(a) Falcons fly
(b) Things that use differential pressures on an airfoil to produce lift fly.
The predicate fly is per se to both, but it is per se and first only of (b), that is, (a) has the predicate first of all because it is a (b). This is the same relationship between
(a) The universe exists
(b) God exists.
But then aren’t we saying that the universe exists so far as it is God? In fact, since the predicate in the first (b) is clearly commensurate, one gets a Barbara syllogism:
(a) Falcons fly
(b) What flies uses differential pressures on an airfoil to produce lift.
(c) Therefore, falcons use differential pressures, etc…
And mutatis mutandis, we’d get
(1) the universe exists
(2) what exists primo and per se is God
(3) The universe is God.
But (1-3) is a case of the fallacy of the secundum quid and simpliciter whereas (a-c) is not.
Start with the fact that the subject term in (a) has something other than what the predicate expresses. This “something other” stands to the predicate as a receiving something from it, i.e. whatever is in (subject of a) that is other than (predicate in a and b) receives (predicate in a and b) from (subject in b) But what we just said about the subject of (a) can’t be said about the subject of (b), since b is a subject only semantically and in the mode of understanding. This is the same reason a definition is not a predicate of the defined word, but only a clarification or distinct apprehension of it.
Taken concretely, a falcon is an ontological and not merely semantic subject with many properties to which flight modidies, like being warm blooded, having feathers, being carnivorous, etc. Analogously, the universe has more to it than what exists simpliciter. The universe obviously does not have “more than existence” by containing more existent things, but because there is something other than what exists simpliciter. This “something other” is an indetermination allowing for different things in the same order, or essence. In the case of the universe, this indetermination is either the logical possibility of another universe (i.e. the non-contradiction of a multiverse) or, what would be truer to say, that “universe” not a substance but the order (i.e. the set of relations) of a multitude of substances in one created order.
So the reason conclusion (3) above is false is because “exists” in (1) is used in a way that is open to composition with what exists secundum quid while “exists” in (2) is used as only existing simpliciter in a way that is repugnant to any composition with what exists secundum quid. This intrinsic repugnance to composition is what Thomas means by saying God is ipsum esse subsistens.
We hit the same conclusion with more Platonic ideas if we see that “one” is a transcendental predicate of being simpliciter, meaning the many is not being simpliciter. Essence-distinct-from-existence allows for “this thing that is not that thing” in one and the same order. Seen from this angle, the real distinction is simply Thomas’s answer to the problem of the one and the many.