Because human beings know things that are physical and know things that are spiritual both are seen as realities. Interestingly, as the lower aspirations – which tend to be physical needs – are fulfilled higher aspirations are sought. The higher aspirations like love, loyalty, and friendship are not physical things; nevertheless they are the realities, the realities of human beings who are in a state of higher aspirations.
Methodological dualism defines the seemingly insurmountable bridge between “the external world of physical, chemical and physiological phenomena and the internal world of thought, feeling, valuation and purposeful action.” [Mises in Human Action, Fourth Revised Edition (San Francisco: Fox & Wilkes, 1996), p. 18] The cusp of social sciences is always near this enigma. The exploration of the human mind is the task at hand and it will be a task of primary importance for the foreseeable future.
Consider one evident manifestation of dualism: the act of rational self-interest that ultimately leads to the well-being of all. This is the very essence of the divine economy! As you can see the divine economy must be profound indeed!