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.