certain places, do really exist in those places. And, if this is so, then we must admit that neither he who believes (with Reid) in the existence of other minds and of matter also, nor he who believes in the existence of other minds and denies that of matter, can have, in his own observations, the slightest reason either for his assertion or for his denial: we must admit that he can have no reason for either assertion or denial, except one which consists in the assumption of the existence or non-existence of something which he does not observe— something, therefore, of the very same kind as that for which he gives it as a reason. I am very unwilling to suppose that this is the case: I am very unwilling to suppose that he who believes that Sindbad the Sailor really saw what the “Arabian Nights” represent him as seeing, has just as good reason (so far as his own observation goes) for believing this as he who denies it has for denying it. Still this may be the case. We must, perhaps, be content to assume as certain that for which our observation gives no reason: to assume such propositions as that Sindbad did not see a Roc, and that you do hear my voice. But if it is said that these things are certain; then it also appears to me to be certain that the colours which I perceive do exist (some of them) where I perceive them. The more I look at objects round me, the more I am unable to resist the conviction that what I see does exist, as truly and as really, as my perception of it The conviction is overwhelming.

This being, then, the state of the case, I think I may at least plead that we have grounds for suspense of judgment as to whether what I see does not really exist; grounds, too, for renewed enquiry, more careful than such enquiry has sometimes been in the past.