284. Act and Potency. -- Here, as with Aristotle (41), the theory of the actual and the potential occupies a prominent place: more prominent even than with the Grecian philosopher, seeing that scholasticism derived from the development and application of his idea, consequences quite undreamt of by himself. Act and potency, together with the kindred notion of movement (motus), are primarily employed for the purpose of explaining CHANGE. But the theory was extended. The two notions became synonymous with "being determined and being determinable". Outstepping its original domain -- the process of organic change -- the theory was applied universally within the real order, and pervaded and penetrated every possible composition of contingent being, of being limited in its reality.^{1} The act and potency couple is found in such fundamental compositions as those of substance and accident, matter and form, common essence and individualized essence. To it, moreover, is attached the theory of causes.

{1} It is even found in the domains of logic and moral; and everywhere it expresses the same primordial relation of determinable and determinate: the genus is to its species, the free act is to its intrinsic end, as potency is to act.