The Ontological Argument

The Ontological Argument attempts to prove God’s existence through pure logic, with no appeal to evidence, experience, or observation of the physical world. First formulated by Anselm of Canterbury in 1078, the argument claims that the very concept of God - defined as the greatest conceivable being - logically requires that God exists in reality. Alvin Plantinga revived it using modal logic in the 20th century, and Kurt Godel developed an independent version using mathematical logic. With a soundness score of 15/100, the argument is ingenious but faces what most philosophers consider 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 understanding (even an atheist understands the definition).
3. A being that exists both in the understanding and in reality is greater than one that exists only in the understanding.
4. If God existed only in the understanding, we could conceive of 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 derives a real-world existence claim from a definition. No telescope, no experiment, no observation required - only thought. This is precisely what makes it both fascinating and deeply suspect.

Gaunilo’s Immediate Objection

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

1. The greatest conceivable island is defined as an island 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 obviously absurd. We cannot conjure islands into existence through definitions. Gaunilo’s parody demonstrates that Anselm’s logical structure, 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 conclusions are clearly false, something must be wrong with the argument’s structure.

Anselm responded that his argument applies only to a being with maximal greatness in every respect - not to objects within a category like islands, which are inherently limited. Whether this response succeeds is debated, but most philosophers find Gaunilo’s objection compelling: if the logical structure generates absurd results when applied to other concepts, the structure itself is flawed.

Kant’s Critique: Existence Is Not a Predicate

The most influential philosophical 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 assert that the concept of a furry cat has an instance in reality. A hundred real dollars, Kant observed, contain no more conceptual content than a hundred imagined dollars. The concept is identical; the only difference is whether it is instantiated.

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 argument’s third premise collapses. A God that exists and a God that does not exist are conceptually identical - the only difference is whether the concept is instantiated, and that cannot be settled by analyzing the concept.

Kant’s critique remains the dominant philosophical response to the ontological argument. Modern logic largely supports his position: in predicate logic, existence is expressed by the existential quantifier, not as a predicate applied to individuals.

Descartes’ Version

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

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

Plantinga’s Modal Version

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

1. It is possible that a maximally great being exists (there is a possible world in which such a being exists).
2. A maximally great being is one that possesses 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 genuinely possible that a maximally great being exists? Plantinga himself acknowledged that the argument is not a proof - it is logically valid, meaning 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 because the concept is incoherent), the argument fails.

Critics point out that 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 works only if one already accepts that maximal greatness is genuinely possible, which is precisely the point in dispute.

Godel’s Ontological Proof

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

Godel’s proof has been verified as logically valid by automated theorem provers, confirming that the conclusion follows from the axioms. However, the axioms themselves are contestable. Whether Godel’s “positive properties” are coherently defined, whether they can all be instantiated in a single being, and whether the axioms correspond to genuine features of reality rather than artifacts of the logical system are all open questions. The proof demonstrates 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 the philosophy of religion. Even many theist philosophers - including Thomas Aquinas, who offered five of his own arguments for God - rejected it. The core objections are:

Defining things into existence is illegitimate. No matter how carefully one constructs a definition, definitions describe what something would be if it existed. They cannot establish that something actually exists. This is a fundamental limitation of conceptual analysis.

The argument proves too much. If the logical structure were valid, 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 generates obviously false conclusions when applied elsewhere indicates the structure is flawed.

Existence and conceivability are different. We can conceive of many things that do not exist and cannot conceive of things that do exist. Conceivability is a feature of minds; existence is a feature of reality. The gap between these cannot be bridged by logical manipulation.

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

Historical Significance

Despite its low soundness score, the ontological argument holds immense historical importance in philosophy. It was one of the first attempts to apply rigorous logical reasoning to theological questions. It has generated nearly a thousand years of philosophical debate and has contributed to major developments in logic, metaphysics, and the philosophy of language - particularly regarding the nature of existence, necessity, and predication.

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

Relationship to Other Arguments

The ontological argument is unusual among pro-God arguments because it operates entirely in the realm of abstract logic, requiring no empirical input. This 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 consequently, the world provides no support for it.

Plantinga’s modal version connects to the broader theistic project: if one already finds God’s existence plausible on other grounds (cosmological arguments, fine-tuning, moral arguments), the modal ontological argument provides a logical framework for moving from “possible” to “necessary.” In isolation, however, 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 considered illegitimate by the vast majority of philosophers. Kant’s critique that existence is not a predicate, combined with Gaunilo’s parody demonstrating that the logical structure generates absurd results, gives strong grounds for rejecting the argument. 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 were sound, a maximally great being would necessarily exist as the ground of all reality. A being possessing maximal greatness would be the ultimate source and sustainer of everything that exists - fitting both the Creator and Higher Power definitions exceptionally well. Maximal greatness, by definition, encompasses 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 for any argument. This is because a maximally great being, as defined in the argument, would possess all perfections, including omniscience, omnipotence, and moral perfection. These attributes map closely onto the Personal God concept of an omniscient, omnipotent, omnibenevolent being. The score is slightly lower than Creator and Higher Power only because the concept of “personal” involvement in human affairs is not strictly entailed by maximal greatness - a maximally great being might be maximally great without intervening in the specific concerns of individual humans.