The ontological argument: Anselm's a priori argument from the concept of the greatest possible being, Descartes's version, and the criticisms from Gaunilo and Kant (existence is not a predicate).

What this key area is asking

The ontological argument is the one a priori argument you study: uniquely, it tries to prove God exists from the concept of God alone, with no appeal to experience or to the world. You must understand Anselm's argument from the idea of "that than which nothing greater can be conceived," Descartes's version (existence as a perfection), and the two famous criticisms, Gaunilo's perfect island and Kant's claim that existence is not a predicate, then evaluate whether the argument can possibly work.

Anselm's argument

The argument is a reductio: deny that God exists in reality, and you contradict the very concept of God as the greatest conceivable being. Because it reasons from the concept alone, it is a priori, and if sound it would yield a necessary truth, which is what makes it so striking and so suspicious.

Descartes's version

Descartes's version makes the crucial assumption explicit: that existence is a property (a perfection) that a being can have or lack, and that a perfect being must have it. This assumption is exactly what Kant later denies, so the two versions share a single point of failure.

Gaunilo's criticism

Gaunilo's island shows that something is wrong with arguing from a concept to existence in general, even if it does not pinpoint what. Defenders reply that the argument works only for a necessarily existing being, God, not for contingent things like islands, which raises whether God is a special case.

Kant's criticism

Kant's point goes to the heart of both Anselm and Descartes: both treat existence as a perfection or property that the greatest being must have. If existence is not that kind of property, the arguments collapse, because the move from "the concept of the greatest being" to "this being exists" is blocked.

Evaluating the argument

The standard verdict is that the ontological argument is ingenious but unsuccessful as a proof, chiefly because Kant's objection is hard to answer: defining a thing as existing does not make it exist. Gaunilo shows the form proves too much; Kant explains why. Defenders (such as later modal versions) argue that necessary existence is different from ordinary existence and may be a genuine perfection, keeping the debate alive. A strong judgement explains why the criticisms bite, rather than merely citing them, and weighs whether any version escapes them.

Worked example

Try this

Q1. How does Anselm define God in the ontological argument? [2 marks]

• Cue. As "that than which nothing greater can be conceived," from which he argues God must exist in reality, not only in the understanding.

Q2. What is Kant's objection to the ontological argument? [2 marks]

• Cue. That existence is not a predicate; saying a thing exists adds nothing to its concept, so God cannot be defined into existence.