London: Routledge & Kegan Paul, 1952; Chicago: University of Chicago Press, 1952, xxii, 209 pp; new edition 1963/1976. Reprinted Chicago: University of Chicago Press, Phonenix Book paperback, 1963 (out of print). University of Chicago Press has reissued the paperback in a Midway Reprint, 1976, with the Heinrich Klüver Introduction.

A great deal of explanation would be necessary were I to justify why an economist ventures to rush in where psychologists fear to tread. But this excursion into psychology has little connexion with whatever competence I may possess in another field. It is the outcome of an idea which suggested itself to me as a very young man when I was still uncertain whether to become an economist or a psychologist. But though my work has led me away from psychology, the basic idea then conceived has continued to occupy me; its outlines have gradually developed, and it has often proved helpful in dealing with the problems of the methods of the social sciences. In the end it was concern with the logical character of social theory which forced me to re-examine systematically my ideas on theoretical psychology.

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