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A term employed in scholastic philosophy to express the absolute perfection of God. In all finite beings we find actuality and potentiality, perfection and imperfection. Primary matter, which is the basis of material substance, is a pure potentiality. Moreover, change necessarily supposes a potential element, for it is a transition from a state of potentiality to a state of actuality; and material things undergo manifold changes in substance, quantity, quality, place, activity, etc. Angels, since they are pure spirits, are subject to none of the changes that depend on the material principle. Nevertheless, there is in them imperfection and potentiality. Their existence is contingent. 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.
It is true that in the same being the state of potentiality precedes that of actuality; before being realized, a perfection must be capable of realization. But, absolutely speaking, actuality precedes potentiality. For in order to change, a thing must be acted upon, or actualized; change and potentiality presuppose, therefore, a being which is in actu. This actuality, if mixed with potentiality, presupposes another actuality, and so on, until we reach the Actus Purus. Thus the existence of movement (in scholastic terminology, motus, any change) points to the existence of a prime and immobile motor. 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. When we endeavour to account ultimately for the series of phenomena in the world, it is necessary to place at the beginning of the series -- if the series be conceived as finite in duration -- or above the series -- if it be conceived as eternal-a pure actuality without which no explanation is possible. Thus at one extreme of reality we find primary matter, a pure potentiality, without any specific perfection, and having, on this account, a certain infinity (of indetermination). 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.
So that God is not a becoming, as in some pantheistic systems, nor a being whose infinite potentiality is gradually unfolded or evolved. But He possesses at once all perfections. He is simultaneously all that He can be, infinitely real and infinitely perfect. What we conceive as His attributes or His operations, are really identical with His essence, and His essence includes essentially His existence. For all intelligences except His own, God is incomprehensible and indefinable. The nearest approach we can make to a definition is to call Him the Actus Purus. It is the name God gives to Himself: "I am who am", i.e., I am the fullness of being and of perfection.
ARISTOTLE, esp. "Metaphysics", Bk. XI (Berlin, ed. 1831); "Physics", Bks. VII, VIII; ST. THOMAS, "Comment. In lib. VII, VIII Physic." And "in lib. XII Metaphysic." (XI of Berlin ed.); "Summa theologica", esp. P. I. QQ. ii, iii, iv, etc., "Contra Gent." L. I, c. xiii, xvi, etc.; PIAT, "Dieu et la nature d'apres Aristotle" in "Revue neo-scholastique", VIII, 1901, p. 167 (reproduced in his book "Aristotle", L, II, c., ii Paris, 1903); WATSON, "The Metaphysic of Aristotle", IV-"The divine Reason", in "Philos. Rev." VII (1898), p. 341.