Nothing is more surprising to the ordinary reflective man than the concept of a finite universe, yet it is to such a concept that the latest development of mathematical science, so admirably expounded in Jeans's Astronomy and Cosmogony, requires us to adapt ourselves. A finite universe is one in which anything moving outwards in a straight line, and maintaining its direction, must after an interval return to its starting-point. This seems a self-contradiction. A straight line, we think, must go on for ever.

The transition from a vague generalization to an accurate statement is the first step on the road to science. It is a step of great importance. Vague generalizations find a ready entrance into many minds, and produce a comfortable sense of satisfaction that is easily mistaken for knowledge, and that stops further questioning. An exact statement of fact, on the other hand, draws attention to detail, and shows itself to be set in a mass of further detail that it challenges you to explore. “Nature abhors a vacuum” sounds final; it explains why water will rise in an ordinary suction-pump rather than allow a vacuum to be produced; why the schoolboy’s “sucker” will lift a stone; and many other phenomena of a similar kind. It explains everything by a phrase and stimulates no inquiry. But when Galileo noted that the pump would raise water only to a height of about 32 feet, and pointed out that this gives a measure of the extent to which Nature abhors a vacuum in its dealings with water, new questions arose, stimulating further investigation.

I was led to philosophy by politics. There can be no foundation for political action except in ethics; and there can be no foundation for ethics except in some form of metaphysics, whether religious or other. And one cannot travel very far along the philosophic road— particularly if one has in mind the need of arriving at some definite destination—without finding as an obstacle the perennial problem of Free Will. It is an obstacle which has somehow to be crossed. It cannot be evaded or ignored. The man who is dealing with public affairs—with the principles of Criminal Law, for example, or with the factors that make for peace or war, and indeed with any of the major questions that confront our society—if he tries to think things out, is faced constantly by the problem of individual human responsibility; just as the man of religion is faced by it constantly.

Of all the sciences, philosophy is the most concrete and comprehensive. The sense of cold, remote spaces which it is apt to generate in us is the result of this very width and concreteness. The philosopher has to condense the many-sided variety of human life and express it through the symbols of a common language. The symbols are at best only semi-transparent. Abstractness descends upon him the moment they become opaque. Philosophy, in fact, is useless to us unless we can see through it. If we cannot, the fault may be ours. If it is the philosopher's, then his failure is a failure to be wide enough or concrete enough. He needs above all to be sympathetically sensitive to the wide range of human interests, because it is this as a whole that he is bidden to systematize. He must give a single answer, and for that he must formulate a single question. Therefore his first problem is to discover the unity of problems.

I propose in this article to consider the question of the relation between mind and body. This question raises some of the most difficult issues in philosophy and constitutes the main problem of psychology.