The ontological argument is an attempt to prove that God exists from the idea of God alone, without appealing to any evidence from the world. Its classic form, set out by Anselm of Canterbury in 1078, says that God is that than which no greater can be conceived, and that such a being must exist in reality, because a being that existed only in the mind would not be the greatest conceivable. The argument has fascinated philosophers for nearly a thousand years. Most, including many believers, think it fails, although modern versions are still defended. This guide explains the argument, its main versions, the classic objections, and where the debate stands.
The Ontological Argument at a Glance
| Version | Thinker and date | Main idea |
|---|---|---|
| Classic | Anselm, Proslogion, 1078 | A greatest conceivable being must exist in reality, not only in the mind |
| Cartesian | René Descartes, Meditations, 1641 | Existence belongs to the idea of a supremely perfect being, as three angles belong to a triangle |
| Modal | Alvin Plantinga, The Nature of Necessity, 1974 | If a maximally great being is possible, it exists necessarily |
| Logical | Kurt Gödel, published in his Collected Works, 1995 | A formal proof from the definition of positive properties |
Anselm’s Argument
Anselm, a Benedictine monk who later became Archbishop of Canterbury, presented the argument in his Proslogion. He addressed it to the fool of the Psalms, who says in his heart that there is no God. Even the fool, Anselm said, understands what is meant by “that than which no greater can be conceived”, so this being exists at least in the understanding. But a being that exists in reality as well as in the understanding is greater than one that exists only in the understanding. So if the greatest conceivable being existed only in the mind, we could conceive of something greater, which is a contradiction. Therefore, Anselm concluded, it exists in reality.
The argument is called ontological, from the Greek word for being, because it reasons from the concept of a being rather than from observations of the world. That sets it apart from the cosmological argument, which reasons from the existence of the universe, and the design argument, which reasons from its order.
Gaunilo’s Perfect Island
The first famous objection came soon afterwards from Gaunilo, a monk of Marmoutier, in a reply titled On Behalf of the Fool. Gaunilo asked his readers to imagine an island more perfect than any other. By Anselm’s reasoning, he said, such an island would have to exist, because an island existing only in the mind would be less perfect than one existing in reality. Since that conclusion is absurd, something must be wrong with the reasoning, even if it is hard to say exactly what. This kind of reply is called a parody objection: it shows that an argument of the same shape proves things nobody believes.
Anselm answered that his reasoning applies only to the greatest conceivable being, not to the greatest island, because an island, however perfect, could always be imagined greater. Philosophers still debate whether that reply works.
Descartes and Leibniz
René Descartes gave his own version in the Fifth Meditation of his Meditations on First Philosophy, published in 1641. He argued that existence is part of the idea of a supremely perfect being, just as having three angles that add up to two right angles is part of the idea of a triangle. A supremely perfect being without existence would lack a perfection, so it would not be supremely perfect. Gottfried Leibniz later accepted the general approach but argued that it needs an extra step: it must first be shown that the idea of a supremely perfect being is consistent, and he offered an argument for that.
Not every believer has accepted the argument. Thomas Aquinas, the most influential medieval Christian philosopher, rejected it, arguing that we do not know God’s nature well enough to see that God must exist simply from the concept. He preferred arguments that start from the world.
Kant: Existence Is Not a Predicate
The most influential objection comes from Immanuel Kant’s Critique of Pure Reason. Kant argued that ontological arguments treat existence as a real predicate, a property that can be added to a concept and make the thing described greater or more perfect. In his view it is not. Saying that something exists does not add anything to our idea of what it is; it says that the idea corresponds to something real. A hundred real coins, in his famous example, contain no more coins than a hundred imagined ones. If existence is not a property, then it cannot be part of the definition of God, and the argument does not get started.
Many philosophers think Kant’s objection decisive against Anselm’s and Descartes’s versions. Others argue that it is less clear than it looks, and that some versions of the argument do not depend on treating existence as a property.
Plantinga’s Modal Ontological Argument
In the twentieth century, philosophers revived the argument using modal logic, the logic of possibility and necessity. The best-known version is Alvin Plantinga’s, in his 1974 book The Nature of Necessity. Plantinga defines a maximally great being as one that has maximal excellence, including all knowledge, all power and perfect goodness, in every possible world. The argument then runs: it is possible that a maximally great being exists; if it is possible, then it exists in some possible world; but a maximally great being exists in every possible world if it exists in any; so it exists in the actual world.
Plantinga himself did not claim that the argument proves God’s existence. He claimed that it shows belief in God to be rational, because accepting its key premise is not unreasonable. Critics focus on that premise. To grant that a maximally great being is possible, in this strong sense, is effectively to grant that it exists, so they argue that the argument assumes its conclusion. They also point out that the same pattern can be run in reverse: if it is possible that no maximally great being exists, then by the same logic none exists. Which premise is more plausible cannot be settled by the argument itself.
Gödel’s Ontological Proof
The logician Kurt Gödel, best known for his incompleteness theorems, wrote a short formal version of the argument that was published in his Collected Works in 1995. It defines God as a being with every positive property and uses axioms about positive properties to derive that such a being necessarily exists. Logicians have since checked the proof by computer and found it valid given its axioms. The debate is over whether the axioms are true, and some critics have shown that, as originally stated, they lead to the conclusion that nothing could have been otherwise than it is, which most philosophers reject.
Why the Argument Still Matters
Few philosophers today think the ontological argument proves God’s existence, but it keeps being studied because it raises basic questions about existence, necessity and the relation between concepts and reality. It also matters for other arguments for God. Immanuel Kant claimed that the cosmological argument depends on it, and some defenders of belief combine it with other arguments, for example with the claim that faith is a reasonable bet, discussed in the site’s analysis of Pascal’s Wager and the rationality of theistic belief.
It is also a different kind of argument from those that point to unexplained features of the world. Sceptics often criticise appeals to the god of the gaps, where God is placed wherever science has no answer yet. The ontological argument is not open to that charge, because it does not depend on any gap in knowledge, and nor does it say anything about whether theists genuinely maintain that everything came from nothing. Its strength and weakness are the same: it works, if it works, by logic alone.
Frequently Asked Questions
What is the ontological argument in simple terms?
It says that God, by definition, is the greatest possible being, and that a being which really exists is greater than one which is only imagined. So, the argument claims, God must really exist.
Who came up with the ontological argument?
Anselm of Canterbury set out the classic version in his Proslogion in 1078. Descartes, Leibniz, Plantinga and Gödel later produced their own versions.
What is the main objection to the ontological argument?
The best known is Kant’s: existence is not a property that can be part of a definition, so you cannot define something into existence. Gaunilo’s perfect island is the classic parody objection.
Do any philosophers still accept it?
A minority defend modern versions, especially Plantinga’s modal argument, usually as showing that belief in God is rational rather than as a conclusive proof. Most philosophers, believers and non-believers alike, think the argument does not succeed.
Summary
The ontological argument tries to prove God’s existence from the concept of God alone. Anselm’s classic version, Descartes’s version and the modern modal and logical versions of Plantinga and Gödel all aim to show that a greatest or maximally perfect being must exist. The classic objections, Gaunilo’s perfect island and Kant’s claim that existence is not a predicate, persuade most philosophers that the older versions fail, and critics argue that the modern versions assume what they set out to prove. The argument remains one of the most discussed in philosophy because of what it reveals about logic, possibility and existence. For how it fits with other proofs that have been offered, see proof of God.
Sources
- Stanford Encyclopedia of Philosophy, “Ontological Arguments”. plato.stanford.edu.
- Anselm of Canterbury, Proslogion (1078), with Gaunilo, On Behalf of the Fool.
- René Descartes, Meditations on First Philosophy (1641), Fifth Meditation.
- Immanuel Kant, Critique of Pure Reason, “The Impossibility of an Ontological Proof of the Existence of God”.
- Alvin Plantinga, The Nature of Necessity (1974).
- Kurt Gödel, “Ontological Proof”, in Collected Works, volume III (1995).