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 ethers 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 nonexistence 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.