Mulla Sadra is considered unanimously as the most influential philosopher in the Islamic Philosophy tradition in the last four hundred years. Mulla Sadra’s philosophy is founded on existence as the unique constituent of reality and its primacy, the intensity of existence, and finally transubstantiality or substantial motion of being. Mulla Sadra made the primacy of existence as the main basis of his philosophy. He distinguishes between the concept of being and the reality of being. The first, is the most obvious of all concepts and the most universal, while the second, is the most ambiguous, since it requires presence knowledge and pure intellect, which would be able to discern existence as reality. Farabi and Ibn Sina perceived that in the existence-quiddity relationship, existence is an accident. Al-Suhrawardi holds the theory more radical that existence is merely a mental concept with no corresponding reality and it is quiddity, which constitutes reality. Ibn Rushd had criticized this approach. Mulla Sadra despite all Islamic philosophy tradition and his teacher, Mir Damad adopts an opposite and new outlook. His fundamental doctrine is principality of existence, and then quddities are the mental constructs. Reality is then the base of existence, which is graded and existentiating the reality of all things. Mulla Sadra at first followed his teacher and only after visionary and Gnostic existence, came to realize that it is existence, which bestows reality and has primacy on quiddity.