In his Die beiden Grundprobleme der Erkenntnistheorie, Popper explains that different epistemological positions can be regarded as attempts to solve the problems of induction and demarcation. Inspired by Popper's approach, the author considers Ludwig von Mises' epistemological position as an endeavor to resolve those problems with respect to the special situation of the social sciences. Mises states that the theoretical social sciences can be justified only as an a priori discipline, which he calls ‘Praxeology’. In his view, all statements of praxeology presuppose an a priori valid ‘category of human action’. Given his argument, this paper suggests that the ‘category of action’ may be interpreted in four different ways; as (1) an observational statement or a statement describing experiences from introspection; (2) a proposition about the basic ontological form of the social universe, describing its essential characteristics; (3) a definition, adopted as a convention; (4) a methodological principle, of the kind required by methodological individualism. However, under no interpretation can Mises justify the social sciences as a priori disciplines.