The root and source of the unity of existence is mysticism, and it has been entered into philosophy from this realm. Perhaps the first philosophical explanation of the mystics’ unity of existence doctrine has been done by Dawwani. But this philosophical explanation has some unjustified and irrational consequences and accessories. Sadra, also, makes his best efforts to give a philosophical explanation of mystics’ unity of existence of doctrine. He did that through abandoning the theory of analogical unity of existence and suggesting a new philosophical system, whose pieces have been mentioned in Asfar (Volume II) and other places of the book and also in the book of Igaz an-naimin (Awakening the sleepers). By this way, he not only has not ended philosophy, but has brought it into a new realm of philosophical thoughts. So, we must design this new philosophical system to understand this important stage of his philosophy, especially to understand his philosophical explanation of personal unity and its valuable accessories. In this paper we describe briefly the philosophical system and the ontology of Dawwani’s approach, and attempt, as much as possible, express his philosophical explanation of unity of existence through an optimistic outlook. Then we will show that even though there are some the similarities between this explanation and Sadra's philosophical explanation, Dawwani’s approach is self-inconsistent and that is why we will criticize it ultimately. Afterwards, we continue to express the Sadra's philosophical explanation of mystical unity of existence. So the object of this paper is to compare these two philosophical explanation, reveal their strengths and weaknesses, also the relationship between these two explanations. It will be clear that Sadra's philosophical explanation is worthy and reasonably defensible through a new philosophical system which has been followed by Sadra and he has referred to it in his books, not in the analogical unity of existence