It is a curious fact that in the many writings on the theory of probability, one rarely finds an instance of a clear cut definition of the fundamental problem of the subject. On intuitive grounds, one grasps the intent of most of the authors: it is to define the concept of probability and to show its relationship to the other concepts of science; and yet since the criterion of adequacy in the answering of this question is rarely stated, one must feel that the problem itself has remained undefined.