A central assumption in this essay, in terms of both historical development and methodological approach, is that later Islamic philosophy is characterized by a shift from a substance-based metaphysics to a processoriented metaphysics. Defenders of substance metaphysics often focus on the nature of causation to attack process metaphysics. If there is no substance or substratum for process, then how can events have any causal nature? If neither cause nor the caused are somehow stable in terms of their essence and essential features, then how can one be said to act upon the other? After considering the function of causation in other metaphysical systems and certain skeptical denials of causation, its role in the mystical thought and onto-theology of the Iranian philosopher Mullā Sadrā (d. 1641) is examined.