The propositional language of Aristotelian logic concerning the existents of the world is based on his 10 categories. The habit of employing this language in everyday life and in the field of philosophy has resulted in the emergence of certain difficulties regarding some philosophical problems. From among them, we can refer to the problems related to essence or quiddity and the quality of acquiring the knowledge of existents in the outside world. The purpose of the present paper is to reveal the quality of the effect of this kind of language and, following it, essentialism in the realm of epistemology with special reference to two philosophers of existence in the East and West, Mulla Sadra and Martin Heidegger, respectively, concerning the terms “essence” and “quiddity”. Here, the writers conclude that, based on Heidegger’s philosophy and Mulla Sadra’s Transcendent Philosophy, particularly as reflected in the words of some of the commentators of the second school, the logic of essentialism and its tools are not capable of solving the problem of the conformity of mental forms with external realities and providing a justifiable criterion for defending this conformity.