Their actions are successive, and are distinct from the faculty of acting. The fact that all things have in themselves some potentiality warrants the conclusion that there must exist a being, God, from whom potentiality is wholly excluded, and who, therefore, is simply actuality and perfection, Actus Purus . But, absolutely speaking, actuality precedes potentiality. This actuality, if mixed with potentiality, presupposes another actuality, and so on, until we reach the Actus Purus . Causality leads to the conception of God as the unproduced cause.
Contingent beings require a necessary being.
The limited perfection of creatures postulates the unlimited perfection of the Creator. The direction of various activities towards the realization of an order in the universe manifests a plan and a divine intelligence. It needs to be completed by a substantial form, but does not of itself, demand any one form rather than another. At the other extreme is God, pure actuality, wholly determined by the very fact that He is infinite in His perfection. Between these extremes are the realities of the world, with various degrees of potentiality and actuality.