It has become one of the most interesting and important problems in the modern philosophy to examine whether knowledge is idealistic or realistic scince Descartes declaired contrary to the aristotelianthomistic noetic, his philosophical, revolutionary position, presenting the well-known formula: cogito ergo sum. In this article, according to the thomistic noetic which is insisting that truth and knowledge must be realistic, I especially intended to investigate the reason why truth of judgement is called true in the proper sense of the word. A proposition and a judgement are logically true when things actually are as they say they are. It can be deduced from the above that being true of the proposition and the judgement is nothing other than their correspondency to things, i.e. the essential adequacy between the composition of the intellect and the disposition of things. Then, in what way can this kind of being true be recognized ? In order to recognize this kind of being true, that is, truth of propositions and judgements, we must examine if things actually are (esse rei) as they say they are, with actual help of sensitive perception which is cognitio experimentalis. We are thus led to the conclusion that the recognition of truth of judgement is founded not in quiddity or entity i.e. modus essendi, but in existence of things, i.e. actus essendi.