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 1928, this book contains the substance of the Gifford Lectures which were delivered by Eddington at the University of Edinburgh in January to March 1927. The text discusses the philosophical outcome of fundamental changes in scientific thought related to areas such as the theory of relativity, quantum theory and thermodynamics. 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.