Belief in substantial motion in material world is one of the intellectual foundations of Mulla Sadra’ s philosophy. Proving this theory by him is in the first place dependent on answering the arguments for impossibility of substantial motion, which are mainly based on some foundations put forward in transcendental philosophy. Two important and effective foundations in resolving the difficulties of substantial motion are that the motion becomes existential in Mulla Sadra’ s philosophy and he believes in cutting-motion. The present study has resolved, by explaining these two foundations, the two difficulties extracted from Avicenna’ s words: ‘ no survival of the subject’ and ‘ the necessity of actuality of infinite species between quantifiers’ . This is because considering the abovementioned foundations, the category has one individual amid motion, but an extended and fluid individual as long as the time of motion, and the motion is nothing but the fluidity of this very individual. In other words, it is an extended, connected and fluid process, and since connection and continuity are the criteria for individual unity, motion is automatically a unique individual process and its unity is essential and it needs no protector of unity, not an accidental one needing the protector of unity. Similarly, although various types of essence are abstracted from different stages of fluid existence through closing up its frontiers, existence of types of actualized species is not necessitated from essences. This is because [the position of] principality is for existence and the unique existence is gradually connected, wherein types of potential limitless essences are present there, and since they are potential, limitation of infinite between two quantifiers is not realized.