It is often said that there is no "philosophy of science", but only the philosophies of certain scientists. But in so far as we recognize an authoritative body of opinion which decides what is and what is not accepted as present-day physics, there is an ascertainable present-day philosophy of physical science. It is the philosophy to which those who follow the accepted practice of science stand committed by their practice. This book contains the substance of the course of lectures which the author Eddington delivered as Tarner Lecturer of Trinity College Cambridge in the Easter Term 1938. The lectures have afforded him an opportunity of developing more fully than in his earlier books the principles of philosophic thought associated with the modern advances of physical science.