The present book has for its object the presentation of the lectures which I delivered as foreign lecturer at Columbia University in the spring of 1909 under the title: “The Present System of Theoretical Physics.” It also includes the 1920 Nobel Prize Address on the Origin and Development of the Quantum Theory. The points of view which influenced me in the selection and treatment of the material are given at the beginning of the first lecture. Essentially, they represent the extension of a theoretical physical scheme, the fundamental elements of which I developed in an address at Leyden entitled: “The Unity of the Physical Concept of the Universe.” Therefore I regard it as advantageous to consider again some of the topics of that lecture. The presentation will not and cannot, of course, claim to cover exhaustively in all directions the principles of theoretical physics.