In the main, therefore, experience is, according to common sense, a very simple affair. We see something, or we feel something, or, like Dr. Johnson, we kick something, and “there’s an end on’t.” Experience is the source of all knowledge, and therefore of all explanation; but, in itself, it seems scarcely to require to be explained. Common sense is prepared to leave it where it finds it. No doubt the occurrence of optical or other illusions may disturb this mood of intellectual tranquillity. Common sense, when it has to consider the case of appearances, some of which are held, on extraneous grounds, to be real and others to be illusory, may feel that there are, after all, problems raised by perception—by the direct experience of things—which are not without their difficulties. But the case of illusions is exceptional, and rarely disturbs the even tenor of our daily round.
II
Now science, as it gladly acknowledges, is but an extension of common sense. It accepts, among other matters, the common-sense view of perception. Like common sense, it distinguishes the thing as it is from the thing as it appears. Like common sense, it regards the things which are experienced as being themselves unaffected by experience. But, unlike common sense, it devotes great attention to the way in which experience is produced by things. Its business is with the causal series. This, to be sure, is a subject which common sense does not wholly ignore. It would acknowledge that we perceive a lamp through the light which it sheds, and recognise a trumpet through the sound which it emits; but the nature of light or sound, and the manner in which they produce our experience of bright or sonorous objects, it hands over to science for further investigation.
And the task is cheerfully undertaken. Science also deems perception to be the source of all our knowledge of external nature. But it regards it as something more, and different. For perception is itself a part of nature, a natural process, the product of antecedent causes, the cause of subsequent effects. It requires, therefore, like other natural facts, to be observed and explained; and it is the business of science to explain it.
Thus we are brought face to face with the contrast on which so much of the argument of these lectures turns: the contrast between beliefs considered as members of a cognitive series, and beliefs considered as members of a causal series. In the cognitive series, beliefs of perception are at the root of our whole knowledge of natural laws. In the causal series, they are the effects of natural laws in actual operation. This is so important an example of this dual state that you must permit me to consider it in some detail.
We may examine what goes on between the perceiving person and the thing he perceives from either end; but it is by no means a matter of indifference with which end we begin. If we examine the relation of the perceiver to the perceived it does not seem convenient or accurate to describe that relation as a process. It is an experience, immediate and intuitive; not indeed infallible, but direct and self-sufficient. If I look at the sun, it is the sun I see, and not an image of the sun, nor a sensation which suggests the sun, or symbolises the sun. Still less do I see ethereal vibrations, or a retinal image, or a nervous reaction, or a cerebral disturbance. For, in the act of perceiving, no intermediate entities are themselves perceived.
But now if we, as it were, turn round, and, beginning at the other end, consider the relation of the perceived to the perceiver, no similar statements can be made. We find ourselves concerned, not with an act of intuition, but with a physical process, which is complicated, which occupies time, which involves many stages. We have left behind cognition; we are plunged in causation. Experience is no longer the immediate apprehension of fact; it is the transmission of a message conveyed from the object to the percipient by relays of material messengers. As to how the transmission is effected explanations vary with the growth of science. They have been entirely altered more than once since the modern era began, and with each alteration they become more complicated. They depend, not on one branch of science only, but on many. Newtonian astronomy, solar physics, the theory of radiation, the optical properties of the atmosphere, the physiology of vision, the psychology of perception, and I daresay many other branches of research, have to be drawn upon: and all this to tell us what it is we see, and how it is we come to see it.
III
Now there is no one who possesses the least smattering of philosophy who does not know that the views I have just endeavoured to describe are saturated with difficulties: difficulties connected with the nature of perception; difficulties connected with the nature of the object as perceived; difficulties connected with its unperceived physical basis; difficulties connected with the relation in which these three stand to each other. For common sense the material object consists of a certain number of qualities and aspects which are perceived, an inexhaustible number which might be perceived, but are not, and (perhaps) a vaguely conceived “somewhat” lying behind both. The medieval Aristotelian, if I rightly understand him (which very likely I do not), developed this “somewhat” into the notion of substance—an entity somewhat loosely connected with the qualities which it supported, and in no way explaining them. There was “substance” in a piece of gold, and “substance” in a piece of lead; but there was nothing unreasonable in the endeavour to associate the qualities of gold with the substance of lead, and thus for all practical purposes to turn lead into gold.
Modern science teaches a very different lesson. It has, perhaps, not wholly abandoned the notion of material substance, if this be defined as the unperceivable support of perceivable qualities; but it persistently strives to connect the characteristics of matter with its structure, and, among other characteristics, that of producing, or helping to produce, in us those immediate perceptions which we describe as our experience of matter itself.