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Metaphysics · Foundation lesson

Act and potency

Learn the distinction Aquinas uses to make change intelligible: what a thing already is in act and what it can really become in potency.

Lesson

Foundation lesson

Estimated time

15 min

Prerequisites

No prerequisites

Start with an acorn

An acorn is not an oak tree. Yet an acorn can become an oak in a way that a stone cannot. That difference is not captured by saying only that the oak does not exist yet.

The acorn already has an actual nature and, because of that nature, real capacities for further development. Aquinas calls what is already realized act and a capacity that can be realized potency.

The distinction lets us describe genuine change without saying that the same thing is both oak and not-oak in the same respect at the same time.

Make the distinction

Actus, potentia, motus

actus

Act

A perfection or determination that is already realized in a thing.

Example

The acorn is actually an acorn; warm water is actually warm at its present temperature.

potentia

Potency

A real capacity in a subject to receive or exercise a further actuality.

Example

The acorn can mature into an oak; water can become hotter than it is now.

motus

Motion or change

The actuality of what exists in potency precisely insofar as that potency is being realized.

Example

Heating is not merely the earlier cold state or the finished hot state, but the ongoing actualization between them.

How change becomes intelligible

Aquinas inherits the distinction from Aristotle and uses it as a piece of metaphysical grammar. The point is not that a thing is simultaneously actual and potential in exactly the same way, but that one subject can be actual in one respect and potential in another.

  1. 1

    Name the subject as it actually is

    Begin with something that already exists and has determinate features. Change does not begin from sheer nothing; there is a subject that is actual in some respect.

  2. 2

    Identify a real capacity

    Ask what this subject can become or receive because of what it is. Potency is grounded in the subject and its powers, not merely in our ability to imagine an outcome.

  3. 3

    Describe the potency being actualized

    During change, a previously unrealized capacity is being made actual. What is actual and what is potential are therefore distinguished according to different respects or stages, avoiding contradiction.

  4. 4

    Look for what is already actual

    A potency does not actualize itself in the same respect in which it is merely potential. Aquinas therefore asks what actual factor accounts for the transition, a principle that later becomes central to the First Way.

Read Aquinas

In Physic., lib. 3, lect. 2, n. 6

motus est actus possibilis inquantum est possibile

Translation

Motion is the actuality of what is potential insofar as it is potential.

Commentary on Aristotle's Physics

Common misunderstanding

Potency just means that something is logically possible.

For this lesson, potency means a real capacity of an existing subject. It is not enough that a sentence describing an outcome contains no contradiction. An acorn has a natural potency to mature as an oak; a stone does not have that same potency simply because we can imagine an oak where the stone now lies.

Test the distinction

Objection 1

If one subject is both actual and potential, does that violate the law of non-contradiction?

Reply

No. The subject is not actual and potential in the same respect at the same time. Water can be actually 20 °C while being potentially 80 °C. The distinction is precisely what prevents the description of change from collapsing into contradiction.

Objection 2

Does modern physics make act and potency unnecessary by describing changes as transitions between physical states?

Reply

A physical model can describe states, rates, fields, or interactions with far greater precision. Act and potency ask a different question: what must be true of a changing subject for a transition from an unrealized state to a realized state to be intelligible at all. The metaphysical distinction does not substitute for a physical theory.

Objection 3

If potency needs something actual, does Aquinas deny that living things move themselves?

Reply

No. His restriction is that the same thing cannot be merely potential and actualize that very potency in the same respect. A living thing can be called self-moving when one actual part, power, or principle moves another. The act–potency distinction still applies within the process.

Check your understanding

1.Why is potency not the same as nothing?Show answer

Because potency belongs to an existing subject as a real capacity. Nothing has no subject, nature, or capacity to be actualized.

2.How can the same water be actual and potential without contradiction?Show answer

It is actual with respect to the temperature it presently has and potential with respect to a different temperature it can acquire. The respects are different.

3.Why does Aquinas insist that what moves is in potency while what moves it is actual, insofar as it moves?Show answer

Because a capacity considered merely as unrealized does not explain its own transition to actuality. The transition must involve something already actual in the relevant respect.

Continue studying

Related arguments

The Five Ways for God's existenceGod as Pure Act

Primary work

Commentary on Aristotle's Physics

Further reading

De principiis naturaeCommentary on Aristotle's Metaphysics