Ontological Argument

The problem it answers

Can the existence of God be demonstrated from the concept of God alone, without appeal to any empirical observation of the world? The other major arguments for God in the Western tradition — the cosmological argument, the teleological argument, the moral argument — all begin from some feature of the observed world (motion, contingency, design, the moral law) and infer to God as the cause. The ontological argument is the family of arguments that does not make this move. It begins from the concept of God and tries to show that the concept itself entails God's existence.

The claim is audacious. If it succeeded, it would demonstrate God's existence with the certainty of mathematics, since the conclusion would follow from the concept by analysis alone. Most philosophers since Kant have judged that no version of the argument actually succeeds, but the argument has continued to attract serious philosophers in every generation, and the contemporary modal version developed by Alvin Plantinga in the 1970s has produced an active research program.

The core claim

The core claim has three parts:

Existence can be analyzed from a concept. Some predicates belong to a concept by analysis — triangles have three sides is analytic; the predicate three-sided is part of what triangle means. The ontological argument claims that exists belongs to the concept of God in the same way: God's existence is part of what God means.

The concept of God is the concept of a being than which no greater can be conceived. This is Anselm's formula. God is by definition the maximally great being; any being that could be conceived to be greater would not be God.

A being that exists is greater than a being that does not exist. From this third premise the argument follows: if God exists only in the understanding (as a concept) and not in reality, then we can conceive of a greater being (one that also exists in reality), which contradicts the definition of God as that than which no greater can be conceived.

History in one paragraph

The argument was first formulated by Anselm of Canterbury in chapter 2 of the Proslogion (1078). It attracted immediate critique from Gaunilo, a Benedictine monk who wrote On Behalf of the Fool arguing that the same reasoning would prove the existence of a perfect island. Anselm replied that the argument applies only to God because only God is by definition maximally great. Thomas Aquinas rejected the argument in the Summa Theologiae (Ia q.2 a.1) on the grounds that the concept of God is not naturally available to us in the way the argument requires. Descartes revived the argument in the Meditations on First Philosophy (1641). Leibniz sharpened the Cartesian version. Kant gave the canonical critique in the Critique of Pure Reason (1781), arguing that existence is not a predicate that can be analyzed from a concept. Hegel defended the argument against Kant. The argument was widely treated as defeated through the nineteenth and early twentieth centuries until Norman Malcolm (1960) and Charles Hartshorne (1962) developed modal versions exploiting necessary existence, followed by Alvin Plantinga's modal version in The Nature of Necessity (1974). Plantinga's argument continues to organize contemporary work.

Anselm's argument

The original argument in the Proslogion runs as follows. Even the fool of Psalm 14 (The fool has said in his heart, There is no God) understands what is meant by God when he hears the word. So God exists at least in the fool's understanding. But if God existed only in the understanding and not in reality, we could conceive of a greater being — namely, one that exists both in the understanding and in reality. This would be greater than the God who exists only in the understanding. But by definition God is that than which no greater can be conceived; so God cannot exist only in the understanding; so God must exist in reality.

The argument turns on the comparison: a being that exists in reality is greater than a being that exists only in the understanding. This premise has been the most-contested element of the argument from Gaunilo onward.

Descartes's argument

Descartes's version in Meditation V proceeds differently. The concept of God is the concept of a supremely perfect being. Existence is a perfection. So existence belongs to the concept of God by analysis, in the same way that having three sides belongs to the concept of a triangle. Therefore God exists.

The Cartesian version makes explicit what the Anselmian version had presupposed: that existence is the kind of property that can belong to a concept by analysis. Kant's critique would target precisely this assumption.

Kant's critique

Kant's argument in the Critique of Pure Reason (A592–602/B620–30) is the canonical critique. The central claim: existence is not a real predicate. When we say the triangle is three-sided, we add information to the concept triangle; when we say the triangle exists, we do not add any information to the concept of the triangle but assert that something falls under the concept. Existence is a logical predicate (in the sense that it can appear in subject-predicate sentences) but not a real predicate (in the sense of adding content to the concept).

The consequence: existence cannot belong to the concept of God by analysis in the way the ontological argument requires. The Kantian critique has been the dominant philosophical objection for over two centuries.

The modal version

Norman Malcolm's 1960 paper Anselm's Ontological Arguments and Charles Hartshorne's The Logic of Perfection (1962) revived the argument in a modal framework. Alvin Plantinga's version in The Nature of Necessity (1974) is the canonical modern statement.

The Plantinga argument runs roughly as follows. Maximal greatness is the property of being maximally excellent in every possible world. A being with maximal greatness exists necessarily — it exists in every possible world. Now consider the property possibly exemplified maximal greatness: the property of being such that some possible world contains a maximally great being. If this property is exemplified, then by the structure of modal logic (specifically S5), the maximally great being exists in every possible world, including the actual world.

The argument shifts the locus of dispute. The Kantian critique does not apply directly because the argument does not analyze existence as a predicate of the concept of God. The dispute becomes whether maximal greatness is possibly exemplified — whether the concept is coherent in the sense required for there to be a possible world that contains an instance.

Critics (J. L. Mackie, Graham Oppy, William Rowe) argue that there is a parallel argument from possibly exemplified maximal anti-greatness that would prove the existence of an evil necessary being; since the two arguments are symmetric, at most one premise can be true. Defenders (Plantinga himself, William Lane Craig, Robert Maydole) argue that the modal version still gives the believer a coherent rational framework even if it does not amount to a public demonstration. Plantinga's own assessment is that the argument is rationally acceptable though not demonstrative.

Common confusions

The ontological argument is not the cosmological argument. The cosmological argument proceeds from features of the observed world; the ontological argument proceeds from the concept of God alone.

Existence is not always a predicate, but sometimes is. Some philosophers have pressed back on the Kantian analysis by arguing that there are contexts in which existence functions as a real predicate.

Gaunilo's perfect island objection is not decisive. Anselm's reply distinguishes the perfect island case (in which the concept of island does not entail maximal greatness across every dimension) from the God case (in which the concept of God is the concept of maximal greatness without restriction).

Live debates

Whether the modal version succeeds. The Plantinga argument and its critics continue to anchor contemporary work.

Whether existence is a predicate. Contemporary defenders of the pre-Kantian arguments (especially in the analytic Thomist tradition through Barry Miller, From Existence to God, 1992) have argued that the Kantian analysis is wrong.

The relation between the ontological argument and the broader project of natural theology. Even philosophers who reject the ontological argument may accept the cosmological or teleological argument, and vice versa.

Contemporary engagement

The contemporary literature is substantial. Alvin Plantinga's The Nature of Necessity (1974) and God, Freedom, and Evil (1974) gave the canonical modern statements. Graham Oppy's Ontological Arguments and Belief in God (1995) is the major systematic critical study. The Cambridge Companion to the Ontological Argument (Szatkowski, ed., 2018) anchors the introductory literature. Major recent work includes Yujin Nagasawa's Maximal God (2017), Tyron Goldschmidt's Ontological Arguments (2020), and Brian Leftow's work on the metaphysics of theistic perfections. The Philosophy of Religion journal and the International Journal for Philosophy of Religion document continuing scholarship.

Further reading

• Anselm of Canterbury — the originator of the argument
• Descartes — the early modern reviver of the argument
• Kant — the canonical critic
• Cosmological Argument — the other major argument for God's existence
• Problem of Evil — the major argument against God's existence
• God — the broader concept the argument concerns

This is a Pillar concept. Satellite concepts should link here as their parent.