Since the publication of this book, a little more than a year ago, the interest in Einstein and the principle of relativity has very greatly increased. There are now a large number of popular expositions, and the theory itself has undergone some notable advances in its philosophical, mathematical and physical application. In pure philosophy Lord Haldane's Reign of Relativity has applied it to the direct interpretation of the theory of knowledge. In mathematical physics the important work of Hermann Weyl, Space-Time-Matter, is an advance even on Einstein, and applies the principle not only to the geometry of masses, but to that of intermolecular movements, and to electro-magnetic phenomena generally, thus making it embrace the physical world in its entirety. The main purpose of this book is to show the historical relations of the new principle to the old philosophical problems and to the classical theories of space and time. I have, with the exception of a few important emendations, left the historical chapters unaltered, but I have added a new chapter on "Einstein's Theory" in order to show more clearly than the original work did the nature of the revolution which it effects in scientific method and in the concept of physical reality.

A problem of philosophy is completely different from a problem of science. In science we accept our subject-matter as it is presented in unanalysed experience; in philosophy we examine the first principles and ultimate questions that concern conscious experience itself. The problem of truth is a problem of philosophy. It is not a problem of merely historical interest, but a present problem-a living controversy, the issue of which is undecided. Its present interest may be said to centre round the doctrine of pragmatism, which some fifteen years ago began to challenge the generally accepted principles of philosophy. In expounding this problem of truth, my main purpose has been to make clear to the reader the nature of a problem of philosophy and to disclose the secret of its interest. My book presumes no previous study of philosophy nor special knowledge of its problems. The theories that I have shown in conflict on this question are, each of them, held by some of the leaders of philosophy. In presenting them, therefore, I have tried to let the full dialectical force of the argument appear. I have indicated my own view, that the direction in which the solution lies is in the new conception of life and the theory of knowledge given to us in the philosophy of Bergson. If I am right, the solution is not, like pragmatism, a doctrine of the nature of truth, but a theory of knowledge in which the dilemma in regard to truth does not arise. But, as always in philosophy, the solution of one problem is the emergence of another. There is no finality.