In philosophy, as late J. Marechal has pointed out, there would not be such problems difficult and misunderstood as those of Epistemology. Since Descartes has presented his proposition: cogito, ergo sum (I think, therefore I am), it seems to me that the most of modern thinkers, though there are nuances among them respectively, have started their philosophy from the order of self-consciousness which scholastic philosophy called intentio reflexiva, and, moreover, have reposed on it. According to St. Thomas, however, it is really impossible for human intellect to acknowledge itself before congnizing being, ens, which is extra-mental and corporeal. St. Thomas, telling that our intellect intettectus, intends being first of all; 'primum quod cadit in intellectum est ens.' expresses his ontology of cognition that cognition, first, should be the cognition of being, in another word, that of the extra-mental and corporeal world (intentio prima) and then, self-consciousness (intentio reflexiva) can be gained after the reflexion on such act of knowing being. In the first section of this treatise, I inquired St. Thomas' conception of truth and researched in what sense truth was the cause of cognition. By him, truth is properly in relation to the intellect. All the natural things are true as far as they are brought on existence by divine Intellect. God is Being Itself and cognizes Himself perfectly without containing any potency and, therefore, He is the very Truth and is first Truth, by the participation with which other intellects can cognize. Human intellect can potentially cognize every existence as it, being created, participates this first Truth. Accordingly, cognition is resulted by correspondence, that is, adaequacy between human intellect and things; this is what is defined as truth. In the second section, the distinction and the relation between cognition of quiddity and cognition by judgement, are investigated. Cognition of quiddity is the result of adaequacy between intellect and things, and cognition by judgement is that between intellect and conceptions. The former has a character of confirmation, and the intellect is not aware of the acaequacy between itself and things. The adaequacy is only recognized in the latter which has a character of understanding. And yet, as far as it concerns the order of act, cognition by judgement can not exist without the precedence of cognition of quiddity. In the third section, thomistic conception of truth is defined according to the three phases of cognition of things. Idealistic theory of knowledge not only neglecting the scholastic distinction between the first intention and the second intention, but methodologically adopting the second intention as an only intention, missed the way to extra-mental being. To know is to know being when it is said primarily, and the second intention or reflexive intention can not exist without the pre-existence of the first intention as conciousness of being. Idealistic theory of knowledge is the extremity of the second intenion which is one of two organic and indespensable intentions in ontological theory of knowledge, and so the former, in the long run, should be included in the latter.