A point of view is presented concerning the psychological concept of subjective probability, both to study its relation to the corresponding mathematical and philosophical concepts and to provide a framework for the rigorous investigation of problems unique to psychology. In order to do this the empirical implications of axiom systems for measurement are discussed first, relying primarily on Krantz's work, with special emphasis, however, on some similarities and differences between psychological and physical variables. The psychological variable of uncertainty is then examined in this light, and it is concluded that few, if any, current theories are satisfactory when viewed from this perspective, particularly those deriving from the mathematical work in the axiomatic foundations of probability. This might appear to pose difficulties for applications to real problems of normative decision theory when those applications require numerical probability judgments from individuals. Two possible solutions are discussed briefly. (Author)