Ontological Argument

God is the greatest conceivable being, and a being that exists is greater than one that does not, so God must exist.

The ontological argument begins from the idea of God rather than from any observation of the world. Anselm of Canterbury, writing in the Proslogion in 1078, defined God as the being than which nothing greater can be conceived. Even the person who denies that God exists must hold that idea in mind, Anselm argued. A being that exists in reality is greater than a being that exists only as an idea. Therefore, the greatest conceivable being cannot exist only as an idea, and must exist in reality as well. Later versions sharpen the argument with modal logic, treating existence in every possible world as part of what it means to be a necessary being.

The Argument in Form

God is the being than which nothing greater can be conceived.
A being that exists in reality is greater than a being that exists only in the understanding.
If God existed only in the understanding, a greater being could be conceived: one just like God, but existing in reality.

Therefore, God exists in reality, not only in the understanding.

Proponents and History

Anselm of Canterbury gave the argument its classical form in the Proslogion of 1078, and it has been debated ever since. René Descartes offered a version based on the idea of a supremely perfect being in the seventeenth century. In the twentieth century, Alvin Plantinga recast it in modal terms, arguing that if a maximally great being is possible, it exists in every possible world, including the actual world. Kurt Gödel drafted a formal version in higher-order logic, published after his death. Each version keeps Anselm's central move: existence belongs to the very concept of the greatest being.

Standard Objections

Gaunilo, a contemporary of Anselm, answered with a parody: the same reasoning would prove a perfect island, since an island that exists is greater than one that exists only in thought. Immanuel Kant later argued that existence is not a predicate, so adding existence to a concept does not make the thing greater, and the argument cannot move from a definition to a real being. J. N. Findlay turned the argument around, arguing that the concept of a necessary being is itself incoherent. These objections are developed on the ontological disproof page.

References

Stanford Encyclopedia of Philosophy — Ontological Arguments Free / open access
Internet Encyclopedia of Philosophy — Ontological Argument Free / open access
Oxford Bibliographies — Ontological Arguments Subscription

Related Across the Site

Aesthetic Argument Argument — The experience of beauty and sublimity points beyond the natural world to a source of beauty itself.
Argument from Consciousness Argument — Conscious, intentional minds are difficult to explain on a purely physical account of the world, and theism explains them better.
Argument from Miracles Argument — Reports of miracles, above all the resurrection of Jesus, provide evidence that a supernatural power acts in the world.
Argument from Reason Argument — The reliability of human reason, and the laws of logic themselves, point to a rational ground beyond the natural world.
Argument from Religious Experience Argument — Widespread and persistent religious experiences are best explained by a transcendent reality that people genuinely encounter.
Cosmological Argument Argument — Everything that begins to exist has a cause, and the universe began to exist, so the universe has a cause.
Moral Argument Argument — Objective moral values and duties exist, and they are best explained by a moral lawgiver.
Teleological / Design Argument Argument — The order, complexity, and fine-tuning of the universe point to a designer rather than to chance alone.

Related Arguments

Ontological Disproof — The ontological argument is said to fail because existence is not a property that can be built into a definition.
Argument from Reason — The reliability of human reason, and the laws of logic themselves, point to a rational ground beyond the natural world.
Transcendental Argument — Logic, science, and objective morality presuppose the Christian God, without whom they have no final ground.