As the above example illustrates, we do not, in fact, experience many things that we think we experience. This makes it necessary to ask, without too much assurance, in what sense physics can be based upon experience, and what must be the nature of its entities and its inferences if it is to make good its claim to be empirically grounded. We shall begin this inquiry in the next chapter.
[CHAPTER XII]
PHYSICS AND PERCEPTION
It will be remembered that we regarded perception, in Chapter V, as a species of “sensitivity”. Sensitivity to a given feature of the environment we defined as consisting in some characteristic reaction which is exhibited whenever that feature is present, but not otherwise; this property is possessed more perfectly, in given directions, by scientific instruments than by living bodies, though scientific instruments are more selective as to the stimuli to which they will respond. We decided that what, from the standpoint of an external observer, distinguishes perception from other forms of sensitivity is the law of association or conditioned reflexes. But we also found that this purely external treatment of perception presupposes our knowledge of the physical world as a going concern. We have now to investigate this presupposition, and to consider how we come to know about physics, and how much we really do know.
According to the theory of [Chapter V], it is possible to perceive things that are not in a spatial contact with the body. There must be a reaction to a feature of the environment, but that feature may be at a greater or less distance from the body of the percipient; we can even perceive the sun and stars, within the limits of the definition. All that is necessary is that our reaction should depend upon the spatial relation between our body and the feature of the environment. When our back is towards the sun, we do not see it; when our face is towards it, we do.
When we consider perception—visual or auditory—of an external event, there are three different matters to be examined. There is first the process in the outside world, from the event to the percipient’s body; there is next the process in his body, in so far as this can be known by an outside observer; lastly, there is the question, which must be faced sooner or later, whether the percipient can perceive something of the process in his body which no other observer could perceive. We will take these points in order.
If it is to be possible to “perceive” an event not in the percipient’s body, there must be a physical process in the outer world such that, when a certain event occurs, it produces a stimulus of a certain kind at the surface of the percipient’s body. Suppose, for example, that pictures of different animals are exhibited on a magic lantern to a class of children, and all the children are asked to say the name of each animal in turn. We may assume that the children are sufficiently familiar with animals to say “cat”, “dog”, “giraffe”, “hippopotamus”, etc., at the right moments. We must then suppose—taking the physical world for granted—that some process travels from each picture to the eyes of the various children, retaining throughout these journeys such peculiarities that, when the process reaches their eyes, it can in one case stimulate the word “cat” and in another the word “dog”. All this the physical theory of light provides for. But there is one interesting point about language that should be noticed in this connection. If the usual physical theory of light is correct, the various children will receive stimuli which differ greatly according to their distance and direction from the picture, and according to the way the light falls. There are also differences in their reactions, for, though they all utter the word “cat”, some say it loud, others soft, some in a soprano voice, some in a contralto. But the differences in their reactions are much less than the differences in the stimuli. This is still more the case if we consider various different pictures of cats, to all of which they respond with the word “cat”. Thus language is a means of producing responses which differ less than the stimuli do, in cases where the resemblances between the stimuli are more important to us than the differences. This fact makes us apt to overlook the differences between stimuli which produce nearly identical responses.
As appears from the above, when a number of people simultaneously perceive a picture of a cat, there are differences between the stimuli to their various perceptions, and these differences must obviously involve differences in their reactions. The verbal responses may differ very little, but even the verbal responses could be made to differ by putting more complicated questions than merely “What animal is that?” One could ask: “Can the picture be covered by your thumb-nail held at arm’s length?” Then the answer would be different according as the percipient was near the picture or far off. But the normal percipient, if left to himself, will not notice such differences, that is to say, his verbal response will be the same in spite of the differences in the stimuli.
The fact that it is possible for a number of people to perceive the same noise or the same coloured pattern obviously depends upon the fact that a physical process can travel outward from a centre and retain certain of its characteristics unchanged, or very little changed. The most notable of such characteristics is frequency in a wave-motion. That, no doubt, affords a biological reason for the fact that our most delicate senses, sight and hearing, are sensitive to frequencies, which determine colour in what we see and pitch in what we hear. If there were not, in the physical world, processes spreading out from centres and retaining certain characters practically unchanged, it would be impossible for different percipients to perceive the same object from different points of view, and we should not have been able to discover that we all live in a common world.
We come now to the process in the percipient’s body, in so far as this can be perceived by an outside observer. This raises no new philosophical problems, because we are still concerned, as before, with the perception of events outside the observer’s body. The observer, now, is supposed to be a physiologist, observing, say, what goes on in the eye when light falls upon it. His means of knowing are, in principle, exactly the same as in the observation of dead matter. An event in an eye upon which light is falling causes light-waves to travel in a certain manner until they reach the eye of the physiologist. They there cause a process in the physiologist’s eye and optic nerve and brain, which ends in what he calls “seeing what happens in the eye he is observing”. But this event, which happens in the physiologist, is not what happened in the eye he was observing; it is only connected with this by a complicated causal chain. Thus our knowledge of physiology is no more direct or intimate than our knowledge of processes in dead matter; we do not know any more about our eyes than about the trees and fields and clouds that we see by means of them. The event which happens when a physiologist observes an eye is an event in him, not on the eye that he is observing.
We come now at last to the question of self-observation, which we have hitherto avoided. I say “self-observation” rather than “introspection”, because the latter word has controversial associations that I wish to avoid. I mean by “self-observation” anything that a man can perceive about himself but that others, however situated, cannot perceive about him. What follows is only preliminary, since the subject will be discussed at length in [Chapter XVI].