But although this counter has some force, it only shows that we need to do a little more work and think through the notion of whether ipsum esse subsistens is subject to multiplication.

But your atheist friend may counter that we still haven’t proven that God exists, because there’s still the possibility of there being more than one such reality, in which case they couldn’t be God. The God of classical theism is absolutely unique.

In “A God-Bathed World” we saw that there exists at least one efficient cause of which essence and existence are identical. We called this ipsum esse subsistens—subsistent being itself. Such a being we also call God.

So let’s do that!

One mode of multiplication is the multiplication of a genus into species. Consider, for example, the genus animal and the species human. A human belongs to the genus animal, but is set apart from other species in that genus by the specific difference rationality. Animal signifies a wider domain of animals than some specific type of animal. Human restricts the understanding of animal to only those types that have the distinguishing feature of rationality. Without such a feature, human couldn’t be specified as a distinct type of animal.

A being of which essence and existence are identical cannot be multiplied in this way. Suppose that pure existence were a genus of which there were multiple species. In order for pure existence to be multiplied into its species, the multiple instances would have to have a specifying feature that differentiates one instance of pure existence from another.

But if we add a distinguishing feature to an instance of pure existence to differentiate it from other instances of pure existence, then no instance would be pure existence itself. Just as human is not pure animality but animality plus the form of human, so too an instance of pure existence wouldn’t be pure existence itself but existence plus a particular form of existence.

Moreover, the instance of pure existence that had the differentiating feature would limit existence to existing in this way and not some other way. But for existence to be limited to some particular mode means a lack of some existence, and thus the instance of pure existence wouldn’t be just pure existence itself.

Therefore, we must deny multiplication of this type for a first efficient cause of which essence and existence are identical. Otherwise, we would lose the notion of pure existence.

Another way to have more than one thing of a certain kind is when there are individual members of a species. For example, both Plato and Socrates are individual members of the species human. But neither one of them is human by virtue of being the individual that he is. In other words, neither Plato nor Socrates exhausts the species human.

If Plato were human by virtue of being the individual Plato, then every human being would be Plato, including Socrates. But obviously that is not the case. Therefore, neither Plato nor Socrates can be individuated by the species that they are members of. There must be some other differentiating factor.

That further element of individuation is matter. Plato is distinct from Socrates because the substantial form human is united to this clump of matter (Plato’s body) in this space at this time, and not that clump of matter (Socrates’s body) in that space at that time. Therefore, the reception into matter of a type brings about individuals of that type.

Can pure existence be multiplied in this way? It cannot. The first reason is the same as given above concerning multiplication of a genus into its various species. To differentiate one instance of pure existence from another by the matter that it informs is to make that instance of pure existence a composite of existence plus that parcel of matter. It would also limit existence to a particular mode, in which case that instance of pure existence would lack existence. But in either case that instance of pure existence would no longer be pure existence, which cannot be.

Another reason why pure existence cannot be multiplied through reception into matter is that all material beings are subject to generation and corruption. The matter that makes up a corporeal being is always in potency to losing the form it has (in which case the thing ceases to exist) and receiving a new form (in which case a new thing begins to exist).

But a being for which existence belongs to its nature cannot possibly be subject to generation or corruption. How could a being, whose nature it is to exist, not exist at some moment in time? To assert that such a being could not exist at some moment would be like saying there could be a moment when a triangle doesn’t have three straight sides. But that’s absurd.

There is another, more fundamental reason why a cause of which essence and existence are identical cannot be multiplied. Notice that both modes of multiplication articulated above signify some reality indeterminately, which is then further determined by some principle distinct from itself. For example, the generic notion animal signifies all animals, but in an indeterminate way. It stands to be further determined by the addition of a specifying feature or features—e.g., rationality—that is formally distinct from the generic notion.

The same is true for the multiplication of a species into its individual members. The species human signifies all individuals of its kind, but in an indeterminate way. For the species to be further determined to the individuals, recourse to another principle besides the species is necessary. And as we saw above, this principle is matter.

Pure existence cannot possibly have a potency to be further determined by something formally distinct from itself. This is because there is nothing other than existence by which it can be determined—except non-existence. And to be determined by nothing is not to be determined at all. Philosopher Gaven Kerr explains,

[A]nything not envisaged by pure esse is precisely an impossibility of being and beyond the scope of being. Given the latter, there can be nothing distinct from pure esse which stands to determine it in some fashion.

Therefore, there can be nothing distinct from pure existence that could possibly determine it in a way that would make multiple instances of it. Pure existence is intrinsically indeterminable, and therefore in principle cannot be multiplied.

Credit is due to our atheist friends who recognize the gap between the conclusion that at least one efficient cause of which essence and existence are identical and the conclusion that God exists. But when you think it through, the gap is almost non-existent, since the attribute of uniqueness is embedded in the notion of pure existence itself. It just requires a little unpacking (the same is true for the other divine attributes—immutability, infinity, eternality, omnipotence, omnibenevolence, omniscience, etc.).

Thus, the absolutely unique first efficient cause of which essence and existence are identical, and on which every thing else that exists depends, is not only what we call God, but is God.