What Is the Ontological Argument for God?

What Is the Argument?

The ontological argument is unique among the classical arguments for God because it doesn't need a premise about the world. The cosmological argument starts from the existence of the universe; the teleological argument starts from its order; the moral argument starts from the fact of moral obligation. The ontological argument starts from the concept of God and tries to show that the concept itself entails existence. The name comes from ontology, the study of being, because the argument moves in the realm of what something is rather than what it does.

The deep idea is that God, if God exists, exists necessarily—God can't be a contingent thing that happens to exist. If that's part of the concept of God, then either God exists necessarily or God doesn't exist at all. The argument tries to close the third option (God exists contingently) by arguing that the very concept of a maximally great being entails existence in every possible world, including the actual one. Critics have spent centuries trying to show that the inference from concept to existence is a trick.

Anselm's Original Proof

Anselm of Canterbury presents the argument in chapters two and three of the Proslogion, written around 1078. The proof runs like this:

1. God is "that than which nothing greater can be conceived" (id quo maius cogitare non potest).
2. Even the fool who "says in his heart, there is no God" understands this concept, since otherwise he couldn't deny it.
3. What exists both in the understanding and in reality is greater than what exists in the understanding alone.
4. So if God existed only in the understanding, a greater being could be conceived (one existing in reality).
5. But God is that than which nothing greater can be conceived.
6. Therefore God exists in reality.

Anselm's argument turns on the idea that existence is a "great-making" property: a being that has it is greater than one that lacks it. If God is the greatest conceivable being and God lacked existence, you could conceive something greater (God-plus-existence), contradicting the definition. So God must exist. The argument is short, elegant, and has the eerie quality of seeming to prove a substantive conclusion from definitions alone.

Gaunilo and Aquinas

Gaunilo, a monk contemporary with Anselm, objected on behalf of the fool. His famous counterexample is the perfect island: define an island than which none greater can be conceived. By Anselm's reasoning, this island must exist in reality, since an island existing only in the understanding is less great than one existing in reality. But clearly the perfect island doesn't exist. So there's something wrong with the form of the argument. Gaunilo's point is that the structure lets you prove the existence of anything by defining it as maximally great.

Anselm's reply is that the island case doesn't parallel the God case, because God is the greatest being conceivable, whereas the greatest conceivable island is still just an island—a limited, contingent thing. The inference to necessary existence works only for a being whose concept includes maximal greatness without limit. Whether this reply succeeds is disputed. The standard modern view is that Gaunilo exposes a real worry about the argument's form, even if Anselm's specific reply has some force.

Thomas Aquinas rejected the ontological argument in the Summa Theologiae. For Aquinas, existence isn't part of a thing's essence in the way the argument requires: we can understand what a thing is (its essence) without knowing whether it exists. The concept of God, even as the greatest conceivable being, doesn't show that such a being actually exists—it only shows what God would be like if God existed. To get existence, you need an argument from effects, like the cosmological arguments. Aquinas's five ways, in effect, are his alternative to the ontological argument.

Kant's Objection

Immanuel Kant gave what most philosophers regard as the decisive objection to the classic version. In the Critique of Pure Reason, Kant argues that existence is not a real predicate—it doesn't add anything to the concept of a thing. "A hundred real thalers" doesn't contain more than "a hundred possible thalers" as a concept; the difference is whether the thalers exist, not what they are. Similarly, "God exists" doesn't add to the concept of God; it only says that the concept has an instance. So you can't derive existence from the concept, because existence isn't a property the concept can include.

Kant's point is contested. Some philosophers defend existence as a predicate in a sense the argument needs, and some read Anselm as not relying on existence-as-predicate at all but on the modal notion of necessary existence. But the Kantian objection remains the standard textbook reply, and it captures a deep intuition: defining something into existence seems to be cheating. The concept of a thing can be coherent and complete without showing the thing exists.

Plantinga's Modal Version

Alvin Plantinga revived the ontological argument in a modern modal form in The Nature of Necessity (1974) and God, Freedom, and Evil. Plantinga's version uses the logic of possible worlds. The argument runs roughly:

1. There is a possible world in which maximal greatness is instantiated.
2. Maximal greatness entails existence in every possible world (a maximally great being exists necessarily).
3. So if a maximally great being is possible, it exists in every possible world.
4. The actual world is a possible world.
5. Therefore a maximally great being exists in the actual world.

The key move is the inference from "maximal greatness is possible" to "maximal greatness is actual." Modal logic licenses this inference if maximal greatness entails necessary existence, which it does by definition. The weakness is premise 1: why think maximal greatness is even possible? A critic can deny that the concept of a maximally great being is coherent, in which case the modal argument fails. Plantinga himself is modest about the argument's force, treating it as showing that rational belief in God is not irrational, rather than as a proof that compels assent.

Why It Matters

The ontological argument matters even if it fails. It forces a clear question: can the existence of a thing be derived from its concept? Most philosophers now think not, at least for ordinary things, but the case of God is special because the concept includes necessary existence. If anything could be argued into existence from its concept, God would be the best candidate, because the concept is built to include existence. The argument, in effect, tests the limits of conceptual reasoning.

It also matters as a model of philosophical precision. The argument has a short logical form that can be examined step by step, and the objections to it—Gaunilo, Aquinas, Kant, the modal objections—are models of philosophical criticism. Studying the ontological argument is one of the best ways to learn how philosophical analysis works, because the moves are explicit and the stakes are visible. Whether or not it proves God, it clarifies what it would take to prove anything from concepts alone.

Sources

• Stanford Encyclopedia of Philosophy, "Ontological Arguments" — https://plato.stanford.edu/entries/ontological-arguments/
• Stanford Encyclopedia of Philosophy, "Anselm of Canterbury" — https://plato.stanford.edu/entries/anselm/
• Stanford Encyclopedia of Philosophy, "Kant" — https://plato.stanford.edu/entries/kant/
• Stanford Encyclopedia of Philosophy, "Philosophy of Religion" — https://plato.stanford.edu/entries/philosophy-religion/

Related Topics

• Does God Exist? — the broader debate this argument belongs to.
• What Is the Cosmological Argument? — the argument from the world's existence.
• What Is the Teleological Argument? — the argument from the world's order.
• What Is the Moral Argument for God? — the argument from moral value.
• Philosophy of God — the wider framework.