According to the Thomistic picture, individual created beings are compositions of essence and existential act (esse). The essence stands as potency to the esse which actuates it, resulting in an existing substance.
In a paper I am working on and conferencing, I have made the point that the esse of finite (created) beings contains no admixture of potency, i.e. it itself is not a composition of act and potency. Several people have objected that this would seem to make finite esse pure act (actus purus), which, on the Thomistic view, should be true of God alone: God and only God is actus purus. So must finite esse be composed of act and potency?
My answer is firmly non. Any esse whatsoever, even that of finite, created beings, can contain no admixture of potency. In this sense (to be qualified shortly), it is indeed correct that finite esse is “purely actual.” But this resoundingly does not mean that finite esse is actus purus in the sense in which that term is predicated of the Divine Being. How so?
First, let me answer why the Thomist must conclude that no esse whatsoever, even that of finite beings, contains any admixture of potency. It is because the very ratio of esse is that of ultimate actuality which lacks any admixture of potency. This is indeed precisely why God is actus purus: because God is identical with God’s own esse, and the ratio of esse entails absence of potency — hence God lacks all potency whatsoever. Esse, according to its ratio, is “the actuality of all things, even of forms themselves” (ipsum esse est actualitas omnium rerum, et etiam ipsarum formarum, ST I.4.1.ad3). If esse were itself a composition of act and potency, then something else would be the actuality of esse, and thus esse itself would not be the ultimate actuality, the actuality of all things. If the intrinsic structure of esse were essentially a composition, then one could not infer from ipsum esse subsistens to actus purus, or from actus purus to ipsum esse subsistens.
Furthermore, if esse were essentially composed, this would also seemingly introduce an infinite regress of composition. For, one prominent reason why one might be inclined to introduce such a composition in finite esse is that one might want to say that any finite item is necessarily composed of act and potency (more on this point below). But then, if finite esse is composed of potency and some further act a1, that act itself would have to also be composed of potency and yet another act, a2, and a2 would have to be composed of potency and another act a3, and so on ad infinitum. This would be make the intrinsic structure of finite being infinitely complex; and would involve precisely the sort of infinite regress to which Thomists are opposed.
So finite esse cannot be composed. Does that make finite esse absolutely simple, or actus purus? Not according to the technical meanings of these terms. Thomas makes this very point in the following passage from his Sentences commentary:
“From this, that [something] falls short of [God’s] simplicity, it is not necessary that it should fall into a composition . . . I say therefore that a creature is twofold. For there is a certain [type of creature] that has complete being in itself, as ‘man’ and [others] of this sort; and such a creature thus falls short from divine simplicity that it falls into composition. For, since in God alone his esse is his own quiddity, it is necessary that in whatever creature, whether in bodily or spiritual, there be found its quiddity or nature, and its esse, which is acquired to it from God, the essence of whom is his own esse; and thus [the creature] is composed from esse, or that by which it is, and what it is. There is also a certain [type of] creature that does not have esse in itself, but only in another, as primary matter, as every form, as the universal; for esse is not of something, except of a particular thing subsisting in a nature; and this [second] sort of creature does not fall short of simplicity so that it is composed . . . but nevertheless [it does yet] fall short from the simplicity of the first [being]: and that falling-short is weighed from two things: either because it is divisible in potency or per accidens, as primary matter, and form, and the universal; or because it is composable with another — which the divine simplicity does not suffer” [InSent. I.8.5.1].
The article from which this passage is taken is considering the question of whether anything created can be simple. Aquinas answers that necessarily, every creature falls short of the divine simplicity; but not such that necessarily every creature is composed. He distinguishes between two sorts of “creatures”: first are creatures in the fundamental sense — complete, subsistent beings. These are individual substances, which do fall short of the divine simplicity by being essentially composed. But then there are creatures which are not complete beings in themselves, but rather are only “constituents” of complete beings. The examples given are primary matter, forms, and universals. The esse of finite beings would fall into this second category of creatures. These items compose other beings, but are not themselves composed of further constituents. Nevertheless, they still fall short of the divine simplicity, in one of two ways: either because they are potentially or per accidens divisible, even though not actually or per se divisible; or because they enter into composition with other things, which is not true of the divine simplicity.
In effect, the difference between divine esse and finite esse is that the former is subsistent while the latter is not. The former is complete and unlimited in itself, while the latter is incomplete and limited, not by being composed, but by entering into composition with something else. Indeed, finite esse is limited precisely insofar as it is received into the potency of an essence. Finite esse is the esse of something, not a subsistent something in its own right. It is thus clear that when Aquinas refers to God as actus purus, he does not mean by this merely to point out that God has no admixture of potency, but also that God is a complete and subsistent actuality. Created esse is also purely actual in the qualified sense of lacking intrinsic admixture of potency; but it is not actus purus because it is not subsistent actuality — it is received or limited actuality.