In these lectures the author Eddington discusses some of the results of modern study of the physical world which give most food for philosophic thought. This will include new conceptions in science and also new knowledge. In both respects we are led to think of the material universe in a way very different from that prevailing at the classical physics. This book is substantially the course of Gifford Lectures which the author 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. 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.