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.