Sir Arthur Eddington (1882-1944) was both a key figure in the development of astrophysics during the early part of the twentieth century and a notable philosopher of science. Originally published in 1939, this book contains the substance of the course of lectures delivered by Eddington as Tarner Lecturer of Trinity College, Cambridge in the Easter term of 1938. The text discusses the principles of philosophical thought associated with advances in physical science, developing the ideas contained in earlier titles such as The Nature of the Physical World (1928) in line with discoveries in quantum mechanics and group theory. This is a highly insightful title that will be of value to anyone with an interest in Eddington, the development of physics and the philosophy of science.