From the beginning of his career, Reichenbach studied the role that probability played both in modern physical theory and in epistemology.¹ He was, with Richard von Mises, one of the foremost proponents of the frequency theory of probability and axiomatized a very general form of it. He further claimed that all reasonable uses of the concept of probability, both in physical theory and as an evaluative concept in epistemology, are to be explicated in terms of the frequency theory.