The material of this essay is largely obtained by observation of the actual currents of opinion in physics; much of what I have to say is more or less common property and doubtless every reader will find passages that he will feel have been taken out of his own mouth. On certain broad tendencies in present day physics, however, I have put my own interpretation, and it is more than likely that this interpretation will be unacceptable to many. But even if not acceptable, I hope that the stimulus of combatting the ideas offered here may be of value. Certain limitations will have to be set to our inquiry in order to keep it within manageable compass. It is of course the merest truism that all our experimental knowledge and our understanding of nature is impossible and non-existent apart from our own mental processes, so that strictly speaking no aspect of psychology or epistemology is without pertinence. Fortunately we shall be able to get along with a more or less naive attitude toward many of these matters. We shall accept as significant our common sense judgment that there is a world external to us, and shall limit as far as possible our inquiry to the behavior and interpretation of this "external" world. We shall rule out inquiries into our states of consciousness as such. In spite, however, of the best intentions, we shall not be able to eliminate completely considerations savoring of the metaphysical, because it is evident that the nature of our thinking mechanism essentially colors any picture that we can form of nature, and we shall have to recognize that unavoidable characteristics of any outlook of ours are imposed in this way. CONTENTS Preface Introduction CHAPTER I. BROAD POINTS OF VIEW New Kinds of Experience Always Possible The Operational Character of Concepts Einstein's Contribution in Changing Our Attitude toward Concepts Detailed Discussion of the Concept of Length The Relative Character of Knowledge Meaningless Questions General Comments on the Operational Point of View II. OTHER GENERAL CONSIDERATIONS The Approximate Character of Empirical Knowledge Explanations and Mechanisms Models and Constructs The Rôle of Mathematics in Physics III. DETAILED CONSIDERATION OF VARIOUS CONCEPTS OF PHYSICS The Concept of Space The Concept of Time The Causality Concept The Concept of Identity The Concept of Velocity The Concepts of Force and Mass The Concept of Energy The Concepts of Thermodynamics Electrical Concepts The Nature of Light and the Concepts of Relativity Other Relativity Concepts Rotational Motion and Relativity Quantum Concepts IV. SPECIAL VIEWS OF NATURE The Simplicity of Nature Determinism On the Possibility of Describing Nature Completely in Terms of Analysis A Glimpse Ahead INDEX THE LOGIC OF MODERN PHYSICS CHAPTER I BROAD POINTS OF VIEW