The term “act” figures in the philosophical analysis of becoming, of the phenomenon of change. Parmenides (d. after 480 b.c.) regarded all change as appearance, while Heraclitus (ca. 500 b.c.) considered all that is unchanged as appearance. Aristotle (384–322 b.c.) made an extensive analysis of becoming in his doctrines of substance and accident, dynamis and energeia, potency and act. These doctrines were greatly refined in medieval metaphysics, especially by Thomas Aquinas (1224/25–74), who related them to the doctrines of God and creation.

A potency seems to precede every act (e…