What was Kant's objection to the ontological argument?

Immanuel Kant argued that existence is not a predicate, not a property that adds to the concept of a thing. Saying a hundred real coins contains no more coins than a hundred imagined ones; it just says the concept is instantiated. So you cannot build existence into a definition and read it back out as a fact. This is the most influential objection.

Does anyone still defend the ontological argument?

Yes. Modern logicians including Alvin Plantinga and Kurt Godel developed modal versions using the logic of possibility and necessity: if a maximally great being is even possible, then it exists necessarily. These versions are taken seriously, though critics question the key premise that such a being is genuinely possible.

The Ontological Argument: Can You Define God Into Existence?

Q: What is the ontological argument?

It is an argument that tries to prove God exists from the concept of God alone, without appealing to the world. Anselm defined God as that than which nothing greater can be conceived, then argued that such a being must exist in reality, since a God that existed only as an idea would be less great than one that also existed in fact.

Q: What is Gaunilo's objection?

The monk Gaunilo replied to Anselm with a parody: imagine the greatest possible island. By the same reasoning, it would have to exist, since an island that existed would be greater than one that did not. Since that conclusion is absurd, Gaunilo argued the form of Anselm's reasoning must be flawed.

The proof that needs
no world at all.

Every other argument for God starts by pointing at something: a cause, an order, a moral fact. The ontological argument points at nothing outside your own head. From the mere idea of a perfect being, it claims, existence follows. It has been called a charm, a fallacy, and a work of genius, sometimes by the same philosopher. It is the strangest move in the whole debate.

From a definition to a fact

Anselm — that than which nothing greater can be conceived

In the eleventh century, Anselm of Canterbury defined God as that than which nothing greater can be conceived. Even the skeptic, he said, understands the phrase, so God at least exists as an idea in the mind. But now compare two versions of that being: one that exists only as an idea, and one that also exists in reality. The second is plainly greater. So if God existed only in the mind, we could conceive of something greater, a God that also existed in fact, which contradicts the definition. Therefore God must exist in reality, not just in thought.

Descartes and the modal versions

Descartes ran a cleaner version: existence belongs to the idea of a perfect being the way three angles belong to the idea of a triangle, so a perfect being that lacked existence would be a contradiction. Centuries later, logicians including Kurt Godel and Alvin Plantinga rebuilt it in modal logic: if a maximally great being is even possible, then by the logic of necessity it exists in every possible world, and so in this one. The whole weight then rests on a single premise: is such a being genuinely possible?

That than which nothing greater can be conceived cannot exist in the understanding alone. For if it exists only in the understanding, it can be conceived to exist in reality as well, which is greater.
Anselm of Canterbury, Proslogion

Two objections that have stuck

Gaunilo's perfect island

Almost at once, a monk named Gaunilo handed Anselm a parody. Picture the greatest possible island, more lush and blessed than any real one. By Anselm's reasoning, it would have to exist, because an island that existed would be greater than one that did not. Since that is obviously silly, something in the form of the argument must be wrong. Defenders reply that the move only works for a being that is greatest by its very nature, not for islands, but the parody has never fully gone away.

Kant — existence is not a predicate

The objection most philosophers find decisive came from Immanuel Kant. Existence, he argued, is not a property that adds anything to a concept. A hundred real coins contain no more coins than a hundred imagined ones; calling them real does not enrich the idea, it just claims the idea is instantiated. So you cannot slip existence into a definition and then pull it back out as an established fact. The argument, on this view, mistakes a feature of our concepts for a feature of the world.

Being is evidently not a real predicate. A hundred real dollars contain not a coin more than a hundred possible dollars.
Immanuel Kant, Critique of Pure Reason

Where it stands

Most philosophers think Kant broke the classic version: you cannot define a thing into existence. The modal rebuilds are alive, but they only push the mystery back one step, onto whether a perfect being is possible in the first place.

The proof that needs
no world at all.

The claim

Thinkers

The objection