Semantic Requirements of the Fundamentality of Existence and Its Role in Making Distinction Between Mir Damad and Mulla Sadra's Thoughts

Mir Damad was Mulla Sadra's immediate master in intellectual and traditional sciences and he undoubtedly had the greatest influence on shaping his thoughts. But a fundamental difference between their thoughts is considered by taking a glance at their books. Mulla Sadra's philosophy, not only is not the continuation of Mir Damad’s theosophy, but it is in contrast of it. The problem is: "Why Mulla Sadra has not followed the ideas of his master and established his new wisdom?" In this article, we have analyzed the basic difference between them on the question of principality of existence or quiddity. Then we showed the role of this difference on the other their philosophical thought. That which is meant by Islamic existentialist is the external reality consists of a single thing which is called the "being or existence" But according to Islamic essentialist the universe consists of various facts that is called "quiddity or essences".