In this chapter, we shall seek an answer to two questions: First, how do we know about the world dealt with in physics? Secondly, what do we know about it, assuming the truth of modern physics?

First: How do we know about the physical world? We have already seen that this question cannot have a simple answer, since the basis of inference is something that happens in our own heads, and our knowledge of anything outside our own heads must be more or less precarious. For the present, I shall take it for granted that we may accept testimony, with due precautions. In other words, I shall assume that what we hear when, as we believe, others are speaking to us, does in fact have “meaning” to the speaker, and not only to us; with a corresponding assumption as regards writing. This assumption will be examined at a later stage. For the present, I will merely emphasise that it is an assumption, and that it may possibly be false, since people seem to speak to us in dreams, and yet, on waking, we become persuaded that we invented the dream. It is impossible to prove, by a demonstrative argument, that we are not always dreaming; the best we can hope is a proof that this is improbable. But for the present let us leave this discussion on one side, and assume that the words we hear and read “mean” what they would if we spoke or wrote them.

On this basis, we have reason to know that the worlds of different people are alike in certain respects and different in certain others. Take, for example, the audience at a theatre: they all, we say, hear the same words and see the same gestures, which, moreover, are those that the actors wish them to hear and see. But those who are near the stage hear the words more loudly than those further off; they also hear them somewhat earlier. And those who sit on the right do not see quite what is seen by those who sit on the left or in the centre. These differences are of two sorts: on the one hand, some people can see something invisible to others; on the other hand, when two people, as we say, see the “same” thing, they see it differently owing to effects of perspective and of the way the light is reflected. All this is a question of physics, not of psychology; for if we place a camera in an empty seat in the theatre, the perspective in the resultant photograph is intermediate between the perspectives that are seen by persons sitting on either side; indeed the whole matter of perspective is determined by quite simple geometrical laws. These laws show also what is common to the shapes that two people see when they see the “same” thing from different points of view: what is common is what is studied by projective geometry, which is concerned with what is independent of measurement in geometrical figures. All the differences in appearance due to perspective have to be learned in learning to draw: for this purpose, it is necessary to learn to see things as they really seem, and not as they seem to seem.

But, it will be said, what can you mean by how things “really” seem and how they “seem” to seem? We come here upon an important fact about learning. When, in early infancy, we are learning to correlate sight and touch, we acquire the habit of reacting to a visual stimulus in a manner which is more “objective” than that in which a camera reacts. When we see a coin not directly in front of us, we judge it to be circular, although the camera would show it as oval, and a man would have to make it oval in a picture of a scene which contained it. We learn, therefore, to react to a visual shape in a manner corresponding to how it would appear if it were in the centre of the field of vision, provided we do not immediately focus upon it, which is what we naturally do when anything visible interests us. As a matter of fact, we are constantly looking in different directions, and, as a rule, only noticing what, at the moment, is in the centre of our field of vision. Thus our visual world consists rather of a synthesis of things viewed directly in succession than of things seen simultaneously while the centre of the field is kept fixed. This is one reason why the rules of perspective have to be learned, although a picture which ignores them makes an impression of being “wrong”.

Another reason for the objectivity of the impressions we derive from sight is correlation with other senses, especially touch. Through this correlation we soon get to “know” that a man twenty yards away is “really” just as big as a man one yard away. When children are learning to draw, they find it very difficult to make distant objects sufficiently small, because they know they are not “really” small. We soon learn to judge the distance of a visual object, and to react to it according to the size that it would have if touched—or travelled over—in the case of very large objects such as mountains. Our sense of size is not derived from sight, but from such sources as touch and locomotion; our metrical judgments, when the stimulus is only visual, are a result of previous experience.

By the time a child can speak well, he has had a great deal of this kind of experience. Consequently our verbal reactions contain a great deal more objectivity than they would if they came at an earlier stage of infancy. The result is that a number of people can view a scene simultaneously, and use exactly the same words about it. The words which we naturally use in describing what we see are those describing features that will also be evident to others in our neighbourhood. We say, “there is a man”, not, “there is a coloured shape whose visual dimensions are such-and-such an angle vertically, and such another horizontally”. The inference is a physiological inference, and only subsequent reflection makes us aware that it has taken place. We can, however, become aware of it through occasional mistakes; a dot on the window-pane may be mistaken for a man in a distant field. In this case, we can discover our error by opening the window, or by moving the head. In general, however, physiological inferences of this sort are correct, since they have resulted from correlations which are very common, and are likely to be present on a given occasion. Consequently our words tend to conceal what is private and peculiar in our impressions, and to make us believe that different people live in a common world to a greater extent than is in fact the case.

We have been using the word “objectivity” in the preceding pages, and it is time to consider exactly what we mean by it. Suppose some scene—say in a theatre—is simultaneously seen by a number of people and photographed by a number of cameras. The impression made upon a person or a camera is in some respects like that made upon other persons and cameras, in other respects different. We shall call the elements which are alike “objective” elements in the impression, and those which are peculiar we shall call “subjective”. Thus those features of shapes which are considered in projective geometry will be objective, whereas those considered in metrical geometry (where lengths and angles are measured) cannot be made objective through sight alone, but demand the use of other senses. In the photographs, a man on the stage will be longer if the camera is near the stage than if it is far off, assuming all the cameras to be alike. But if four actors are standing in a row in one photograph, they will be standing in a row in another; this is an “objective” feature of the impression. And the differences in the visual impressions of a number of spectators with normal eyesight are exactly analogous to the differences in the photographs; so also are the likenesses. Thus the “subjectivity” that we are speaking about at present is something belonging to the physical world, not to psychology. It marks the fact that the stimulus, whether to an eye or to a camera, is not exactly the same wherever the eye or the camera may be placed; there are features of the stimulus which are constant (within limits), but there are others which are different from any two different points of view.

The tendency of our perceptions is to emphasise increasingly the objective elements in an impression, unless we have some special reason, as artists have, for doing the opposite. This tendency begins before speech, is much accentuated after speech has been acquired, and is prolonged by scientific physics. The theory of relativity is only the last term, so far, in the elimination of subjective elements from impressions. But it must not be supposed that the subjective elements are any less “real” than the objective elements; they are only less important. They are less important because they do not point to anything beyond themselves as the others do. We want our senses to give us information, i.e. to tell us about something more than our own momentary impression. We acquire information through our senses if we attend to the objective elements in the impression and ignore the others; but the subjective elements are just as truly part of the actual impression. This is obvious as soon as we realise that the camera is as subjective as we are.

Such considerations lead irresistibly to the scientific view that, when an object can be seen or photographed from a number of points of view, there is a connected set of events (light-waves) travelling outward from a centre; that, moreover, there are some respects in which all these events are alike, and others in which they differ one from another. We must not think of a light-wave as a “thing”, but as a connected group of rhythmical events. The mathematical characteristics of such a group can be inferred by physics, within limits; but the intrinsic character of the component events cannot be inferred. The events constituting light-waves are only known through their effects upon our eyes, optic nerves, and brains, and these effects are not themselves light-waves, as is obvious from the fact that nerves and brains are not transparent. Light in the physical world, therefore, must consist of events which are in some way different from the events which happen when we see; but we cannot say more than this as to the intrinsic quality of these external events. Moreover, when a number of people, as we say, “see the same thing”, what we have reason to believe is that light-waves emanating from a certain region have reached the eyes of all these people. As to what is in the region from which the light-waves come, we cannot tell.

But—so the plain man is tempted to argue—we can tell quite well, because we can touch objects that we see, and discover that there is something hard and solid in the place from which the light-waves come. Or, again, we may find that there is something there which, though not solid, is very hot, and burns us when we try to touch it. We all feel that touch gives more evidence of “reality” than sight; ghosts and rainbows can be seen but not touched. One reason for this greater sense of reality is that our spatial relation to an object when we touch it with our finger-tips is given, and therefore an object does not give such different impressions of touch to different people as it does of sight. Another reason is that there are a number of objects that can be seen but not touched—reflections, smoke, mist, etc.—and that these objects are calculated to surprise the inexperienced. None of these facts, however, justify the plain man in supposing that touch makes him know real things as they are, though we are verbally forced to admit that it brings him into “contact” with them.

We have seen on an earlier occasion how complex is the physical and physiological process leading from the object to the brain when we touch something; and we have seen that illusions of touch can be produced artificially. What we experience when we have a sensation of touch is, therefore, no more a revelation of the real nature of the object touched than what we experience when we look at it. As a matter of fact, if modern physics is to be believed, sight, prudently employed, gives us a more delicate knowledge concerning objects than touch can ever do. Touch, as compared with sight, is gross and massive. We can photograph the path of an individual electron. We perceive colours which indicate the changes happening in atoms. We can see faint stars even though the energy of the light that reaches us from them is inconceivably minute. Sight may deceive the unwary more than touch, but for accurate scientific knowledge it is incomparably superior to any of the other senses.

It is chiefly through ideas derived from sight that physicists have been led to the modern conception of the atom as a centre from which radiations travel. We do not know what happens in the centre. The idea that there is a little hard lump there, which is the electron or proton, is an illegitimate intrusion of common-sense notions derived from touch. For aught we know, the atom may consist entirely of the radiations which come out of it. It is useless to argue that radiations cannot come out of nothing. We know that they come, and they do not become any more really intelligible by being supposed to come out of a little lump.

Modern physics, therefore, reduces matter to a set of events which proceed outward from a centre. If there is something further in the centre itself, we cannot know about it, and it is irrelevant to physics. The events that take the place of matter in the old sense are inferred from their effect on eyes, photographic plates, and other instruments. What we know about them is not their intrinsic character, but their structure and their mathematical laws. Their structure is inferred chiefly through the maxim “same cause, same effect”. It follows from this maxim that if the effects are different, the causes must be different; if, therefore, we see red and blue side by side, we are justified in inferring that in the direction where we see red something different is happening from what is happening in the direction where we see blue. By extensions of this line of argument we arrive at the mathematical laws of the physical world. Physics is mathematical, not because we know so much about the physical world, but because we know so little: it is only its mathematical properties that we can discover. For the rest, our knowledge is negative. In places where there are no eyes or ears or brains there are no colours or sounds, but there are events having certain characteristics which lead them to cause colours and sounds in places where there are eyes and ears and brains. We cannot find out what the world looks like from a place where there is nobody, because if we go to look there will be somebody there; the attempt is as hopeless as trying to jump on one’s own shadow.

Matter as it appears to common sense, and as it has until recently appeared in physics, must be given up. The old idea of matter was connected with the idea of “substance”, and this, in turn, with a view of time that the theory of relativity shows to be untenable. The old view was that there is one cosmic time, and that, given any two events in any two parts of the universe, either they are simultaneous, or the first is earlier than the second, or the second earlier than the first. It was thought that the time-order of the two events must always be objectively definite, although we might be unable to determine it. We now find that this is not the case. Events which can be regarded as all in one place, or all parts of the history of one piece of matter, still have a definite time-order. So do events in different places if a person situated where the second takes place can see the first before the second happens, or, more exactly, if light can travel from the place of the one to the place of the other so as to reach the other place before the second event. (Here we mean by a “place” the position of a given piece of matter: however the matter may move relatively to other matter, it is always in the same “place” from its own point of view.) But if light travelling from the place of the one event to the place of the other event arrives at the place of the other event after the other event has taken place, and conversely, then there is no definite objective time-order of the two events, and there is no reason for regarding either as earlier than the other; nor yet for regarding the two as simultaneous; ideally careful observers will judge differently according to the way in which they are moving. Thus time is not cosmic, but is to some extent individual and personal for each piece of matter.

What do we mean by a “piece of matter” in this statement? We do not mean something that preserves a simple identity throughout its history, nor do we mean something hard and solid, nor even a hypothetical thing-in-itself known only through its effects. We mean the “effects” themselves, only that we no longer invoke an unknowable cause for them. We find that energy in various forms spreads outwards from various centres; we find also that such centres have a certain degree of persistence, though this persistence is not absolute—the modern physicist faces cheerfully the possibility that an electron and a proton may mutually annihilate each other, and even suggests that this may be the main source of the radiant energy of the stars, because when it happens it makes an explosion. What is asserted may be put as follows: When energy radiates from a centre, we can describe the laws of its radiation conveniently by imagining something in the centre, which we will call an electron or a proton according to circumstances, and for certain purposes it is convenient to regard this centre as persisting, i.e. as not a single point in space-time but a series of such points, separated from each other by time-like intervals. All this, however, is only a convenient way of describing what happens elsewhere, namely the radiation of energy away from the centre. As to what goes on in the centre itself, if anything, physics is silent.

What Dr. Whitehead calls the “pushiness” of matter disappears altogether on this view. “Matter” is a convenient formula for describing what happens where it isn’t. I am talking physics, not metaphysics; when we come to metaphysics, we may be able, tentatively, to add something to this statement, but science alone can hardly add to it. Materialism as a philosophy becomes hardly tenable in view of this evaporation of matter. But those who would formerly have been materialists can still adopt a philosophy which comes to much the same thing in many respects. They can say that the type of causation dealt with in physics is fundamental, and that all events are subject to physical laws. I do not wish, as yet, to consider how far such a view should be adopted; I am only suggesting that it must replace materialism as a view to be seriously examined.