Heidegger's critique of natural science and its domination in the area of psychiatry, as it is formulated in his Zollikoner Seminare, is confronted with Foucault's epistemological analysis of the classical thought that is presented in Les mots et les choses and with the picture of classical medicine that Foucault presents in his Naisannce de la clinique. This confrontation brings to the fore the Cartesian idea of mathesis universalis which functions as a general matrix of scientific thought. The play of conceptual identities and differences based on the general matrix of mathesis universalis, however, leaves no place for the individuality of human existence. To grasp the individuality of human existence, both phenomenology and medicine must turn away from the conceptual scheme of mathesis universalis and from the classical notion of thought. Together with the individuality of human existence, phenomenology also uncovers the phenomenon of the lived body which reflects the psychosomatic nature of human existence. In Zollikoner Seminare, Heidegger then integrates the individuality of the human existence with the phenomenon of the lived body in the complex structure of being-in-the-world. To avoid the use of the German term Dasein with whose help Heidegger describes human existence and its being-in-the-world, the term "sojourn" (der Aufenthalt) is introduced, which, in Zollikoner Seminare, serves as synonym for Dasein.