Act

That which is opposed to potency; that which is accomplished, completed, or fulfilled. Act designates the effective realization of a possibility and the perfection by which a being is what it is. Whereas potency is the capacity to be or to become, act is being actually possessed. It is the fulfillment of what previously existed only as a possibility.

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The distinction between act and potency, developed by Aristotle, constitutes one of the foundations of Western metaphysics. It makes it possible to understand both change and permanence. A being can become other than it presently is because it possesses potencies; yet such becoming is intelligible only because it tends toward an act that constitutes its fulfillment.

Thus, the seed is potentially the tree it will become; the mature tree is the act of that potentiality. Likewise, the child is potentially the adult he or she will become, and the ignorant person is potentially learned. The passage from potency to act characterizes motion, understood in the metaphysical sense as the progressive actualization of a possibility.

Act enjoys a certain primacy over potency. Potency can be defined only in relation to the act it is capable of receiving. A capacity is intelligible only in view of its possible realization. For this reason, Aristotle holds that act is prior to potency in perfection, intelligibility, and ultimately in being itself.

Scholastic philosophy deepened this doctrine by distinguishing different levels of actuality. Existence itself is understood as the fundamental act of being (actus essendi), that by which an essence ceases to be merely possible and comes to exist in reality. According to Thomas Aquinas, the act of being is the supreme perfection of every thing, for it actualizes all other determinations.

This doctrine leads to the conception of God as Pure Act (Actus Purus). In God there is no passive potency, since every potentiality implies a certain imperfection or the possibility of becoming other than one is. God, however, is the absolute fullness of being: He is what He is from all eternity, without change or becoming. He is thus the infinite actuality from which all creatures receive their being according to limited modes.

The distinction between act and potency also makes it possible to understand the hierarchy of beings. The more actualized a being is, the more it participates in the perfection of being; the more it remains in the indeterminacy of potency, the further it is from its full realization. Every growth, every act of knowledge, and every form of life may therefore be understood as a passage from potency to act.

Finally, this distinction has a spiritual significance. Human beings are not only what they presently are; they are also what they are called to become. Their existence contains virtualities awaiting realization. Human vocation may thus be understood as the progressive actualization of the powers inscribed in human nature, culminating in their ultimate fulfillment through knowledge of and union with the Principle from which all being proceeds.

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