The ontological argument for God's existence, first formulated by Anselm of Canterbury in the Proslogion (1077-78) and revived in modal logic form by Alvin Plantinga in The Nature of Necessity (1974), is one of the most discussed arguments in the history of philosophy. Its claim - that the very concept of God as the greatest conceivable being entails God's existence - has attracted both powerful objections and sophisticated defenses for nine centuries. Its biblical grounding is less obvious than that of the cosmological or design arguments, but Anselm derived his key formulation from Psalm 14:1's 'fool who says in his heart there is no God' and from the account of divine being in Exodus 3:14, making the ontological argument an exercise in philosophical reflection on the biblical concept of God.
The Thinkers and Their Works
Anselm of Canterbury (1033-1109), Archbishop of Canterbury, philosopher, and theologian, developed the argument in Proslogion, a text organized as an extended meditation addressed to God - not an academic argument but a prayer of philosophical reflection. The context is crucial: Anselm is not constructing a proof to persuade an atheist but exploring what it means to say that God exists, starting from the address of faith ('I believe, that I may understand,' his adaptation of Augustine's 'faith seeking understanding').
Rene Descartes revived the argument in the Fifth Meditation (1641), arguing that the concept of God as supremely perfect necessarily includes existence, just as the concept of a triangle necessarily includes having angles summing to 180 degrees. Leibniz added the premise that God is a possible being (internally consistent) to shore up the argument.
Immanuel Kant's Critique of Pure Reason (1781) offered what has been the standard objection: 'existence is not a predicate.' You cannot derive existence from a concept, because adding 'exists' to a concept does not describe any property of the thing but merely asserts that there is something answering to the concept. Kant's objection dominated philosophy of religion for two centuries.
Alvin Plantinga's modal version (The Nature of Necessity, 1974; God, Freedom, and Evil, 1974) argues using possible worlds semantics: a maximally great being - one that has maximal greatness in all possible worlds - exists in some possible world; maximal greatness includes necessary existence; therefore a maximally great being exists in all possible worlds, including the actual world. The argument is valid given its premises, and Plantinga acknowledges that the key question is whether the premise 'a maximally great being possibly exists' is true.
Biblical Texts Engaged
Psalm 14:1 - 'The fool says in his heart, "There is no God"' - is Anselm's starting text. The Psalm characterizes the denial of God's existence as folly rather than reasonable skepticism, and Anselm takes this as an invitation to philosophical inquiry: why is atheism foolish? What is it about the concept of God that makes its denial not merely false but epistemically disordered? His argument is an attempt to answer this question.
Exodus 3:14 - 'God said to Moses, "I AM WHO I AM."' - grounds Anselm's understanding of divine being as necessary being. God's self-identification with pure existence ('I am what I am,' or 'I will be what I will be') has been interpreted in the tradition of negative theology as the claim that God does not have existence as a property but is existence itself - or, in Anselm's terms, that God cannot be conceived not to exist. Aquinas, following Maimonides, argued that God's essence and existence are identical, and the ontological argument can be read as an attempt to make this theological claim philosophically rigorous.
Isaiah 40:25 - 'To whom then will you compare me, that I should be like him? says the Holy One' - grounds the 'maximality' premise: God is by definition the being who has no superior and to whom nothing can be compared. This biblical concept of divine incomparability is the theological starting point from which the philosophical concept of 'that than which nothing greater can be conceived' is derived.
Core Argument
Anselm's original argument (Proslogion II): God is that than which nothing greater can be conceived. Even the fool who denies God's existence understands the concept of that than which nothing greater can be conceived. A being that exists in reality is greater than one that exists only in the mind. Therefore, if God existed only in the mind, something greater - a God that exists in reality - could be conceived. But this contradicts the assumption that God is that than which nothing greater can be conceived. Therefore God must exist in reality.
Proslogion III contains a stronger form: God cannot be conceived not to exist. Necessary existence is greater than contingent existence; therefore God, as that than which nothing greater can be conceived, exists necessarily - God's non-existence is not merely false but inconceivable.
Intellectual Context
Anselm wrote in the Augustinian tradition of 'faith seeking understanding' - the conviction that theological reflection on the content of faith can yield genuine philosophical knowledge. His argument is not a proof addressed to unbelievers but a philosophical exploration addressed to God himself, attempting to understand why the faith already held by the believer is rationally well-grounded.
Descartes's version was part of his project of building certain knowledge from the cogito outward, using God's existence as a guarantee of the reliability of clear and distinct ideas. Kant's rejection of the ontological argument was part of his larger project of limiting knowledge to possible experience and criticizing the 'dialectical illusions' of traditional metaphysics.
Reception and Critique
Gaunilo, a contemporary of Anselm, offered the first objection: the same form of argument would prove the existence of a perfect island, since a perfect island that exists in reality is greater than one that exists only in the mind. Anselm's response - that only a being that possesses necessary existence, not merely maximal greatness, is proven by the argument - anticipates Plantinga's modal version.
David Hume argued that no existential claim can be proved a priori - that 'whatever we can conceive as existent, we can also conceive as non-existent.' Kant's 'existence is not a predicate' is the canonical version of this objection. Philosophers sympathetic to the argument (Norman Malcolm, Charles Hartshorne, Plantinga) argue that necessary existence - the kind of existence at issue in the ontological argument - is a genuine property, even if contingent existence is not.
Legacy
The ontological argument has generated more sustained philosophical discussion than almost any other argument in philosophy. Its importance lies not primarily in its persuasive force (most philosophers find it unconvincing) but in what it reveals about the concept of God and the relationship between conceptual analysis and existential claims. It is the clearest example of the philosophical examination of the biblical concept of God - the attempt to make explicit what is implicit in the claim that God is the ultimate being.
Key Passages
'And so, Lord, do thou, who dost give understanding to faith, give me, so far as thou knowest it to be profitable, to understand that thou art as we believe; and that thou art that which we believe... Even the fool, then, is forced to agree that something-than-which-nothing-greater-can-be-thought exists in the mind, since he understands this when he hears it, and whatever is understood, exists in the mind.' (Proslogion II, trans. Deane)
Contemporary Relevance
The ontological argument remains active in contemporary analytic philosophy of religion. Graham Oppy's Arguing about Gods (2006) provides the most comprehensive recent critical analysis. Plantinga's modal version has generated a cottage industry of responses, defenses, and modifications. The argument also intersects with the metaphysics of modality: questions about what possible worlds exist, what makes a being necessarily existent, and what 'maximal greatness' requires are live issues in contemporary metaphysics that the ontological argument helps to sharpen. Anselm's grounding of the argument in the biblical concept of God - the God who says 'I am who I am' - ensures that the philosophical debate remains connected to its theological source.