The Ontological Argument

In 1078, Anselm of Canterbury was praying in the monastery at Bec in Normandy when he arrived at an argument he had been seeking for some time — an argument for God's existence that did not depend on observation of the world but flowed from the concept of God itself. He recorded it in a short work called the Proslogion (Discourse). The argument Anselm offered has been discussed, attacked, defended, reformulated, and attacked again by virtually every major philosopher in the Western tradition: Gaunilo and Aquinas objected, Descartes and Leibniz defended, Kant demolished, and Plantinga reconstructed. It is still debated. It is, in one philosopher's description, the most annoying argument in the history of philosophy — because it refuses to die cleanly, and because everyone who encounters it feels that something must be wrong with it without being able to say precisely what. The honest account of it requires sitting with that difficulty rather than resolving it too quickly in either direction.

Anselm's argument

Anselm begins with a definition. God, he says, is "that than which nothing greater can be conceived" — the greatest conceivable being, the being beyond which no more perfect or complete being is imaginable. This definition is not controversial; it is roughly what believers mean when they speak of God, and even an atheist can understand the concept without affirming the object.

Now the argument: this greatest conceivable being exists at minimum in the understanding — we can think about it. The question is whether it also exists in reality. Anselm argues that if it existed only in the understanding and not in reality, then we could conceive of something greater: namely, a being exactly like it but which also exists in reality. A being that exists in reality is greater than a merely conceptual being in exactly the same way that a real sum of money is greater than an imaginary sum. But if we can conceive of something greater than God, then God is not the greatest conceivable being — which contradicts our definition. Therefore, the greatest conceivable being must exist in reality, not only in the understanding. Therefore, God exists.

The argument is a reductio ad absurdum: assume God does not exist, derive a contradiction, conclude that God must exist. Its strength is that it requires no empirical premises — no observations of the world, no scientific data, no cosmological history. The concept alone, Anselm claims, compels the conclusion.

Gaunilo's objection: the perfect island

A fellow monk named Gaunilo wrote a response to Anselm almost immediately, in a treatise called "On Behalf of the Fool" (referencing Psalm 14:1, "The fool hath said in his heart, There is no God"). Gaunilo's objection was not so much a philosophical refutation as a reductio of the reductio: if Anselm's argument form is valid, it proves too much. Consider the most perfect island imaginable — the greatest island that can be conceived. By Anselm's logic, this island must exist in reality, because an island that exists in reality is greater than one that exists only in the mind. But we have no reason to believe such an island exists. If the argument proves the existence of the greatest conceivable island, it proves nothing interesting about God specifically.

Anselm's response was that his argument applies specifically to a necessary being — one whose non-existence is logically impossible — and that islands and all finite things are contingent: their existence or non-existence is a factual matter, not a logical one. The greatest conceivable island is still a contingent thing; the greatest conceivable being is one whose nature requires existence, making the analogy fail. Whether this response fully satisfies Gaunilo's objection is contested.

Kant's objection: existence is not a predicate

The most influential criticism of the ontological argument came from Immanuel Kant in his Critique of Pure Reason (1781). Kant argued that Anselm's argument rests on a mistake about what existence is. The argument treats existence as a property — something that can be added to or subtracted from a concept, making the concept "greater" or "lesser." But existence, Kant argued, is not a property in that sense. "A hundred real thalers," he wrote, "do not contain the least coin more than a hundred possible thalers." The concept of a hundred real thalers is not greater than the concept of a hundred merely possible thalers; what differs is whether the concept has an instance in the world, not what the concept contains. To say "God exists" is not to add a property to the concept of God; it is to say that the concept has an instantiation in reality. Therefore, the move from "the greatest conceivable being" to "the greatest conceivable being necessarily exists" is illegitimate — it treats existence as a property that can be analytically derived from a concept, when in fact existence is never analytically derivable from a concept but must be established by experience.

Kant's objection is widely regarded as the strongest response to the classical form of Anselm's argument. The logical positivists of the twentieth century extended it: any meaningful statement about the world must be verifiable by experience, and the existence of God is not. On this view, the ontological argument is not even meaningful.

Plantinga's modal version

The philosopher Alvin Plantinga reformulated the ontological argument in the 1970s using the tools of modal logic — the logic of necessity and possibility. His version runs as follows:

1. A maximally great being is possible — that is, there is at least one possible world in which a maximally great being exists.
2. A maximally great being, by definition, is one that is maximally excellent in all possible worlds — it is omnipotent, omniscient, and morally perfect not only in this world but necessarily, in every possible world.
3. If a maximally great being exists in some possible world, it exists in all possible worlds (because its maximal greatness entails necessary existence).
4. Therefore, a maximally great being exists in the actual world — our world.

Plantinga himself acknowledges that his argument is valid but not what philosophers call "coercive" — that is, a rational person can deny the first premise (that a maximally great being is even possible) without contradiction. The argument shows that if God is possible, God is necessary — which is not the same as showing that God is possible in the first place. What Plantinga claims is that the first premise — the possibility of a maximally great being — is not itself unreasonable to affirm, and therefore a person who affirms it and accepts the argument's logic is within their rational rights to conclude that God exists. Critics respond with a parallel argument: if a necessarily non-existent being (an impossible being) is conceivable — if it is even possible that God is impossible — then by similar modal logic, God does not exist in any possible world. The argument's conclusion tracks which of the two premises you find more plausible to begin with.

The honest verdict

The ontological argument does not have the same probative force as the cosmological or moral arguments. The cosmological argument rests on empirically established facts about the universe; the moral argument rests on the widely shared intuition that objective moral standards exist. The ontological argument rests entirely on conceptual analysis, and it is contested at the level of whether its conceptual moves are legitimate. Kant's objection — that existence is not a predicate — has not been decisively answered. Plantinga's modal version is technically valid but shifts the weight to a contested premise about possibility.

What the argument illuminates, even if it does not prove, is something important about the concept of God: God is not one being among others whose existence happens to be the case, as the existence of chairs or planets happens to be the case. God, if God exists, exists necessarily — it could not be otherwise. This is what the Hebrew tradition says when God reveals his name to Moses as "I AM THAT I AM" (Exodus 3:14): the name is not a biographical datum but a statement about the kind of being God is. God's existence is not contingent. The ontological argument is an attempt to turn that theological conviction into a philosophical proof. Whether the attempt fully succeeds is honestly open.

The wiser posture is probably Anselm's own, expressed at the opening of the Proslogion, before the argument. He wrote that he was not seeking to understand in order to believe, but believing in order to understand — credo ut intelligam, "I believe so that I may understand." The ontological argument, on this reading, is not a path from unbelief to belief, but an exploration of what is already implied by the belief already held. It is faith examining its own content, not a proof meant to coerce assent. Taken that way, it says something true and important: the God of the Bible is not a contingent fact of the universe but the ground of all being, in whom existence and essence are one. The argument's philosophical form is disputed. The theological insight it reaches for is not.

Related: The Cosmological Argument (the universe began to exist, therefore a cause outside space and time), The Moral Argument (objective morality requires a moral ground), Not Meant to Be Blind (faith and reason as companions, not enemies), The Character of God. Primary sources: Anselm of Canterbury, Proslogion (1078); Gaunilo, On Behalf of the Fool (c. 1078); Immanuel Kant, Critique of Pure Reason (1781), "The Impossibility of an Ontological Proof of the Existence of God"; Alvin Plantinga, The Nature of Necessity (Oxford, 1974), ch. 10. Scripture from the King James Version, linked to BibleHub.