Energeia

Energeia (ἐνέργεια) is a word Aristotle coined, built from en ('in') and ergon ('work', 'deed'): it means 'being-at-work', activity, actuality. He set it against dynamis, potentiality — and the pair became one of the most fruitful distinctions in all philosophy.

Potential and actual

A thing's potentiality is what it can be; its energeia is its actively being that. The seed has the potentiality to be a tree; the grown tree is that potentiality made actual. A person asleep knows only potentially; the same person thinking knows in actuality. Change and life are the passage from potency to act — and for Aristotle actuality is prior, in being and in worth, to mere potential.

The activity of the divine

Aristotle coined a companion term, entelecheia — 'having its end within' — for what has fully attained its completion. At the summit of his cosmos the divine Intellect is pure energeia with no unrealized potential, an eternal activity of thought thinking itself. The same idea shapes his ethics: happiness is not a possession or a mood but an energeia, an activity of the soul. (Our word 'energy' descends from the term, though in a sense Aristotle would not recognize.)