Ontological Argument Anselm

Historical Context and Formulation

The ontological argument associated with Saint Anselm of Canterbury (1033–1109) is one of the most famous and debated arguments for the existence of God. It appears primarily in Proslogion (c. 1077–1078), where Anselm seeks a single, necessary proof for God’s existence based solely on reason, independent of sensory experience. The argument is thus a priori: it relies on conceptual analysis rather than empirical observation.

Anselm’s starting point is a conception of God shared, he claims, even by “the fool” who says in his heart that there is no God (an allusion to Psalm 14:1). Even the atheist, Anselm argues, must at least understand what the word “God” means. On this basis, Anselm defines God as “that than which nothing greater can be conceived” (id quo nihil maius cogitari potest).

From this definition, Anselm argues that God cannot exist merely as an idea in the mind. If God existed only in the understanding and not in reality, then a greater being—one existing in reality as well as in thought—could be conceived. This would violate the very definition of God as the greatest conceivable being. Hence, Anselm concludes, God must exist in reality as well as in the understanding.

Anselm also offers a related argument in Proslogion III, appealing to God’s necessary existence. If God could be thought not to exist, then a greater being—one whose non-existence is impossible—could be conceived. Therefore, God must exist necessarily, not contingently. While often treated together, this necessary-existence argument is conceptually distinct from the more commonly cited Proslogion II formulation.

Logical Structure of Anselm’s Argument

Anselm’s ontological argument can be reconstructed more formally as follows:

1. Definition of God: God is that than which nothing greater can be conceived.
2. Conceptual Existence: Even the “fool” understands the concept of God; thus God exists in the understanding (as an idea).
3. Greatness and Existence: A being that exists in reality as well as in the understanding is greater than a being that exists only in the understanding.
4. Reductio Assumption: Suppose God exists only in the understanding and not in reality.
5. Conceivability of a Greater Being: Under that supposition, one could conceive of a being exactly like God but also existing in reality; this being would be greater than the God who exists only in the understanding.
6. Contradiction with Definition: But then a being greater than “that than which nothing greater can be conceived” would be conceivable, which contradicts the original definition.

Conclusion: Therefore, the supposition that God exists only in the understanding must be false; God must exist in reality as well as in the understanding.

In Proslogion III, Anselm’s argument for necessary existence can be represented as:

1. God is that than which nothing greater can be conceived.
2. A being that cannot be conceived not to exist (i.e., a necessarily existing being) is greater than one that can be conceived not to exist.
3. If God could be conceived not to exist, then a greater being—one that cannot be conceived not to exist—would be conceivable.
4. This would contradict the definition of God as the greatest conceivable being.

Conclusion: Therefore, God’s non-existence is inconceivable; God exists necessarily.

These formulations are influential prototypes for later modal ontological arguments, which use the logic of possibility and necessity to argue from the possibility of a maximally great being to its necessary existence.

Major Criticisms and Responses

Anselm’s argument immediately provoked criticism, most notably from his contemporary Gaunilo of Marmoutiers, and has remained a focal point of debate in philosophy of religion.

Gaunilo’s “Perfect Island” Objection: Gaunilo proposed a parody argument in his work On Behalf of the Fool. By analogous reasoning, one might argue for the existence of a perfect island—the greatest conceivable island. If existence in reality is greater than existence in the mind alone, then the perfect island must exist in reality. Since such a conclusion seems absurd, Gaunilo contended that Anselm’s form of reasoning is flawed and can be used to prove the existence of any “perfect” object.

Anselm’s Reply: Anselm responded that his argument applies uniquely to a necessary being, not to contingent objects like islands. An island, by nature, can always be conceived as greater—more fertile, larger, more pleasant—so it does not admit of a maximum in the same way as the concept of “that than which nothing greater can be conceived.” Defenders of Anselm argue that the notion of God is uniquely apt for this kind of argument, whereas the “perfect island” is not.

Kant’s Critique of Existence as a Predicate: In the 18th century, Immanuel Kant offered one of the most influential criticisms. Kant argued that “existence is not a real predicate”—it does not add a property to a concept in the way that, for example, “red” or “tall” do. Saying that something exists does not enrich the concept; it only asserts that the concept is instantiated. Therefore, Kant claimed, one cannot move from a definition of God to God’s existence simply by treating existence as a “great-making” property.

From this perspective, Anselm’s key step—that a being existing in reality is “greater” than one existing only in the understanding—misconstrues the logical role of existence. Critics conclude that the argument illegitimately builds existence into the concept of God and then claims to derive existence from that very concept.

Modern Modal Versions and Evaluations: In the 20th century, philosophers such as Norman Malcolm, Charles Hartshorne, and Alvin Plantinga developed modal ontological arguments inspired by Anselm. These versions emphasize necessary existence and employ modal logic. A common structure is:

• If it is possible that a maximally great being exists, then it exists in some possible world.
• If it exists in some possible world as maximally great, it exists in every possible world.
• Therefore, it exists in the actual world.

Defenders regard these arguments as more rigorous descendants of Anselm’s second (Proslogion III) line of thought. Critics reply that such arguments often shift the burden to the coherence and possibility of the concept of a “maximally great being,” which is itself contested.

Overall Assessment: Philosophers generally regard Anselm’s ontological argument as valid in form under certain interpretations—its conclusion follows from its premises—but controversial in soundness, since its key premises about greatness, existence, and the nature of God are widely disputed. The argument has nonetheless played a central role in discussions of a priori theistic reasoning, the logic of existence, and the interface between conceptual analysis and metaphysical reality.