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In this book Planck has followed in the footsteps of those two talented English popularizers of the philosophical implications of modern physics, Eddington and Jeans, but he has not entered a field hitherto foreign to him. On the contrary, Planck has always been interested in the borderline problems of physics and philosophy and has published many addresses and articles in this domain. Especially important for him has been the significance of causality in physics in its bearing upon freedom of will; this problem has long occupied him, and it is discussed again in his latest book, two of the six chapters of which are devoted to it. Next in importance among the topics of the book is that of the reality of the external world. The two remaining chapters (Fifty Years of Science, and From the Relative to the Absolute) are of slighter significance.
It is not necessary to reproduce in this review the development of the author's ideas. The problems that concern him, and his procedures belong to an antiquated academic philosophy, which, it must be said, is not even employed correctly. In discussing the question "Is the external world real?" he attacks positivism, identifies it with a misrepresented solipsism that no one defends, and uses in the process a "Kantianism" that equates the concepts "objective" and "transcendental". With the principle that observation is the sole source of science and sensory experience the only source of physics he is unable to find a connection to the existence of a real external world except by a "jump into the metaphysical realm". He opposes to the positivistic postulate that there is no other source of knowledge in physics except within the restricted range of perception through the senses, the two metaphysical postulates: "(1) that there is a real outer world which exists independently of our act of knowledge, and (2) the real outer world is not directly knowable." To this adds: "to a certain degree these two statements are mutually