1929. The course of Gifford Lectures that Eddington delivered in the University of Edinburgh in January to March 1927. It treats of the philosophical outcome of the great changes of scientific thought which have recently come about. The theory of relativity and the quantum theory have led to strange new conceptions of the physical world; the progress of the principles of thermodynamics has wrought more gradual but no less profound change. The first eleven chapters are for the most part occupied with the new physical theories, with the reasons which have led to their adoption, and especially with the conceptions which seem to underlie them. The aim is to make clear the scientific view of the world as it stands at the present day, and, where it is incomplete, to judge the direction in which modern ideas appear to be tending. In the last four chapters I consider the position which this scientific view should occupy in relation to the wider aspects of human experience, including religion. Contents: The Downfall of Classical Physics; Relativity; Time; The Running-Down of the Universe; Becoming; Gravitation-the Law; Gravitation-the Explanation; Man's Place in the Universe; The Quantum Theory; The New Quantum Theory; World Building; Pointer Readings; Reality; Causation; and Science and Mysticism.