The issue of causation is one of the most important philosophical issues, so that without having a good understanding of this concept, it is impossible to understand the world. Any kind of discussion, speaking, and experience, or any human action is based on some kind of understanding of the concept of causation and accepting its reality. The issue of causation can be discussed from various aspects. Modern Western philosophers including Hume and Kant have treated this issue from epistemological point of view. But Muslim philosophers consider it as an ontological one. Mullah Sadrā as a great Muslim philosopher has dealt with this issue and has given its correct meaning with reference to its various forms in his works particularly, in his main book al-Asfār. By proving the principality of existence and mentally-positedness of quiddity, he claimed that it is not possible to explain causation on the basis of quiddity and quiddative possibility. At first, he explained causation on the basis of gradational unity of existence, but since this unity may lead to the separation of cause and effect, he finally by following a mystical approach to this principle, shifted from gradational unity to the individual unity of existence. He explained the principle of causation with reference to the mystical oneness of being, since there is no room for multiplicity. In such an approach, effect will be nothing other than a symbol or sign and has no identity other than a kind of mirror
