DR. KEYNES'S book is a searching analysis of the fundamental principles of the theory of probability and of the particular judgments involved in its application to concrete problems. He adopts the view that knowledge may be relevant to our rational belief of a proposition without amounting to complete proof or disproof of it, and treats the probability as a measure of this relevance. NO.Otherwise he does not attempt to define “probability,” regarding it as a concept intelligible without further definition. In this respect, as in several others, he is in agreement with the views expressed by Dr. Wrinch and the present reviewer (Philosophical Magazine, vol. 38, 1919, pp. 715-31), and some comparison of the two presentations may not be out of place. A Treatise on Probability By J. M. Keynes. Pp. xi + 466. (London: Macmillan and Co., Ltd., 1921.) 18s. net.