Act and Potency

Recently I read Ed Feser’s book Aquinas. It’s a great introduction to the philosophy of St Thomas. A warning is in order, however. It is an entry in the publisher’s “Beginner’s Guide” series, but is for beginners in roughly the same way that the Summa Theologiae is (as St Thomas famously wrote). Of course, there are all sorts of beginners, but in this case it’s probably fair to describe it as an introduction to Aquinas that presupposes at least a little familiarity with the philosophy of Aristotle. The reader who lacks this may find the book to be tough sledding at times. This is because of the subject matter, not because of Feser’s presentation, which is wonderfully clear. At any rate, I hope to do a few posts based upon material in the book.

Elsewhere I’ve said, “I’m not a philosopher. I’m not a theologian.” These facts were forcefully driven home for me early in Feser’s book when he wrote (on page 12) of “the famous Twenty Four Thomistic Theses.” Umm…what? Sorry, never heard of those (and this fact ought to tell you something about how well-informed my opinions are, sadly), but you may review them in this article. In this post I want to briefly review the first Thesis (and maybe future posts will look at the rest):

Potency and Act divide being in such a way that whatever is, is either pure act, or of necessity it is composed of potency and act as primary and intrinsic principles.

A better word, maybe, for “potency” is potential: the capacity in a subject to become or to do or to be something that it is not or that it lacks. Water has the potential to become hot; a sitting man has the potential to run. If a subject can’t become or do or be something, it lacks potency for it. Water can’t think; a pig can’t fly. When a potency is realized (i.e., it becomes real), it is actualized. It exists. We might also describe potency as a capacity for change in some way. What the First Thesis says is that there are two kinds of beings: that which is “pure act” and that which is a composition of potency and act.

To be “pure act” is to lack any potencies at all, and this may be said only of God. He does not change, nor is there any sense in which His existence is less than perfect. He is completely “in act” or “actual”. On the other side, all other beings are composed of potency and act. We can change.

The question might be asked about a third category: what about beings that are pure potency? But that is a contradiction in terms. Potency refers to that which is not actual—to what doesn’t exist. Water that has the potential to become hot is by definition not hot. A sitting man is by definition not running. So that which is pure potency would then lack any existence at all (because pure potency would include the potency to exist).

St. Thomas says this in ST I q.77 a.1: “power and act divide being and every kind of being” (“Power” is another word for potency or potential to do or be something).

Posted in Aquinas - Philosophy, Fides et Ratio, Summa Theologiae, Thomistic Theses
9 comments on “Act and Potency
  1. […] speaking of overlap, this post overlaps a lot with this other one. LikeBe the first to like this […]

  2. neiljuliano says:

    I have a brief question, and I’m hoping you can give an adequate reply. If something possesses the mere possibility for change, then in order for that change to be realized it must be instantiated by an external cause, thus requiring the actual existence of an external actualizer. However (and I’ll try to keep this question short), is it not possible that something could exist in a state (let’s call it state X) in which it could possibly change, while not ever actually having that change realized, and still exist in state X without necessarily requiring an external cause to instantiate its existence in state X? Therefore, it would seem that a being could exist with some degree of potentiality and yet, hypothetically speaking, exist necessarily. What are your thoughts on this?

  3. aquinasetc says:

    Hello Chance,

    Thank you for stopping by. I hope I have an adequate reply for you, but I won’t be surprised if I don’t because I am not a real philosopher. :-)

    There are different senses in which one can speak of a thing’s necessary existence: absolutely speaking, and in relation to something else. It seems to me that the mere fact of composition in the being you postulate implies its dependence upon some prior being which is responsible for that composition, since a being cannot create potentialities in itself in any absolute sense. So if you would say that this proposed being is absolutely necessary, I guess you would have to have some alternate explanation for its existence than anything Aquinas might say about God, and I find his arguments sufficiently compelling that I do not see how that could be done.

    Another thing that occurs to me is that if you postulate this being as eternal, and yet as having an eternally unrealized potency, it is no actual potency at all.

    I suppose my thoughts here aren’t going to be satisfactory. I would propose to you that you take a look at the first book of the Summa contra Gentiles (you can find it online here), where Aquinas presents a more extensive argument for the existence of God than he does more famously in the Summa Theologiae; among other things he addresses issues related to act, potency, and necessary beings. You might also try asking Ed Feser or James Chastek (see the “Just Thomism” link in my Blogroll); they are actual philosophers.

    Sorry I can’t be of more help to you.


  4. neiljuliano says:

    Thanks! Your response helps in many respects, I especially loved your brief note on the eternality of a potentially existing entity. But I have one thing I would like to clarify, If a thing is composed (in any respects, be it act and potency, essence existence, parts, etc.) then the thing would be an actualized potentiality given that there must be some sort of actual principle to compose the thing, right? Therefore, even if a composite (in any respect (act/potency; essence/existence; etc.)) thing existed in an eternal state it would still necessarily be contingent, in that its parts would require an external principle to actualize them. Is that about right?

  5. aquinasetc says:

    Yes, I think that’s correct.


  6. neiljuliano says:

    Thanks very much! If I have any additional questions I shall let you know.

  7. […] Source: Act and Potency […]

  8. Noveed Thomas says:

    hi Aquinasetc. I m wishing u my hearty greetings of grace filled season of Lent. I am a student of philosophy at St. Francis Xavier Major Seminary Lahore, Pakistan. I am doing my dissertation on ” Becoming in Act and Potency.” I have read all what you wrote on Act and Potency here in your website its very helpful to grasp the exact meaning of the subject. I request you if you can send me more material on my desired topic via email:

  9. aquinasetc says:

    Hello Noveed,

    I’m very sorry but I only have one suggestion:

    That is a link to St. Thomas Aquinas’ commentary on Aristotle’s Metaphysics. It is also available from And of course Feser’s book (which I reference in the post) is excellent.

    I’m glad you found this post helpful.



Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s


Enter your email address to follow this blog and receive notifications of new posts by email.

Join 146 other subscribers
%d bloggers like this: