Most arguments for God’s existence work by examining the world and inferring a cause. The ontological argument does something more unusual. It argues from the concept of God to the existence of God — from what God would have to be, to the conclusion that such a being must exist.
The argument has been reformulated many times since Anselm of Canterbury first stated it in the eleventh century. Its contemporary form, developed in modal logic, is philosophically rigorous in ways that earlier versions were not. It is also contested — which is itself a mark of philosophical seriousness. Trivially false arguments are not debated for nine centuries.
The basic intuition
Start with the concept of God as classically understood — not the folk-religious God made in various human images, but the God of philosophical theology: the greatest possible being, unlimited in power, knowledge, and goodness, the ground of all existence.
Now consider: could such a being merely contingently exist? Could the greatest possible being be the kind of thing that might happen not to exist — that exists in this universe but not in all possible configurations of reality?
The answer seems to be no. A being that could fail to exist would be limited by that very contingency. A being that necessarily exists — one whose non-existence is impossible — is greater than one whose existence is merely contingent. Therefore, if God is the greatest possible being, God exists necessarily, or not at all. There is no version of God that merely happens to exist.
The modal form
In its contemporary modal logic formulation, the argument runs as follows. It is possible that a maximally great being exists — a being that is all-knowing, all-powerful, and all-good in every possible world. If it is possible that such a being exists, then there is a possible world in which it exists. But a being that is maximally great exists in every possible world by definition — its greatness cannot be bounded by the limits of any particular world. Therefore, if such a being exists in any possible world, it exists in all possible worlds. And since the actual world is a possible world, the being exists actually.
The key premise is the first: it is possible that a maximally great being exists. This seems intuitively defensible. The concept of a maximally great being is not internally contradictory the way the concept of a married bachelor or a square circle is. If it is genuinely possible — not merely imaginable, but metaphysically coherent — then the conclusion follows from the logic.
The objection
The standard objection is to deny that possibility. One can construct a parallel argument: it is possible that a maximally great being does not exist; therefore it necessarily does not exist. But this parallel cuts both ways — it shows that we need independent reason to think the concept is coherent, not simply to assert it.
The asymmetry that the theist appeals to is this: the concept of a maximally great being is a coherent concept, while “the necessary non-existence of anything” is arguably not a coherent concept at all. Something can coherently be necessary. But the necessity of there being nothing is harder to make sense of — if nothing is necessary, then it is necessary that everything including any candidate necessary being fails to exist, which is a very strong claim that seems to require explanation rather than assuming it as a starting point.
Why this matters alongside the other arguments
The ontological argument is strongest not as a standalone proof but as part of the broader case. If the cosmological argument establishes that there must be an uncaused first cause, and the fine-tuning argument establishes that this cause appears to operate with intention, and the consciousness argument establishes that this cause is likely mental rather than merely physical — then the ontological argument adds a further point: the being that fits all these descriptions is the kind of being whose existence is, if possible at all, necessary. It is not an accident. It is not contingent. If it is there, it is there in the deepest possible sense.