These four scientists are unique in their respective elaborations and elucidations of mathematical philosophy. It is not for me to advise the reader what selections to make, for if a thorough knowledge of the subject is desired the reader should read all these books, but not all readers are willing to make that effort toward clear thinking (which in the meantime will remain of the _highest_ importance in science). Some readers will wish to select for themselves and to facilitate their selection I will lay out a “Menu” of this intellectual feast by giving in some cases the chapter heads. For many temporary reasons I was not able, before going into print, to give a fuller list of the writings of those four unique men; but there is no stroke of their pen but which should be read with great attention—besides which there is a very valuable literature about their work. (1) The purely mathematical foundation: RUSSELL, BERTRAND. “The Principles of Mathematics.” Cambridge University, 1903. (I am not giving any selections from the contents of this book because this book should, without doubt, be read by every one interested in mathematical philosophy.) “The Problems of Philosophy.” H. Holt & Co., N. Y., 1912. “Our Knowledge of the External World, as a Field for Scientific Method in Philosophy.” Chicago, 1914. “Introduction to Mathematical Philosophy.” Macmillan, N. Y. Selection from contents: Definition of number. The Definition of order. Kinds of relations. Infinite cardinal numbers. Infinite series and ordinals. Limits and continuity. The axiom of infinity and logical types. Classes. Mathematics and logic. “Mysticism and Logic.” Longmans Green & Co. 1919. N. Y. Selection from contents: Mathematics and the metaphysicians. On scientific method in philosophy. The ultimate constituents of matter. On the notion of cause. WHITEHEAD, ALFRED N. “An Introduction to Mathematics.” Henry Holt & Co. 1911. N. Y. “The Organization of Thought Educational and Scientific.” London, 1917. Selections from contents: The principles of mathematics in relation to elementary teaching. The organization of thought. The anatomy of some scientific ideas. Space, time, and relativity. “An Enquiry Concerning the Principles of Natural Knowledge.” Cambridge, 1919. Selection from contents: The traditions of science. The data of science. The method of extensive abstraction. The theory of objects. “The Concept of Nature.” Cambridge, 1920. Selection from contents: Nature and thought. Time. The method of extensive abstraction. Space and motion. Objects. The ultimate physical concepts. “Principia Mathematica.” By A. N. Whitehead and Bertrand Russell. Cambridge, 1910-1913.