Ontological Argument

The Ontological Argument is the argument from the concept of God to the existence of God. The classical statement is in the Proslogion of Anselm of Canterbury: God is “that than which nothing greater can be conceived”; if such a being existed only in the understanding and not in reality, it would not be that than which nothing greater can be conceived; therefore it must exist in reality.

The argument’s history is one of repeated burials and reappearances. Gaunilo replied to Anselm in his own century with the parody of the perfect island. Aquinas thought the argument failed and rejected it. Descartes revived it in a slightly different form in the seventeenth century. Kant attacked it again in the Critique of Pure Reason, arguing that existence is not a predicate. The Kantian critique was widely thought decisive for nearly two centuries.

The argument’s twentieth-century revival is largely owed to Alvin Plantinga, who reformulated it in modal logic in The Nature of Necessity (1974). Plantinga’s modal version turns on the claim that a maximally great being is possible; if such a being is possible, it exists in some possible world; if it exists in some possible world, by the meaning of “maximally great,” it exists in every possible world, including ours. The premise that a maximally great being is possible is what carries all the weight, and Plantinga is candid that someone who denies the premise has not, by his lights, been refuted.

J. L. Mackie in The Miracle of Theism regards the modal argument as question-begging at the possibility premise. Few philosophers think the argument is sound. Even fewer can specify exactly why.