41. Potency and Act. -- All change implies passing from one state to another. Consider a being B passing from the state a to the state b. If we analyse this change we see it demands that B already possess in a the real principle of its change to b: before being b it was capable of becoming b: it was really susceptible of a new determination: it was in the state of potency to be what it now is actually. Act is, therefore, the present sum of perfection or degree of being (entelecheia, to enteles echein). Potency or potentiality, is the aptitude to receive perfection (dunamis): it is imperfection and non-being if you will; but it is not mere nothingness, because this non-being is endowed with the germ of future actualization. This actualization or passage from a potential to an actual state is called motion or movement, and is defined by Aristotle: "the act of a being in potency, in so far as it is in potency; he tou dunatou, he dunaton, entelecheia phaneron hoti kinêsis estin"^{1} or again: the act of an imperfect subject, actus imperfecti.

Three great theses of peripatetic metaphysics are, as it were, the offspring of this distinction between potency and act: the relation of parentage can be traced in the theory of composition from matter and form; in the composition of universal and individual; and finally in the theory of the four causes, -- a theory which is the fruit of an analysis of motion.