One of the important problems of Islamic philosophy is the demonstration of the substantiality of the soul. In spite of their agreement concerning the substantiality of the soul, Mulla Sadra and Ibn Sina have some basic disagreements in the interpretation and explanation of this principle. The extent of these disagreements has also stretched to some critical issues such the explanation of the soul, its changes, and its relationship with other faculties, which are the focus of this paper. According to Ibn Sina, possible beings consist of two analytic-rational components, that is, existence and quiddity. He also maintains that the source of the division of categories, under which substance and accident fall, is quiddity itself. The difference between substance and accident also lies in the fact that existence is substance by itself, and existence is accident through the other. Therefore, the soul is an immaterial substance that performs voluntary administrative acts and perceives universal affairs. It has some branches or faculties through which it carries out its acts. In this approach, the differences among human souls, from their highest to lowest levels, are rooted in accidents, and no change occurs in their substance. In Mulla Sadra’s view, too, accident has no independence before substance, thus it is a dependent truth enjoying an existence depending on the other. Nevertheless, based on the principle of the principiality of existence, the criterion for individuation is existence; an existence which underlies the individuation of a subject and is the referent for substance itself and a referent for all accidents. In other words, it is a single existence that is a referent for man with various accidents. The soul is a substance commensurate with existence and enjoys an essential and graded existence the accidents of which are considered to be the grades of this truth. The soul’s faculties are its modes and grades, and the perfections attained by the soul originate in its unity with perceptive forms.