Philosophy › Faith and reason › Anselm of Canterbury
Anselm's ontological argument
1077–1078 (Proslogion)· Idea or argument· Reality, mind & God
Well documentedSolid records, texts or physical evidence.
Anselm's argument that God, defined as the greatest conceivable being, must exist, because existing is greater than not existing.
See it on the Philosophy map →The idea
Idea: God is 'that than which nothing greater can be conceived.' Reasoning: even the fool who denies God understands the phrase, so God exists at least in the mind. But a being that also exists in reality is greater than one only in the mind. So if God existed only in the mind, we could conceive something greater, a contradiction. Hence God exists. Main objection: the monk Gaunilo replied that the same logic would prove a perfect island exists.
Why it matters
It is the purest attempt to prove God a priori, from logic alone before any experience; Descartes revived it, Kant rejected it, and logicians still test new versions.
Where it comes from
Chapters 2–4 of Anselm's Proslogion (1077–78), Gaunilo's reply On Behalf of the Fool, and Anselm's answer survive and circulated together.
Still debated
Whether existence can count as a perfection at all, as Kant denied, and whether Plantinga's modal version (1974) succeeds, remain open.
Connections (4)
▸Anselm of Canterbury devised Anselm's ontological argumentWell documented
The connection
Anselm said he wanted one short argument that would prove God exists without help from other arguments. After long frustration, it came to him during night prayers: the definition of God as 'that than which nothing greater can be conceived' itself implies God exists.
When and how
Abbey of Bec, Normandy, 1077–78, in the Proslogion ('Address').
How we know
The Proslogion's preface describes the search, and his biographer Eadmer recounts the night the idea came.
Debates and open questions
Whether chapter 3's version, that God cannot even be conceived not to exist, is a separate and stronger argument is debated.
▸Does God exist? was answered a priori by Anselm's ontological argumentWell documented
The connection
Most arguments for God start from the world: motion, causes, design. The ontological argument answers the question a priori, from reason alone, before any observation. If it worked, denying God would be a contradiction, like a square circle.
When and how
Bec, Normandy, 1077–78; restated by Descartes (1641) and Leibniz, rejected by Kant (1781).
How we know
Anselm's text survives; its purely a priori structure is why philosophers class it apart from cosmological and design arguments.
Debates and open questions
Whether any a priori argument could prove that something exists is the deep dispute; Kant said existence is not a property.
▸Anselm's ontological argument was rejected by Immanuel KantWell documented
The connection
Anselm argued that God, 'that than which nothing greater can be conceived,' must exist, since existing is greater than not existing. Kant replied that 'being is obviously not a real predicate': existence adds nothing to a concept. A hundred real thalers (coins) contain no more than a hundred imagined ones, so no definition can prove existence.
When and how
Kant's Critique of Pure Reason (1781), in the Transcendental Dialectic, aimed mainly at Descartes's and Leibniz's versions; Kant also coined the name 'ontological proof.'
How we know
The Critique's section 'On the Impossibility of an Ontological Proof of God's Existence'; later logicians like Frege and Russell formalized his point.
Debates and open questions
Whether Kant refuted Anselm's own version, and whether modern modal versions, such as Alvin Plantinga's (1974) or Kurt Gödel's, escape his objection.
▸Anselm's ontological argument was revived by René DescartesWell documented
The connection
Anselm argued that God, 'that than which nothing greater can be conceived', must exist. Aquinas rejected the argument, but Descartes revived a version: existence belongs to the idea of a supremely perfect being as inseparably as a valley belongs to the idea of a mountain, so God exists.
When and how
Anselm's Proslogion (1077–78), at Bec in Normandy; Descartes' Fifth Meditation, published in Paris in 1641.
How we know
The Fifth Meditation states the argument, and the theologian Caterus, in the first set of Objections, compared it to Anselm's and raised Aquinas's criticism.
Debates and open questions
Whether Descartes' version is really Anselm's or a new argument, and whether either works, is debated; Kant later objected that existence is not a property.