The Ontological Argument

The Ontological Argument tries to prove God exists through pure logic alone - no evidence, no experience, no observation of the physical world required. First written by Anselm of Canterbury in 1078, it claims the very concept of God - defined as the greatest being you can conceive - logically requires that God exists in reality. Alvin Plantinga revived it with modal logic in the 20th century, and Kurt Godel built a separate version with mathematical logic. We score it 15/100 for soundness. The argument is ingenious but hits what most philosophers see as a fatal flaw: you cannot define something into existence.

Anselm’s Original Version

Writing in the Proslogion around 1078, Anselm proposed this reasoning:

1. God is defined as “a being than which nothing greater can be conceived.”
2. This concept exists at least in the mind (even an atheist understands the definition).
3. A being that exists both in the mind and in reality is greater than one that exists only in the mind.
4. If God existed only in the mind, we could imagine a greater being - one that also exists in reality.
5. But this contradicts God’s definition as the being than which nothing greater can be conceived.
6. Therefore, God must exist in reality.

The argument is remarkable because it pulls a real-world existence claim out of a definition. No telescope, no experiment, no observation required - only thought. That is what makes it both fascinating and deeply suspect.

Gaunilo’s Immediate Objection

The first and still most intuitive objection came from Gaunilo of Marmoutiers, a fellow monk and contemporary of Anselm. Gaunilo applied the same reasoning to a “Lost Island” - the greatest conceivable island. By Anselm’s logic:

1. The greatest conceivable island is one than which no greater island can be conceived.
2. An island that exists in reality is greater than one that exists only in the mind.
3. Therefore, the greatest conceivable island must exist in reality.

This is clearly absurd. We cannot conjure islands into existence with definitions. Gaunilo’s parody shows that Anselm’s logic, if valid, would prove the existence of the greatest conceivable anything - the greatest conceivable pizza, the greatest conceivable unicorn, the greatest conceivable villain. Since these are obviously false, something must be wrong with the structure.

Anselm replied that his argument applies only to a being with maximal greatness in every respect - not to objects within a category like islands, which are limited by their nature. Whether this reply works is debated, but most philosophers find Gaunilo’s objection convincing: if the structure produces absurd results when applied to other concepts, the structure itself is broken.

Kant’s Critique: Existence Is Not a Predicate

The most influential critique came from Immanuel Kant in the Critique of Pure Reason (1781). Kant argued that existence is not a real predicate - it does not add a property to a concept.

When we say “the cat is furry,” we add a property (furriness) to the concept of a cat. But when we say “the cat exists,” we do not add a property. We just claim the concept has a real-world instance. A hundred real dollars, Kant noted, contain no more conceptual content than a hundred imagined dollars. The concept is the same; the only difference is whether it actually exists.

This matters because Anselm’s argument treats existence as a property that makes a being “greater.” If existence is not a property at all, the third premise collapses. A God that exists and a God that does not exist are conceptually identical - the only difference is whether the concept has a real instance, and you cannot settle that by analyzing the concept.

Kant’s critique is still the standard response to the ontological argument. Modern logic largely backs his view: in predicate logic, existence is expressed by the existential quantifier, not as a predicate.

Descartes’ Version

Rene Descartes offered a separate version in his 1641 Meditations. He argued that existence belongs to God’s essence the way three angles belong to a triangle’s essence. Just as you cannot imagine a triangle without three angles, you cannot imagine a supremely perfect being without existence - since a being lacking existence would not be supremely perfect.

This faces the same Kantian objection. You can define a triangle as having three angles, but that does not make triangles exist. The definition says what a triangle would be if one existed; it does not bring triangles into being. Likewise, defining God as a being whose essence includes existence says what God would be if God existed, but does not show God actually exists.

Plantinga’s Modal Version

Alvin Plantinga rebuilt the argument using modal logic - the logic of possibility and necessity:

1. It is possible that a maximally great being exists (there is some possible world where such a being exists).
2. A maximally great being is one that has maximal excellence (omnipotence, omniscience, moral perfection) in every possible world.
3. If a maximally great being exists in some possible world, then by definition it exists in every possible world.
4. If it exists in every possible world, it exists in the actual world.
5. Therefore, a maximally great being exists.

The modal version shifts the debate to premise 1: is it really possible that a maximally great being exists? Plantinga himself admitted the argument is not a proof - it is logically valid (the conclusion follows from the premises), but it is not compelling, because one can reasonably deny premise 1. If a maximally great being is not possible (perhaps the concept is incoherent), the argument fails.

Critics note the argument can be reversed. If it is possible that a maximally great being does not exist - that there is some possible world with no such being - then by parallel modal reasoning, no maximally great being exists in any possible world, including ours. The argument only works if you already accept that maximal greatness is genuinely possible, which is exactly the point in dispute.

Godel’s Ontological Proof

Kurt Godel, one of the greatest logicians of the 20th century, built a formal ontological proof using higher-order modal logic. Godel defined God as a being with all “positive properties” and showed through a series of axioms and theorems that such a being necessarily exists.

Automated theorem provers have verified Godel’s proof as logically valid, confirming the conclusion follows from the axioms. But the axioms themselves are contestable. Whether Godel’s “positive properties” are coherently defined, whether they can all coexist in a single being, and whether the axioms describe real features of reality rather than quirks of the logical system are all open questions. The proof shows the power of formal logic but does not settle the metaphysical question.

Why Most Philosophers Reject It

The ontological argument is among the most rejected arguments in philosophy of religion. Even many theist philosophers - including Thomas Aquinas, who offered five of his own arguments for God - rejected it. The core objections:

Defining things into existence is not allowed. No matter how careful the definition, definitions only describe what something would be if it existed. They cannot show that something actually exists. This is a basic limit of conceptual analysis.

The argument proves too much. If the structure worked, parallel arguments could establish the existence of maximally great objects of any kind, leading to absurd conclusions (Gaunilo’s objection). The fact that the structure produces obviously false conclusions when applied elsewhere shows the structure is broken.

Existence and conceivability are different. We can conceive of many things that do not exist and cannot conceive of things that do. Conceivability is a feature of minds; existence is a feature of reality. Logical manipulation cannot bridge the gap.

Begging the question. Critics argue the key premises (especially in Plantinga’s version, the claim that a maximally great being is possible) essentially assume what they are trying to prove. Granting that maximal greatness is genuinely possible is already very close to granting God exists.

Historical Significance

Despite its low soundness score, the ontological argument is hugely important historically. It was one of the first attempts to apply rigorous logical reasoning to theology. It has produced nearly a thousand years of debate and pushed major progress in logic, metaphysics, and philosophy of language - especially around existence, necessity, and predication.

The argument also illustrates a key point: logical validity is not the same as soundness. An argument can be perfectly valid (the conclusion follows from the premises) while still being unsound (one or more premises are false). The ontological argument is a textbook case of an argument that may be logically valid in some forms but fails because its premises do not match how existence actually works.

Relationship to Other Arguments

The ontological argument is unusual among pro-God arguments because it works entirely in abstract logic, with no input from the physical world. That makes it both uniquely ambitious and uniquely vulnerable. Unlike the Fine-Tuning Argument, which rests on measured physical constants, or the Moral Argument, which appeals to shared moral intuitions, the ontological argument asks nothing of the world - and the world gives nothing back to support it.

Plantinga’s modal version connects to the wider theistic project: if you already find God’s existence plausible on other grounds (cosmological arguments, fine-tuning, moral arguments), the modal ontological argument gives a logical framework for moving from “possible” to “necessary.” On its own, though, it cannot do the heavy lifting.

Our Scoring

The soundness score of 15 is very low because the argument’s central move - treating existence as a property that can be established through definition - is rejected by most philosophers. Kant’s critique that existence is not a predicate, combined with Gaunilo’s parody showing the structure produces absurd results, gives strong grounds for rejection. Its persistence in philosophical discussion reflects its ingenuity and historical importance, not its persuasive power.

The Creator score of 85 and Higher Power score of 85 are among the highest on the site because if the argument worked, a maximally great being would necessarily exist as the ground of all reality. A being with maximal greatness would be the ultimate source of everything - fitting both Creator and Higher Power definitions extremely well. Maximal greatness, by definition, includes every positive attribute to the highest possible degree.

The Personal God score of 80 is also very high - the highest Personal God score on the site. A maximally great being, as defined, would have all perfections, including omniscience, omnipotence, and moral perfection. These match the Personal God concept of an omniscient, omnipotent, omnibenevolent being closely. The score is slightly lower than Creator and Higher Power only because “personal” involvement in human affairs is not strictly required by maximal greatness - a maximally great being might exist without intervening in the lives of individual humans.