This is not an introduction to physics but an analysis of its foundations. Indeed, the aims of this book are: (1) to analyze the form and content of some of the key ideas of physics; (2) to formulate several basic physical theories in an explicit and orderly (i. e. , axiomatic) fashion; (3) to exhibit their presuppositions and discuss some of their philosoph­ ical implications; (4) to discuss some of the controversial issues, and (5) to debunk certain dusty philosophical tenets that obscure the understanding of physics and hinder its progress. To the extent to which these goals are attained, the volume can serve as a companion to studies in theoretical physics aiming at deepening the understanding of the logical structure and the physical meaning of our science. In order to keep the book slender, whole fields of basic physical research had to be excluded - chiefly many-body physics, quantum field theories, and elementary particle theories. A large coverage was believed to be less important than a comparatively detailed analysis and reconstruction of three representative monuments: classical mechan­ ics, general relativity, and quantum mechanics, as well as their usually unrecognized presuppositions. The reader is invited to join the project and supply some of the many missing chapters - or to rewrite the present ones entirely.