Science qua science must enjoy coherence of problems ,and this by itself originates in a unifying element. Most philosophers called this unifying element the subject of science and considered it as the criterion for distinguishing different sciences from each other. The main purpose of the present research is to examine the views of Ibn Sina and Mulla Sadra on determining the subject of philosophy and distinguishing it from other sciences. As we read in logic, the subject of each science is what discusses its essential accidents. In other words, the predicates of the problems of a science are among its essential accidents. Hence, there is a tight relationship between the subject of each science and its problems. According to Ibn Sina and Mulla Sadra, the subject of philosophy, as the definition goes, is “existent qua existent”, and its problems are also among the essential accidents of existence. However, not only is there no proof to demonstrate this point, but also, as research shows, some of the issues existing in the works of these two philosophers are not logically among the problems of this discipline, for example, the issues of the soul, resurrection, prophethood, and leadership. The application of all logical principles to everything that is known as a philosophical problem is highly challenging. This is because, at the level of definition, philosophical problems must be among the essential accidents of “existent qua existent”; however, this is problematic at the level of realization, and it seems that this point has not been observed in some philosophical problems. Therefore, there is a gap between the definition and realization of philosophy, and some efforts are required to fill it. For example, if we wish to demonstrate our ideas philosophically, we can examine them in a separate discipline or in the fields of second-order knowledge. Thus it seems rational to define various fields of philosophy and their branches and, through considering different subjects and observing the principle of essential accidents, to gain the knowledge of particular and general philosophies.