The Ontological Argument

The Ontological Argument is unique among the arguments for God's existence because it is a priori — it does not rely on evidence from the world but instead attempts to prove God's existence through reason and logic alone. The argument claims that the very concept of God entails God's existence — that it is logically impossible for God not to exist. This lesson examines the classical formulations by Anselm and Descartes, the major objections from Gaunilo and Kant, and the modern modal versions that have given the argument new philosophical life.

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Anselm's First Form (Proslogion 2)

St Anselm of Canterbury (1033–1109) presented the first version of the ontological argument in his Proslogion (1077–1078). His argument can be reconstructed as follows:

1. God is defined as "that than which nothing greater can be conceived" (aliquid quo nihil maius cogitari possit)
2. Even the fool who denies God's existence understands this definition — the concept of God exists in the understanding (in intellectu)
3. A being that exists both in the understanding and in reality (in re) is greater than a being that exists only in the understanding
4. If God existed only in the understanding, then a greater being could be conceived — one that also exists in reality
5. But this contradicts the definition of God as that than which nothing greater can be conceived
6. Therefore, God must exist both in the understanding and in reality

Key Definition: A Priori Argument — An argument that can be known to be true or false through reason alone, without reference to experience or observation. The ontological argument is the classic example.

The genius of Anselm's argument is that it seems to derive a factual conclusion (God exists) from a mere definition. If God is the greatest conceivable being, and existing is greater than not existing, then God must exist — otherwise the definition is contradicted.

Anselm's Second Form (Proslogion 3)

Anselm developed a second, arguably stronger version of the argument:

1. God is that than which nothing greater can be conceived
2. A being whose non-existence is logically impossible (a necessary being) is greater than a being whose non-existence is logically possible (a contingent being)
3. If God's non-existence were possible, then a greater being — one whose non-existence is impossible — could be conceived
4. This contradicts the definition of God
5. Therefore, God's non-existence is logically impossible — God exists necessarily

This second form shifts the argument from the claim that existence is a perfection to the stronger claim that necessary existence is a perfection. It anticipates the modern modal versions of the argument.

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Descartes' Version

Rene Descartes (1596–1650) presented his own version of the ontological argument in the Meditations (1641), specifically Meditation V. Descartes argued:

1. I have a clear and distinct idea of God as a supremely perfect being
2. A supremely perfect being must possess all perfections
3. Existence is a perfection
4. Therefore, God must exist

Descartes compared God's existence to a property of a triangle: just as a triangle necessarily has three sides and angles summing to 180 degrees, God necessarily exists. Existence is as inseparable from God's essence as three-sidedness is from a triangle's essence.

Philosopher	Key Claim	Approach
Anselm (Form 1)	God = that than which nothing greater can be conceived; existing in reality > existing only in mind	Reductio ad absurdum
Anselm (Form 2)	Necessary existence > contingent existence	Necessary existence argument
Descartes	Existence is a perfection; God has all perfections	Perfection argument

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Gaunilo's Island

Gaunilo of Marmoutiers (d. c.1083), a contemporary of Anselm, objected in his On Behalf of the Fool. Gaunilo argued that Anselm's reasoning could be used to prove the existence of anything — for example, a perfect island:

1. Conceive of an island than which no greater island can be conceived
2. An island that exists in reality is greater than one that exists only in the mind
3. Therefore, the perfect island must exist in reality

Since this conclusion is absurd — we cannot think perfect islands into existence — Gaunilo argued that Anselm's logic must be flawed. If the argument works for God, it should work for perfect islands, perfect pizzas, or any other perfect thing. Since it clearly does not work for these things, it does not work for God either.

Anselm's response: God is unique — God is the only being whose essence includes necessary existence. Islands, pizzas, and other contingent things have a maximum conceivable greatness that does not include necessary existence. The argument applies only to a being defined as that than which nothing greater can be conceived — and only God fits this definition.

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Kant's Critique: "Existence Is Not a Predicate"