Act and potency according to Aristotle: Definition and examples

Act and potency are fundamental concepts in Aristotle’s Metaphysics. These two metaphysical concepts serve to solve that great problem in the history of ancient philosophy that the pre-Socratic philosophers, Aristotle’s predecessors, sought to solve: the problem of change.

Does everything change, as Heraclitus said, or is change just an illusion of the senses, as Parmenides claimed? Aristotle took up this debate again and proposed a new way of answering.

For Aristotle, change is not a mere appearance or illusion, it actually exists. To explain it, Aristotle turned to the metaphysical principles of act and potency.

According to Aristotle, all change implies a passage from potency to act. But what do these concepts mean?

What is act?

The act means something already realized, that is, what a being already is at a given moment. For example, a seed is a seed in act; a child is a child in act.

Things existing in the world have possibilities to change, to be transformed, and it is precisely this possibility of undergoing changes that Aristotle will call potency.

What is potency?

According to Aristotle, potency means that which at that moment is not, but which can become. In other words, potency is a simple possibility of something becoming something else, or undergoing a change, however simple.

For example, the seed in act has the potency to become a tree; and the child has the potency to become an adult. When the seed becomes a tree, we can say that it is a tree in act, and no longer in potency; and the adult is no longer a child in act.

Potency is related to the nature of beings, for example, a dog does not have the potency to fly; a man does not have the potency to run more than 150 km/h. The nature of each one limits its own possibilities.

Examples of act and potency

See other examples that illustrate the concepts of act and potency, and realize how these concepts can be applied to everything that exists, and in many ways.

  • A healthy man in act, has the potential to get sick;
  • A climate with a clear sky in act, has the potential to become cloudy;
  • An object that is on a table in act, can potentially be on the chair;
  • An unlit lamp does not emit light in act, but has the potential to emit it;
  • A wall the color white in act, has the potential to be painted another color;

Aristotle against Heraclitus and Parmenides

According to Heraclitus’ philosophical theory, it is not possible to bathe in the same river twice, because the river ceases to be the same with movement, with the passing of time. Reality, for him, is constantly changing.

Parmenides, on the other hand, sees being as static, immobile, one. Movement does not exist, because change implies a passage from being to non-being, which is impossible. Being cannot be reduced to nothingness. Being always is, hence his famous phrase: being “is” and non-being “is not”.

How does Aristotle explain movement?

Aristotle tried to reconcile the theses of these two philosophers: he does not deny that changes exist, nor does he deny a certain immutability of being, as argued by Parmenides. Aristotle summarizes these positions as follows:

For a thing to change, it must actualize that which, in its nature, is possible. Marble, for example, has the potential to become a statue. This possibility is already found in the marble itself. Parmenides did not realize that sensible beings are not pure acts; they possess possibilities of becoming, that is, potency. Beings can indeed change without ceasing to exist. The change is not a passage from being to nothingness, but to a new form of being.

As we have seen, everything that exists in the world is a substance composed of matter and form (synolo). Matter is related to potency, because it is an indeterminate principle; and form is act, a principle of determination.

When a substance undergoes a change, it loses its old form and gains a new form (determination). What is implied in this movement is that there is a substratum that does not change, which is the subject itself that loses and gains a new form. This would refute Heraclitus’ thesis, since it would prove that not everything is in constant motion.

Only a being that has no matter in its composition is not capable of change, this being is pure act, which Aristotle calls the Unmoved mover.

What is needed for this change to occur?

For a movement to happen, it is necessary that a being in act actualizes the potency contained in a being.

In order for bronze to become a statue, it is necessary that a sculptor (efficient cause) actualize its potentiality. For a change to occur, a cause is necessary.


Aristóteles. (2001). Metafísica, ensaio introdutório, texto grego com tradução e comentário de Giovanni Reale, (Trad. Marcelo Perine). Vol. I e II. São Paulo: Edições Loyola.


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