It does not normally occur to us that there is any need for us to justify our belief in the existence of material things. At the present moment, for example, I have no doubt whatsoever that I really am perceiving the familiar objects, the chairs and table, the pictures and books and flowers with which my room is furnished; and I am therefore satisfied that they exist. I recognize indeed that people are sometimes deceived by their senses, but this does not lead me to suspect that my own sense-perceptions cannot in general be trusted, or even that they may be deceiving me now. And this is not, I believe, an exceptional attitude. I believe that, in practice, most people agree with John Locke that "the certainty of things existing in rerum natura, when we have the testimony of our senses for it, is not only as great as our frame can attain to, but as our condition needs."
When, however, one turns to the writings of those philosophers who have recently concerned themselves with the subject of perception, one may begin to wonder whether this matter is quite so simple. It is true that they do, in general, allow that our belief in the existence of material things is well founded; some of them, indeed, would say that there were occasions on which we knew for certain the truth of such propositions as "this is a cigarette" or "this is a pen." But even so they are not, for the most part, prepared to admit that such objects as pens or cigarette are ever directly perceived. What, in their opinion, we directly perceive is always an object of a different kind from these; one to which it is now customary to give the name of "sense-datum." These sense-data are said to have the "presentative function of making us conscious of material things. But how they perform this function, and what is their relation to the material things which they present, are questions about which there is much dispute. There is dispute also about the properties of sense-data, apart from their relationship to material things: whether, for example, they are each of them private to a single observer; whether they can appear to have qualities that they do not really have, or have qualities that they do not appear to have; whether they are in any sense "within" the percipient's mind or brain. I shall show later on that these are not empirical questions. They are to be settled by making it clear how the term "sense-datum" is intended to be used. But first 1 must explain why it is thought necessary to introduce such a term at all. Why may we not say that we are directly aware of material things?
The answer is provided by what is known as the argument from illusion. This argument, as it is ordinarily stated, is based on the fact that material things may present different appearances to different observers, or to the same observer in different conditions, and that the character of these appearances is to some extent causally determined by the state of the conditions and the observer. For instance, it is remarked that a coin which looks circular from one point of view may look elliptical from another; or that a stick which normally appears straight looks bent when it is seen in water; or that to people who take drugs such as mescal, things appear to change their colours. The familiar cases of mirror images, and double vision, and complete hallucinations, such as the mirage, provide further examples. Nor is this a peculiarity of visual appearances. The same thing occurs in the domains of the other senses, including the sense of touch. It may be pointed out, for example, that the taste that a thing appears to have may vary with the condition of the palate; or that a liquid will seem to have a different temperature according as the hand that is feeling it is itself hot or cold; or that a coin seems larger when it is placed on the tongue than when it is held in the palm of the hand; or, to take a case of complete hallucination, that people who have had limbs amputated may still continue to feel pain in them.
Let us now consider one of these -examples' say that of the stick which is refracted in water, and see what is to be inferred. For the present it must be assumed that the stick does not really change its shape when it is placed in water. I shall discuss the meaning and validity of this assumption later on. Then it follows that at least one of the visual appearances of the stick is delusive; for it cannot be both crooked and straight. Nevertheless, even in the case where what we see is not the real quality of a material thing, it is supposed that we are still seeing something; and that it is convenient to give this a name. And it is for this purpose that philosophers have recourse to the term "sense-datum." By using it they are able to give what seems to them a satisfactory answer to the question: What is the object of which we are directly aware, in perception, if it is not part of any material thing? Thus, when a man sees a mirage in the desert, he is not thereby perceiving any material thing; for the oasis which he thinks he is perceiving does not exist. At the same time, it is argued, his experience is not an experience of nothing, it has a definite content. Accordingly, it is said that he is experiencing sense-data which are similar in character to what he would be experiencing if he were seeing a real oasis, but arc delusive in the sense that the material thing which they appear to present is not actually there. Or again, when I look at myself in the glass my body appears to be some distance behind the glass; but other observations indicate that it is in front of it. Since it is impossible for my body to be in both these places at once, these perceptions cannot all be veridical. I believe, in fact, that the ones that are delusive are those in which my body appears to be behind the glass. But can it' be denied that when one looks at oneself in the glass one is seeing something? And if, in this case, there really is no such material thing as my body in the place where it appears to be, what is it that I am seeing? Once again the answer we are invited to give is that it is a sense-datum. And the same conclusion may be reached by taking any other of my examples.
If anything is established by this, it can be only that there are some cases in which the character of our perceptions makes it necessary for us to say that what we are directly experiencing is not a material thing but a sense-datum. It has ndt been shown that this is so in all cases. It has not 14" denied, but rather assumed, that there are some perceptions that do present material things to us as they really are; and in their case there seems at first sight to be no ground for saying that we directly experience sense-data rather than material things. But, as I have already remarked, there is general agreement among the philosophers who make use of the term "sense-datum," or some equivalent term, that what we immediately experience is always a sense-datum and never a material thing. And for this they give further arguments which I shall now examine.
In the first place it is pointed out that there is no intrinsic difference in kind between those of our perceptions that are veridical in their presentation of material things and those that are delusive. When I look at a straight stick, which is refracted in water and so appears crooked, my experience is qualitatively the same as if I were looking at a stick that really was crooked. When, as the result of my putting on green spectacles, the white walls of my room appear to me to be green, my experience is qualitatively the same as if I were perceiving walls that really were green. When people whoa legs have been amputated continue to feel pressure upon them, their experience is qualitatively the same as if pressure really were being exerted upon their legs. But, it is argued, if, when our perceptions were delusive, we were always perceiving something of a different kind from what we perceived when they were veridical, we should expect our experience to be qualitatively different in the two cases. We should expect to be able to tell from the intrinsic character of a perception whether it was a perception of a sense-datum or of a material thing. But this is-not possible, as the examples that I have given have shown. In some cases there is indeed a distinction with respect to the beliefs to which the experiences give rise, as can be illustrated by my original example. For when, in normal conditions, we have the experience of seeing a straight stick there; but when the stick appears crooked, through being refracted in water, we do not believe that it really is crooked; we do not regard the fact that it looks crooked in water as evidence against its being really straight. It must, however, be remarked that this difference in the beliefs which accompany our perceptions is not grounded in the nature of the perceptions themselves, but depends upon our past experience. We do not believe that the stick which appears crooked when it stands in water really is crooked because we know from past experience that in normal conditions it looks straight. But a child who had not learned that refraction was a means of distortion would naturally believe that the stick really was crooked as he saw it. The fact, therefore, that there is this distinction between the beliefs that accompany veridical and delusive perceptions does not justify the view that these are perceptions of generically different objects, especially as the distinction by no means applies to all cases. For it sometimes happens that a delusive experience is not only qualitatively indistinguishable from one that is veridical but is also itself believed to be veridical, as in the example of the mirage; and, conversely, there are cases in which experiences that are actually veridical are believed to be delusive, as when we see something so strange or unexpected that we say to ourselves that we must be dreaming. The fact is that from the character of a perception considered by itself, that is, apart from its relation to further sense-experience, it is not possible to tell whether it is veridical or delusive. But whether we are entitled to infer from this that what we immediately experience is always a sense-datum remains still to be seen.
Another fact which is supposed to show that even in the case of veridical perceptions we are not directly aware of material things is that veridical and delusive perceptions may form a continuous series, both with respect to their qualities and with respect to the conditions in which they are obtained.' Thus, if I gradually approach an object from a distance I may begin by having a series of perceptions which are delusive in the sense that the object appears to be smaller than it really is. Let us assume that this series terminates in a veridical perception. Then the difference in quality between this perception and its immediate predecessor will be of the same order as the difference between any two delusive perceptions that are nut to one another in the series; and, on the assumption that I am walking at a uniform pace, the same will. be true of the difference in the conditions on which the generation of the series depends. A similar example would be that of the continuous alteration in the apparent colour of an object which was seen in a gradually changing light. Here again the relation between a veridical perception and the delusive perception that comes next to it in the series is the same as that which obtains between neighbouring delusive perceptions, both with respect to the difference in quality and with respect to the change in the conditions; and these are differences of degree and not of kind. But this, it is argued, is not what we should expect if the veridical perception were a perception of an object of a different sort, a material thing as opposed to a sense-datum. Does not the fact that veridical and delusive perceptions shade into one another in the way that is indicated by these examples show that the objects that are perceived in either case are generically the same? And from this it would follow, if it was acknowledged that the delusive perceptions were perceptions of sense-data, that what we directly experienced was always a sense-datum and never a material thing.
The final argument that has to be considered in this context is based upon the fact that all our perceptions, whether veridical or delusive, are to some extent causally dependent both upon external conditions, such as the character of the light, and upon our own physiological and psychological states. In the case of perceptions that we take to be delusive this is a fact that we habitually recognize. We say, for example, that the stick looks crooked because it is seen in water; that the white walls appear green to me because I am wcaring green spectacles; that the water feels cool because my hand is hot; that the murderer sees the ghost of his victim because of his bad conscience or because he has been taking drugs. In the case of perceptions that we take to be veridical we are apt not to notice such causal dependencies, since as a rule it is only the occurrence of the unexpected or the abnormal that induces us to look for a cause. But in this matter also there is no essential difference between veridical and delusive perceptions. When, for example, I look at the piece of paper on which I am writing, I may claim that I am seeing it as it really is. But I must admit that in order that I should have this experience it is not sufficient that there should actually be such a piece of paper there. Many other factors are necessary, such as the condition of the light, the distance at which I am from the paper, the nature of the background, the state of my nervous system and my eyes. A proof that they are necessary is that if I vary them I find that I have altered the character of my perception. Thus, if I screw up my eyes I see two pieces of paper instead of one; if I grow dizzy the appearance of the paper becomes blurred; if I alter my position sufficiently it appears to have a different shape and size; if the light is extinguished, or another object is interposed, I cease to see it altogether. On the other hand, the converse does not hold. If the paper is removed I shall cease to see it; but the state of the light
or of my nervous system or any other of the factors that were relevant to the occurrence of my perception may still remain the same. From this it may be inferred that the relation between my perception and these accompanying conditions is such that, while they are not causally dependent upon it, it is causally dependent upon them. And the same would apply to any other instance of a veridical perception that one cared to choose.
This point being established, the argument proceeds as follows. It is held to be characteristic of material things that their existence and their essential properties are independent of any particular observer. For they are supposed to continue the same, whether they are observed by one person or another, or not observed at all. But this, it is argued, has been shown not to be true of the objects we immediately experience. And so the conclusion is reached that what we immediately experience is in no case a material thing. According to this way of reasoning, if some perceptions are rightly held to be veridical, and others delusive, it is because of the different relations in which their objects stand to material things, and it is a philosophical problem to discover what these relations are. We may be allowed to have indirect knowledge of the properties of material things. But this knowledge, it is held, must be obtained through the medium of sense-data, since they are the only objects of which, in sense-perception, we are immediately aware.