QuestionsDoes God exist?The ontological proof
Does God exist?TheismOntological

The ontological proof

René Descartes 1596 · 1650Doubt everything to find what survives doubt.
See profile
What it claims

God exists.

Why

I clearly conceive that existence belongs to the perfect being, and what belongs to a thing's nature truly belongs to it.

In short

"The ontological proof" is an argument by René Descartes (Meditation V) on the question "Does God exist?" (answer: theism). Its claim: God exists. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 3 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

Meditation V
Valid with an implicit premise

Second proof of God, without going through the world or causality: the mere idea of a perfect being would be enough.

P1Whatever I clearly conceive to belong to the nature of a thing truly belongs to it.
P2I clearly conceive that existence belongs to the nature of God, the perfect being: a perfect being without existence would lack a perfection.
P3 · implicit premiseIf existence belongs to the nature of a thing, then that thing really exists, not only in our thought.
Therefore God exists.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a possible world where existence is part of the idea of God, but God does not exist outside thought.
With P1 and P2 alone we get "existence belongs to God's nature", not "God exists". We need the bridge P3, which Descartes explicitly defends (the necessity lies "in the thing itself"). That bridge is exactly what Caterus, Gassendi and later Kant dispute.
Analogy. Conceiving a mountain implies a valley, even if no mountain exists. Descartes replies: God is different, because what is inseparable from his idea is existence itself.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

C
Caterus 1641 · attacks premise 3

Even if the concept of a perfect being contains existence, it only follows that the two ideas are linked in our mind, not that this being exists in nature.

Descartes replies
Descartes
The things we conceive contain possible existence; only the idea of God contains necessary existence. This is no fiction of the mind: it is the true nature of such a being.
Where the debate stands (our summary)Descartes replies that only the idea of God contains necessary and not merely possible existence. Caterus and his successors dispute that this necessity in the concept guarantees existence outside thought: the basic disagreement concerns the passage from concept to reality.
Gassendi 1641 · attacks premise 2

Existence is not one perfection among others: it is what any perfection requires. Putting it in the list of God's perfections assumes what must be proved.

Descartes replies
Descartes
Existence may well be called a property. And necessary existence is a property in the strictest sense, since it belongs to God alone and is part of his essence only.
Where the debate stands (our summary)Descartes maintains that necessary existence is a property in the strict sense, proper to God. Gassendi holds that existence is not one perfection among others. The debate on the status of existence continues with Kant, then in modern logic.
Kant 1781 · attacks premise 2

"Being" is not a real predicate: saying a thing exists adds nothing to its concept. A hundred real thalers contain not a cent more than a hundred imagined ones.

Descartes replies
No reply possible: objection raised in 1781, after the author's death (1650).
Norman Malcolm 1960
Malcolm defends Anselm rather than Descartes: he grants Kant that simple existence is not a perfection, but maintains that necessary existence is one: on it, he holds, Anselm's second argument rests. The rest of the debate (Kant, then the modal versions) is treated in Anselm's argument.
see the argument
Where the debate stands (our summary)The proof's supporters distinguish simple existence from necessary existence to escape Kant, as Descartes already did against Caterus; Kant already held that removing the subject removes its necessity without contradiction. The debate has moved to the possibility premise of the modal versions.
In the same family · Ontological
Existence is not a predicateImmanuel Kant
Does God exist?Agnosticism 2
God is not a hundred thalers: Hegel defends the ontological proof against KantGeorg Wilhelm Friedrich Hegel
Does God exist?Theism 3
That than which nothing greater can be conceived existsAnselme de Cantorbéry · 1078 · debate over 3 levels
Does God exist?Theism 5
If it is possible, it exists: the modal ontological argumentAlvin Plantinga · 1974 · debate over 3 levels
Does God exist?Theism 4