The issue of mental mode of existence is one of the old frequently discussed topics in Islamic philosophy and theology playing a striking role in the manner of realization and explanation of knowledge. Emphasizing the point that this issue is a novel one in Islamic philosophy, the present article deals with the arguments behind proving the mental mode of existence with special focus on the doctrine of MullaSadra. According to him, each category occurring to the mind is the very category based on the primary essential predication rather than on the common technical predication; and what occurs to the mind as a concept is a mental quality in its essence and reality. Finally the prevalent views on mental mode of existence are raised and analyzed in the framework of the theory of the theosophers, that of relation, and that of Shabah.