We can already see a correlation between Frege’s three realms and the three modes of investigating human action distinguished by Mises:
Many authors believe that psychology is basic to the social sciences, even that it comprehends them all. ... Insofar as psychology proceeds with the experimental methods of physiology, these claims are manifestly unwarranted. ... But the term “psychology” is applied in another sense too. It signifies the cognition of human emotions, motivations, ideas, judgments of value and volitions, a faculty indispensable to everybody in the conduct of daily affairs .... This popular use of the term “psychology” must not be confused with the psychology of any of the naturalistic schools. ... To prevent mistakes resulting from the confusion of these two entirely different branches of knowledge it is expedient to reserve the term “psychology” for naturalistic psychology and to call the knowledge of human valuations and volitions “thymology.” ...
It is obvious that this knowledge [= thymology] which provides a man with the ability to anticipate to some degree other people’s future attitudes is not a priori knowledge. The a priori discipline of human action, praxeology, does not deal with the actual content of value judgments; it deals only with the fact that men value and then act according to their valuations. What we know about the actual content of judgments of value can be derived only from experience.4
In short, naturalistic psychology, which investigates those aspects of human action (reflexes, neuron firings, and such) that can be grasped by the empirical methods of the natural sciences, corresponds to Frege’s outer realm; thymology, the method of history, which investigates those aspects of human action (subjective meanings and motivations) that can be grasped by the hermeneutical method of verstehen, corresponds to Frege’s inner realm; and praxeology, the method of economic theory, which investigates those aspects of human
3 Cf. Kelly Dean Jolley, Review of C. O. Hill and G. E. R. Haddock, Husserl or Frege? Meaning, Objectivity, and Mathematics (Journal of the History of Philosophy 39, no. 2, April 2001), pp. 311-312: “Frege’s ontological categories supervene on logical ones .... For Frege, there can be no grounding of logic, no justifying of logic from behind.”
4 Ludwig von Mises, Theory and History (Ludwig von Mises Institute, 1985), pp. 264-271, 311.
R. T. Long – Wittgenstein, Praxeology, and Frege’s Three Realms – p. 3