Act & Potency — The Foundation
Aristotle's and Aquinas's distinction between actuality and potentiality.
Before walking the Five Ways, it helps to grasp the single distinction they all lean on: act and potency. Aristotle introduced it to solve an ancient puzzle about change; Aquinas made it the backbone of his demonstrations.
The puzzle of change. The early Greeks asked how anything can change at all. If a thing becomes what it is not yet, where does the new state come from — out of nothing? One thinker, Parmenides, concluded that change must be an illusion. Aristotle solved the puzzle by finding a middle ground between plain being and non-being.
The distinction, in plain terms:
- Act (actuality): the way a thing actually is right now: a cup of coffee that is actually hot.
- Potency (potentiality): the real capacity a thing has to become something it is not yet: that same coffee's ability to become cold.
Potency is not mere nothing, but it is not yet actual either. The cold coffee is potentially hot; the acorn is potentially an oak; you are potentially asleep.
Change is the reduction of potency to act. To change is for a real potential to be made actual. And here is the key principle Aquinas draws out:
Whatever is moved is moved by another ... nothing can be reduced from potentiality to actuality, except by something in a state of actuality.
The cold coffee does not warm itself — the hot stove, already actually hot, turns its potential warmth into real warmth. A potential cannot make itself actual, because then it would already have to possess the very thing it still lacks.
Why this matters for the Five Ways. Each way, especially the First Way from motion, is really an argument about act and potency:
- Change is real: potencies are being actualized all around us.
- No potency actualizes itself; each is actualized by something already in act.
- This cannot regress infinitely through things that are themselves mixtures of act and potency, for the whole chain would then have no source of actuality.
- Therefore there must be something that is Pure Act — actual through and through, with no unrealized potential — the source of all actualization.
This being of Pure Actuality, Aquinas says, is what everyone understands to be God.
Keep this distinction in mind as you read the ways below: wherever you see motion, cause, or dependence, you are watching potency being reduced to act.