This paper contrasts the Thomistic and Cartesian interpretations of what the substantial unity of the body and mind can consist in. A detailed discussion of the Thomistic account of the substantial unity of body and soul identifies especially those principles of the presupposed hylomorphist metaphysical background of this account that Descartes abandoned. After arguing for the consistency of the Thomistic view, briefly outlines how certain developments in late-medieval scholasticism prepared the way for the abandonment of precisely these principles. Finally, the (...) paper shows why from the perspective of the Thomistic principles identified in the first part, Descartes has to appear as someone paying mere lip-service to the thesis of the substantial unity of body and mind. (shrink)