Why One God, Not Many?

From one necessary, unlimited being to monotheism: ruling out polytheism and dualism.

On the unity of God, see Summa Theologiae I, Q. 11, A. 3: "Whether God is one?"

The arguments for God's existence do not merely reach a first cause; they reach a first cause of a very particular kind: one whose existence is not received, not limited, not measured out. Once we see that kind of being, the question "why not several of them?" answers itself.

The ancient world assumed many gods; much of the modern world assumes that how many gods is a matter of taste or tradition. Reason says otherwise. The very features that made this being the ultimate explanation are the features that make it necessarily one.

The argument, step by step:

1. What the earlier ways reach is a being whose existence is not limited or received: Being Itself, not a being that merely has being alongside others.
2. To have two such beings, there would need to be some difference between them, something one had that the other lacked.
3. But a difference is a limit: to lack anything is to be measured, bounded, not unlimited.
4. An unlimited being lacks nothing, so there is no room for a second one to differ from the first.
5. Therefore the ultimate source cannot be many. It is necessarily one. And this all men speak of as God.

Ruling out the rivals. This closes two doors at once. Polytheism — many finite gods dividing up the cosmos — fails, because finite, limited beings are exactly the kind of thing that still needs explaining; they are candles, not the flame's source. Dualism — two ultimate principles, one good and one evil, locked in eternal contest — fails too, because two ultimates would each have to be limited by the other, and a limited ultimate is a contradiction.

"But why not two identical ones?" Here is the objection that feels like an escape hatch: grant that any two beings must differ, so why not simply drop the difference, and posit two unlimited beings exactly alike? The trouble is that this does not give you two beings; it gives you one described twice. To be this one and not that one, there must be something — a location, a role, a property, a boundary — that marks them off from each other. Take away every such difference and you have taken away the very thing that would make them two. Two identical unlimited beings is not a second God kept in reserve; it is the first God under another name. Distinctness is bought only with difference, and difference is bought only with limitation, which unlimited being, by definition, has none of.

See it today: everything there is

Point at everything that exists, the whole of what is. There cannot be two such totalities. A second one would either leave something out, and so not be everything, or include exactly the same things, and so just be the first. And notice this is not a tie waiting to happen: two runners can clock the identical time because fastest is a score two distinct people can share, but all there is is not a score at all: it has no outside from which a rival could match it.

Unlimited being has that same shape, only it is the source of the whole rather than the sum of it. That than which nothing is more ultimate cannot come in a pair: a second one would either fall short — and so not be ultimate — or match it completely — and so just be it. The moment you say unlimited, you have already said one.

Where this leads. So reason does not merely permit monotheism; it requires it. The ultimate source of all being is single, undivided, and unrepeatable. But knowing the source is one still leaves the deepest question untouched: is this one God a someone or a something, a mind that knows and a will that loves, or a blind, impersonal principle? That is the next step.