The present book has grown out of the conviction that no true understanding of contemporary physics and its philosophical implications is possible without first fully realizing in what sense and to what extent modern physical concepts differ from the concepts of classical physics. The classical concepts of space, time, matter, motion, energy, and causality have been radically transformed recently; although the words used by contemporary physicists are the same, their connotations are altogether different from those of their classical counterparts. There is hardly any similarity between the "matter" of modern physics and the traditional material substance of the classical period, and this is true in varying degrees of other concepts as well. The revolutionary character of modern concepts cannot be fully grasped as long as the contrasting background of classical physics is not kept constantly in sight. To bring into full focus an awareness of the contrast between the classical and the modern conceptual frameworks is one of the purposes of this book.