Moreover, as A-T uses the term, a “property” is not part of the essence of a thing, but a feature that flows from its essence. Simply noting that a thing has some feature in every possible world ignores this distinction, and is for that reason too an inadequate way to characterize a thing’s essence. For example, rationality and the capacity to learn languages are both features human beings have in every possible world (if you want to put it that way), but the latter capacity presupposes rationality and is therefore less fundamental than it. While rationality is part of our essence, then, the capacity to learn languages is not, but is rather a “property” – something proper to us in that derives necessarily from our essence. “Property” as used by contemporary philosophers ignores this distinction, and is applied indiscriminately to what is part of a thing's essence, to what is not part of its essence but is nevertheless “proper” to a thing, and to what is neither part of a thing’s essence nor proper to it but merely some contingent feature it has (e.g. the fact that such-and-such a human being was born in Los Angeles or has a blog).
(But might some human being not lose his rationality or capacity to learn languages due to brain damage or the like? Doesn’t that mean the former is not really part of his essence and the latter not really a property? No, that doesn’t follow at all, because to be impeded in the exercise of a power does not entail that one doesn’t have it. From an A-T point of view, every single human being – including one in a so-called “persistent vegetative state” – necessarily has rationality, the capacity to learn languages, etc. Terri Schiavo was a severely damaged rational animal, not a non-rational animal; a human fetus is a rational animal that has not yet been fully formed, not a non-rational animal; and so forth. Restore Terri Schiavo to perfect health and you get someone who can once again exercise her rationality. Restore a rose bush or a dog to perfect health and you still have something that can never exercise reason. Let a human fetus develop fully and you get something that can exercise rationality. Let a rose bush or a dog develop fully and you never get something that can exercise rationality. Thus it is erroneous – not to mention absurd and morally obscene – to compare the likes of Terri Schiavo or a human fetus to a plant or a non-human animal. And thus does bad metaphysics lead to the rationalization of grave immorality, even murder. But I digress.)
It is also often said that for God to be a necessary being is for Him to exist in every possible world. This too is at least very misleading. It leaves the impression that there are these things called “possible worlds” that have some kind of reality apart from God, and it turns out – what do you know! – that God happens to exist in every one of them, right alongside numbers, universals, and other necessarily existing abstract objects. To be sure, since possible worlds other than the actual one are themselves mere abstractions (unless you are David Lewis), they would not exist as concrete entities that God has not created. But the “possible worlds” account of God’s necessity nevertheless insinuates that that necessity is grounded in something other than God Himself – that what is possible or necessary in general is to be determined independently of God, with God’s own necessity in turn defined by reference to these independent criteria. For A-T, this is completely muddled. The reason God is necessary is that He is Pure Act or Subsistent Being Itself, not because He “exists in every possible world.” And since God just is Being Itself – rather than “a being” among other beings, existing in one possible world or in all – all possibilities and necessities whatsoever are themselves grounded in the divine nature, rather than in anything in any way independent of God.
Keep in mind that A-T eschews Platonism and takes a moderate realist approach according to which universals exist only in their instantiations or in an intellect which contemplates them. There is no realm of abstract objects à la Plato’s Forms. What, then, of uninstantiated universals, things that don’t exist but could have? What grounds their possibility? The A-T position is that all universals pre-exist as ideas in the divine intellect. Instantiated universals are those ideas that serve as archetypes for the things God creates, uninstantiated universals are the ones that do not. If we think of a possible but non-actual world on the model of an uninstantiated universal, then possible but non-actual worlds are just the ones which exist as ideas in the divine intellect which God has not used as archetypes in creating.
To be sure, it is only those ideas that do not imply a contradiction that can serve as archetypes in creation – even God cannot make a round square. But the reason is that what God creates are beings, and a “round square” is not any kind of being at all. (Cf. Summa Theologiae I.23.3) A possible being is a possible being, something which “participates” in Being Itself (in the A-T sense rather than the Platonic one – see my discussion of the Fourth Way in Aquinas). Hence, again, possibility is grounded in God (qua Being Itself) rather than in anything outside Him.
As the “participation” language indicates, for A-T anything other than God that exists or might exist is an imitation of God. In creation, that which is unlimited and perfect in God comes to exist in a limited and imperfect way in the natural order. (Recall the doctrine of divine simplicity, as Thomists understand it: Attributes that are distinct in us are analogous to what in God is one.) The divine ideas according to which God creates are therefore to be understood as the divine intellect’s grasp of the diverse ways in which the divine essence might be imitated in a limited and imperfect fashion by created things. As the Scholastic writer Henri Renard sums it up:
From all eternity God contemplating His essence, which is the actuality of all perfection, sees the possibility of limited imitations of that supreme perfection. Thus, from an eternity He conceives the possible essences; consequently, these essences are said to be eternal, immutable, and necessary. The formal realization of the possibles, then, is in the divine intellect. The foundation, however, for this cognition is the essence of God as imitable, for the essence of God is the source of all reality, of all possibles, of all beings. (The Philosophy of Being, p. 112)
Or, as John McCormick puts it:
[I]f anything at all besides God is possible, it is because it can imitate in a finite way some infinite perfection of God. God’s essence as imitable in a finite way in created things is, therefore, the ultimate foundation of the possibles and the final reason why things are possible at all… God’s essence is therefore the Exemplar and Prototype of all reality. (Scholastic Metaphysics, Part I, p. 55)
And just to be ecumenical, let’s also quote a philosopher outside the A-T tradition, albeit one who was influenced by it – the greatest of the moderns, G. W. Leibniz:
It is farther true that in God there is not only the source of existences but also that of essences, in so far as they are real, that is to say, the source of what is real in the possible. For the understanding of God is the region of eternal truths or of the ideas on which they depend, and without Him there would be nothing real in the possibilities of things, and not only would there be nothing in existence, but nothing would even be possible…
For if there is a reality in essences or possibilities, or rather in eternal truths, this reality must needs be founded in something existing and actual, and consequently in the existence of the necessary Being, in whom essence involves existence, or in whom to be possible is to be actual. (Monadology 43-44)
As this last passage indicates, if we agree that some non-actual entities are possible, that any possibility must be grounded in something actual, and that nothing less than what is Pure Act can ultimately be the ground in question, then we have in the notion of possible but non-actual beings the basis of an argument for God’s existence. Or at least some philosophers who would ground all possibilities in the divine nature have thought so (though others have held instead that God’s existence must first be established separately, and then appealed to as an explanation of possibility).
Necessary truths too, for both A-T and Leibniz, are grounded in the divine nature. For suppose we agree that there are such truths – the truths of mathematics, for example – but also that the moderate realist is correct to hold that there are no abstract objects à la Platonism. Such truths have to be grounded in something actual, but it cannot be either the material world or any finite intellect, since neither is necessary. Only that which is infinite, purely actual can do the grounding. What we have here, though, is essentially the “argument from eternal truths” defended by the likes of St. Augustine and Leibniz, which deserves a post of its own, and will get it when I get the chance. Suffice it for now to note that for A-T, as for your more formidable non-A-T writers, necessity no less than possibility must be grounded in God – so that to ground God’s necessity in something other than Him is to get things backwards, and certainly to get them wrong.
I've got your further reading right here:
P. Coffey, Ontology, chapter III
Gyula Klima, “Contemporary ‘Essentialism’ versus Aristotelian Essentialism” (from John Haldane, ed., Mind, Metaphysics, and Value in the Thomistic and Analytic Traditions)
David Oderberg, Real Essentialism, sections 1.1 and 6.2 (only some of which is online, I’m afraid)