Ontological arguments as a priori and deductive, Anselm's argument that God is that than which nothing greater can be conceived, Descartes' argument from God as a supremely perfect being, and the objections of Gaunilo's perfect island, Hume and Kant that existence is not a predicate.

What this dot point is asking

AQA wants you to explain the ontological argument as an a priori deductive argument that moves from the concept of God to God's existence, to set out Anselm's and Descartes' versions, and to evaluate the main objections: Gaunilo's parody of the perfect island and the Hume and Kant claim that existence is not a predicate.

The form of the argument

This makes it unique among the arguments AQA studies. The design and cosmological arguments are a posteriori: they start from observed features of the world (order, causation, contingency) and could in principle be undercut by discovering the world is otherwise. The ontological argument starts from nothing but a concept, so if it works it proves not just that God exists but that God exists necessarily, in every possible world, and that the atheist contradicts himself. That ambition is also its vulnerability: a sound a priori proof of a concrete existing being would be remarkable, and most of the objections charge that the argument smuggles existence into a definition rather than establishing it.

Anselm's argument

Descartes' argument

Objections

• Gaunilo's perfect island. Gaunilo parodies Anselm: by the same reasoning, "the greatest conceivable island" would have to exist, since existing is greater than not existing. Since that conclusion is absurd, the argument form must be flawed. (Anselm replies the argument works only for a being whose greatness is unlimited, not for a contingent thing like an island.)
• Existence is not a predicate (Hume and Kant). Kant argues that existence is not a real predicate: saying a thing exists adds nothing to its concept, it only posits that the concept is instantiated. "A hundred real thalers contain no more than a hundred possible thalers." So you cannot define a thing into existence; the concept of God does not, by itself, entail that God exists.
• Hume's empiricist point. Existence claims are matters of fact, knowable only a posteriori; no existence can be proved by reason alone, since its negation is never a contradiction. Whatever we can conceive as existing we can equally conceive as not existing, so "God does not exist" is never self-contradictory, and the ontological argument's claim of necessity collapses.

These objections target different premises, and a strong evaluation keeps them apart. Gaunilo grants the form of the argument and shows it proves too much, so the theist must explain why the argument works for God but not for islands or any "greatest conceivable" thing (Anselm and later Plantinga appeal to necessary rather than maximal existence, which only a non-contingent being can possess). Kant and Hume attack a premise, denying that existence is the kind of property that can belong to a concept at all; if they are right, no appeal to necessity helps, because the whole strategy of reading existence out of a definition is mistaken. Modern modal versions (Plantinga's argument from a possible maximally great being) reframe the issue around whether God's existence is so much as possible, which is where contemporary debate focuses.