Originally published in 1942, this book discusses an emerging physical science that brought with it a fresh message as to the fundamental nature of the world, and of the possibilities of human free will in particular. The aim of the book is to explore that territory, which forms a borderland between physics and philosophy. The author seeks to estimate the philosophical significance of physical developments, and the interest of his enquiry extends far beyond technical physics and philosophy. Some of the questions raised touch everyday human life closely: can we have knowledge of the world outside us other than that what we can gain by observation and experiment? Is the world spiritual and psychological or material in its ultimate essence; is it better likened to a thought or to a machine? Are we endowed with free will, or are we part of a vast machine that must follow its course until it finally runs down?