Grant that an infinite regress of composition is impossible: it is impossible for some being A to be composed of parts B and C, and B to be composed of parts D and E, and D… etc. This seems self-evident.
(Definition: B is part of A iff B combines with another part(s) C to form A and B is really distinct from C.)
Grant also that, at most, only one absolutely simple being (= a being with no parts) exists. I find Aquinas and Avicenna’s proofs of this claim convincing. (In sum: if there were two simple beings, they’d be similar in some way and different in other ways. But this would mean they have parts, and therefore aren’t simple.)
(Definition: B is a being iff it is logically possible that only B exist (or only B and God, maybe). This is not a very good definition but it makes the point I want it to.)
If both of these are true, then every being (other than the Simple Being, if it exists) is composed of parts which, while really distinct, are not themselves beings.
Proof: Take any being A. Either A has parts or not. If it does not, it is the Simple Being and out of our consideration. If it does not, it has at least two parts. Let B be one of its parts. Is B a being or not? If not, really distinct parts exist which are not beings.
So suppose B is a being. Does B have parts or not? If it does not, it is the Simple Being… etc.
If we allow that every part of a being is a being, then either (a) all parts are the Simple Being, or (b) there is an infinite regress. (b) is impossible. (a) is absurd since in this case only one thing would exist, and self-evidently more than one thing exists.
Therefore, at least some things exist that are not beings, and all beings other than the Simple Being (if it exists) are composed of really distinct parts that are not themselves beings.
(So, for example, if essence and existence are really distinct, it doesn’t follow that essence is a distinct being that is combined with existence to form a third thing. Rather, it’s at least possible that the essence- existence composite is one being, and also its essence and existence are really distinct. Likewise for form and matter.)
(I know I’m playing fast and loose with my terminology here but I think I’m on to something. I’m still trying to figure out what.)