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 1935, this book contains the substance of the Messenger Lectures which were delivered by Eddington at Cornell University in April and May 1934. Using as a nucleus material in addresses and lectures given by the author from 1929 onwards, the text discusses a variety of advances in the physical sciences and scientific thought in general. 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.