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 the present year under the title: "The Present System of Theoretical Physics." 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. CONTENTS The Origin and Development of the Quantum Theory 1 First Lecture. 17 Introduction: Reversibility and Irreversibility. 17 Second Lecture.40 Thermodynamic States of Equilibrium in Dilute Solutions. 40 Third Lecture. 67 The Atomic Theory of Matter. 67 Fourth Lecture. 87 The Equation of State for a Monatomic Gas. 87 Fifth Lecture. 103 Heat Radiation. Electrodynamic Theory. 103 Sixth Lecture. 125 Heat Radiation. Statistical Theory. 125 Seventh Lecture. 137 General Dynamics. Principle of Least Action. 137 Eighth Lecture. 156 General Dynamics. Principle of Relativity. 156