The prospect in. the beginning of Cartesian methodology, the "mathesis universalis," was not only a mere method of the natural science, but also was meant to be the Universal methodical wisdom of human life as the unity of science and moral, of theory and practice. The necessity of the doubt consists in the fact that we cannot arrive at the knowledge of the truth because "we have once been children and have judged things presented to our senses in various ways, while we had not yet the entire use of our reason. "But this fact is due not only to the epistemological incompleteness of our knowledge, but, it arises also from the ontological situation of the human existence. Doubt means the spring-boads for our transcendence from that pastness of man to the future possibilities of our oven existence. But when Descartes explained our knowlecigesi as "aimesis," this consciousness of the ontological situation of man has been placed in that platA,istic traditions which proceed not from "existentia" "essentia" [existentialism], but from "essentia" to "existentia" of [essentialism]. After criticising sensual cognition suspects Descartes, by way of the hyperbolic doubt, all our rational cognitions. And he arrives at the truth that, though all our. cognitions can be suspected,in principle, the existence of suspecting. subject cannot be suspected. So When he says "I doubt all things," the subject "I" is excluded from the sphere of adequacy of the logical statements the proposition. But how can this "logical-non-adequate-sphere," coexist, with the objectivity of the statement? That this existence of myself cannot be proved logically shows, the "vicious circle" which appears in the processes of this proof of the existence of God. The existence of myself' can only be grasped "existentially." In the proposition. "Cogito, ergo sum" the relation of "cogito" and "sum" is synthetic. "Cogito, ergo sum" is, as just says Descartes, an intuition. But this intuition is impossible if its object-myself-does not contain "thinking" and "being" at the same time. When I am grasped not as an abstract "Self " but as a concrete "existent-I," "thinking" and "being" can coexist in it. What is suspected is not the all existences of what is suspected. The being of the intentional objects of the doubt is supposed previously. The being of the objects to which the doubt aims can be denied existentially but essentially not. When I doubt my own being, however, the "I" which is suspected is not only undeniable essentially, but also undeniable existentially. My own existence must have a particularity of being undeniable not only concerning "essentia" but also concerning "existentia." This particularity results from the being-for-itself (etre-pour-soi) of man's existence. Man has arfexisteritial basic structure to think himself for himself. So that When the proposition "I doubt all things," denying existentially, but affirming essentially, all the things outside of myself, falls at last on that denying subject -I myself-; my "existentia" as well as my "essentia" is proved positively of itself. The things outside of myself, though undeniable in their esseritia, are subject to my doubt because they are suspected, about their "existeritia." Only my own being is exempt from all doubt and can provide the criterium of the first truth. In the proof of the existence of God, "clarity and distinction" as the criteriuni of ihe truth is not so coersive as it was in the intuition of "simple natures" described in "Regulm." Thus "clarity and distinction" as the cfiterium of truth remains inconsequent in the, system of Descartes' philosophy. Analysing this notion we find that it contains three qualities with their correlatives: net (vague), simple (complex), separe (total). We can affirm that the mathematical truth is "net," "simple" or "complex" and "separe," the. truth of cogito-sum "simple," "vague" and "total," that of logical propositions "net," "simple" or "complexe" and "separe". The first corresponds to the speciality as the form of the external sense, the second to the tempomrality as the form of the inner sense and the third to the form of the human understanding (to use the term of Kant). The, thinking of the existence of myself and of God is, different from that of the mathematical and logical objects, vague, simple and total. This so-called existential thought is the hidden spring of Catesian philosophy.