There seems no reason to regard causation as a priori, though this question is not simple. Given certain very general assumptions as to the structure of space-time, there are bound to be what we have called causal laws. These general assumptions must really replace causality as our basic principles. But, general as they are, they cannot be taken as a priori; they are the generalisation and abstract epitome of the fact that there are causal laws, and this must remain merely an empirical fact, which is rendered probable, though not certain, by inductive arguments.
[CHAPTER XV]
THE NATURE OF OUR KNOWLEDGE OF PHYSICS
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.