From On the Incarnation c. 2:
[I]f everything has its beginning of itself, and independently of purpose, it follows that everything would only exist, so as to be alike and not distinct.
And, given that body is homogeneous, it would follow that everything must be sun or moon, or that a man would be only a hand, or eye, or foot.
But as it is this is not so; rather, we see a distinction of sun, moon, and earth; and again, in the case of human bodies, of foot, hand, and head.
Now, such arrangement of separate things as this tells us not of their having come into being of themselves, but shows that a cause preceded them; namely God, the one who makes and orders all.
I’ve been fascinated by this argument since the moment I read it, now over fifteen years ago. It is at once more simple than other arguments (proving existence and providence at a single stroke, and apparently from the mere nature of the distinct things we see around us) and at the same time it does not neatly reduce to the other well-known genera of cosmological arguments. There is more to the argument, for example, than a teleological proof, a Platonic One-over-the-many proof, or even a first efficient cause proof.
First, to start with the simpler parts of the argument, Athanasius’s examples appear to be chosen to establish that the cause he is speaking of is (a.) outside the cosmos (this is the sense of using the examples of sun and moon) and (b.) is at work in each of the parts of the universe (this is the sense of using the example of the parts of the body.) The first example points to the existence of something supernatural; the second points to the existence of something involved with natural things.
But the main work of the argument is the first conditional. Let me put the conditional in a slightly more complicated form, defend it, and then show how it is equivalent to Athanasius’s:
If diverse things form a single reality,* there is some cause separate from them.
The best defense of this premise is, oddly enough, in Chapter VI of William James’s Psychology “On the Mind-Stuff Theory” (it’s also in Volume 53 in the Great Books series). James uses it to show that, despite all the best efforts to the contrary, psychology has to posit the existence of a soul separate from all mental reality; though the same premise gets us Athanasius’s conclusion that we need some being separate from all natural or noumenal reality. To give just one of James’s examples, muscle fibers might each all contract, but the only way they cause a leg-bone to move is if there is a tendon uniting the two. More broadly, distinct realities are each contained in themselves, and one can no more get a single effect out of them without an extrinsic cause than you could form a twelve-word sentence by having twelve men each think of a distinct word at the same time.**
It follows that if we deny there is a cause separate from diverse things, that either
a.) there are no diverse things
b.) diverse things do not form a single reality.
Athanasius’s explicitly addresses (a.) but not (b.), which is excusable if it is unintelligible – as I think it is – to speak of diverse things that do not in any way form a single system or reality. In fact, this seems to amount to saying that we have two utterly distinct universes, each a single, utterly homogeneous whole, which do not and cannot interact with each other in any way and yet are both homogeneously physical (which is certainly not what the multiverse theory says, since if this were the claim than an utterly useless, unconfirmable, reality that would explain nothing about the world we live in). That said, denying (a.) also doesn’t make much sense, so this leaves us with some separate cause of all distinct things that form a single system.
When I proposed this argument to a class, one student suggested that the laws of physics could play the role of the separate thing. It was a fascinating suggestion since it points to the peculiar way in which physical law plays a divine role for us – pervading all things and binding them together, while somehow being separate from them. At once perfectly immanent and transcendent! All this is made possible by our unwillingness to take a stand on what physical law is: if it is really separate from the universe, then they are by definition supernatural; if we make them only immanent to the universe, then they become mere parts of the system, but then they become distinct parts of the system, even though the only reason we posited them was to explain the actions of all the parts. Putting the laws of nature into the universe seems like trying to explain spelling by positing more letters.
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*By reality I mean that they either form a single being (like the parts of a body form an animal) or that they coalesce to form a single activity (like vector forces coalescing to form a single vector, or cars colliding at an intersection)
**It’s interesting that this premise is compatible with (though does not require) a complete denial of immanent teleology. One might visualize a cosmological argument that argued to the divine existence both from the affirmation an denial of immanent teleology.