“The doctrine of God’s simplicity,” states James Dolezal, “reaches the zenith of expression and sophistication in the thought of Thomas Aquinas” (God Without Parts, p. 6). One might even argue that it forms the lynchpin of St Thomas’s understanding of divinity. The doctrine enables him to carefully distinguish deity from all created beings, firmly establishing God as absolute reality and ultimate explanation for the existence of the world. In his Summa Theologiae, Thomas examines the divine simplicity under multiple aspects and reaches the following conclusion:
God is not composed of extended parts (since he is not a body), nor of form and matter, nor does he differ from his own nature, nor his nature from his existence. Nor can we distinguish in him genus and difference, nor substance and accidents. It is therefore clear that God is in no way composite. Rather, he is entirely simple. (I.3.7)
Thomas’s predecessors in Christian theology, both in the East and the West, would have been well acquainted with the specific forms of composition that he denies of the eternal Creator; all, that is, except one—the denial in God of a distinction between his existence and his essence.
It is generally conceded that this is a distinction that was unknown, or not clearly known, in antiquity. It is a distinction that Greek seems verbally ill-equipped to make, since the word ousia does double duty for both essence and existence. It is a distinction that is generally thought to have been introduced into Western thought by Muslim thinkers such as Avicenna and Jewish thinkers such as Moses Maimonides, though perhaps hints of it can be found in Boethius’ De Hebdomadibus. (Frederick Bauerschmidt, Thomas Aquinas, p. 89)
This does not mean that Eastern theologians did not have at their disposal manifold ways to speak of God’s nature and aseity—just compare St John of Damascus’s discussion of divinity in On the Orthodox Faith—but apparently they lacked the conceptuality to equate divine existence and divine essence. With the Angelic Doctor a genuinely creative step-forward occurs: there is in God not a whit of difference between what he is (his essence) and that he is (his existence). Ipsum Esse Subsistens—God is identical to his act of Being, or to phrase it a bit more colloquially, God is sheer existing.
Of every created entity, we may ask two questions: what is its nature, and does it actually exist? Answering the first question does not give us an answer to the second. Consider the question “What is ____?” If we fill the blank with the name of any thing we find in the world, we will be able to answer the question (assuming we know the answer) without mentioning its existence at all. If, for example, we ask, “What is a human being?” we might reasonably reply, “A rational animal.” We need not add the phrase “that exists.” It does not improve or clarify our response to the request for a definition. We can grasp the “what-it-is” of an entity, in other words, without having to determine whether it exists. As Bauerschmidt puts it: “Our act of understanding a thing’s quidditas [whatness] is something distinct from the act of judging whether things such as men or phoenixes really exist. And therefore, Thomas concludes, a thing’s existence must be different from its essence” (p. 89).
This distinction between essence and existence also applies to imaginary entities. If you ask me, “What are elves?” I will explain that they are rational children of Ilúvatar, but unlike human beings, they are immortal and exist as long as the world lasts. They do not suffer disease, and after reaching maturity they do not age. If slain, they are reincarnated in the Halls of Mandos. (If my answer perplexes, I refer you to the sacred writings of J. R. R. Tolkien.) I can say all of this while prescinding from the question whether elves exist or not (personally I hope they do). Existence, therefore, appears to be something that an essence receives—that which makes it actual. This is probably not a helpful way for me to express it, as it seems to reify existence and might be interpreted to mean that entities exist before they exist. Be that as it may (I ain’t no philosopher), the important thing to see is that the nature of something does not include the property of existence—except, says Aquinas, for God:
God is not only his own godhead; he is also his own existence. Firstly, properties that do not define a thing derive either from what does define it (when common to a species, like humour in men), or from an outside cause (like heat in water). But existence, if it does not define a thing, cannot derive from what does define it, for that would mean the thing depended on itself for existence. So unless existence defines God he must receive it from outside. Secondly, unless existence defines God he will have a potentially existent nature: for it is existence that realizes forms and nature. (We use the verb is to signify both the act of existing, and the mental uniting of predicate to subject which constitutes a proposition. In the first sense we cannot know the existence of God any more than we can define him; but we can say there is a God, framing a proposition about God which we can know to be true by argument from his effects.) (ST I.3.4; concisely rendered by Timothy McDermott, Summa Theologiae, p. 15)
No creature carries in itself the reason for its being. Every creature, therefore, is ontologically contingent—only by the grace of God does it exist—and therefore a metaphysical composite of essence and existence. Commenting on the above passage from the Summa, Bauerschmidt writes: “Existence is not a part of the definition of any created thing; even more, it cannot be derived from that definition, as the ability to laugh can be derived from the definition of human beings as rational animals, for the existence of a particular thing’s essence presupposes that the thing exist” (Holy Teaching, p. 59, n. 8). But not so with the eternal Creator, who is perfect simplicity. If God is God—that is to say, the ultimate and final answer to the question “Why does anything exist rather than nothing?” (the burden of the Five Ways)—then he cannot suffer from essence/existence composition. God does not potentially exist; he necessarily exists. Deity is the mystery where the ontological buck finally stops. Here is perhaps Thomas’s most important contribution to Christian reflection upon divinity. The eternal Creator and first mover must exist in and of himself. He cannot derive existence from some other source; otherwise the question of “why” would continue ad infinitum. But not only must God exist, he is his existence: the whatness of God is identical to his act of existing (ipsum essendi). “God exists (as is proved by the existence of his effects), explains Timothy McDermott, “but he does not share existence in the way everything else does, he is not a member of the genus thing—he is no thing, though not nothing. He is his own existence: not even the existence that other things share, but the doing of that existence” (p. xxxiii).