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).