One of the most fundamental discussions of philosophy is the discussion surrounding the principality of existence and the unreal nature of quiddity. A proper understanding of this discussion can facilitate the understanding of the personal unity of existence [that the mystics believe in]. In his philosophical system, Mulla Sadra passed four stages in his belief in the unreal nature of quiddity. First, like the majority of philosophers, he believed in the principality of quiddity and the unreal nature of existence. Following this, he proposed what he called the ‘tassul’ of quiddity. Then he said that quiddity is something unreal and he negated it from the external world. In the end, he also negated the reality of quiddity from the mind. In this paper, we will better acquainted with the various stages of the belief in the reality of quiddity in the philosophical system of Mulla Sadra.