The aim of the present book is very simply stated; it is to discuss—and to some extent to explore—that borderland territory between physics and philosophy which used to seem so dull, but suddenly became so interesting and important through recent developments of theoretical physics.

The new interest extends far beyond the technical problems of physics and philosophy to questions which touch human life very closely, such as materialism and free-will. Thus I hope the book may interest many who are neither physicists nor philosophers by profession, and to this end I have made the discussion as simple as possible, avoiding technicalities when I could, and, when I could not, explaining them. I have also tried to arrange the book so that a reading of the first two chapters and the last shall give an intelligible view of the main argument and conclusions of the whole; many readers may prefer to read these three chapters first.

I need hardly add that my acquaintance with philosophy is simply that of an intruder, and nothing could be further from my intentions than to pose as an authority on questions of pure philosophy. If I had to choose a sub-title for my book, it might well be ‘The reflections of a physicist on some of the problems of philosophy”.