Drawing on the methodological character of Mises' dualism and a critical analysis of his defense of praxeology, it is argued that Mises – perhaps unintentionally – champions a form of conventionalism. All other classifications in Popper's scheme are dismissed. The proposed classification resolves a number of interpretational problems in Mises' writings, which otherwise remain open. This is in contrast both to the received view, which interprets Mises' position as synthetic apriorism, and to Tokumaru, who takes the fundamental axiom of praxeology to be a methodological rule. Puster is one of the few interpreters who also take the fundamental axiom to be analytic instead of synthetic.