Chapter III: Part 3

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 A's _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[1] 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 1 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, _β_ _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 _α_ 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.