What is Kant's famous objection that "existence is not a real predicate," and does it succeed in definitively destroying the ontological argument?

This question brings us into the most famous objection to the ontological argument in the history of philosophy. Immanuel Kant (1724-1804) in his "Critique of Pure Reason" (1781/1787) formulated an objection that seemed to many to definitively eliminate the ontological argument developed by Anselm and Descartes. However, the story did not end with Kant, but witnessed remarkable developments in the twentieth and twenty-first centuries.

Inadequate Responses to Avoid

From some defenders of the ontological argument, two common responses are insufficient:

"Kant did not understand the ontological argument." This is an inaccurate accusation. Kant was very precise in his understanding of Descartes' formulation, and even contemporary defenders of the ontological argument (like Plantinga) acknowledge that Kant's criticism succeeds against the classical formulations. The problem is not with Kant's understanding, but rather that the ontological argument evolved after him.

"Existence is of course a predicate! We say 'God exists' so existence is a predicate." This confuses grammar with logic. Kant does not deny that "exists" is a grammatical predicate, but denies that it is a real predicate in the technical sense that adds content to the concept.

From some naturalists, two responses are also insufficient:

"Kant definitively destroyed the ontological argument." This is historical oversimplification. The ontological argument returned forcefully in the twentieth century with new formulations (Hartshorne, Malcolm, Plantinga) that transcend Kant's criticism. Declaring its "death" is premature.

"Any argument that tries to prove existence from concepts is logically doomed." This is hasty generalization. Contemporary logical formulations (Modal Ontological Arguments) use modal logic in a very sophisticated way, and cannot be dismissed with philosophical generalizations.

Kant's Objection: Existence is Not a Real Predicate

Kant distinguished between two types of predicates:

Real Predicates: Add content to the concept. For example, when I say "the triangle has three sides" or "God is omnipotent," I am adding information about the nature of the thing.

Merely Logical Predicates: Do not add content, but affirm or deny the existence of the thing that has the specified attributes. "Existence" according to Kant is of this type.

Kant's famous example: "One hundred real thalers and one hundred imaginary thalers"

When I conceive of one hundred thalers (an old German currency), the concept in my mind has all the attributes: number, value, shape. When I discover that these one hundred thalers actually exist in my pocket, I do not add any new attribute to the concept—I only affirm that something in reality matches the concept.

Therefore, according to Kant, when Descartes says "the perfect being must exist or else it would not be perfect," he commits a logical error: he treats existence as if it were an attribute of perfection like power or knowledge, while existence is merely an affirmation that something matches the concept.

Why Did This Objection Seem Devastating?

The classical ontological argument (Anselm, Descartes) depends on the idea that existence is an attribute of perfection. If existence is not an attribute at all, then the argument collapses from its foundation. We cannot deduce the existence of something from analyzing its concept, because existence is not part of the concept.

This criticism seemed so powerful that the ontological argument almost disappeared from serious philosophical discussion for 150 years after Kant. Even defenders of God's existence (like Thomas Aquinas originally, and later Hume and Kierkegaard) rejected the ontological argument.

The Return of the Ontological Argument: Transcending Kant's Criticism

In the twentieth century, three important developments occurred:

1. Distinguishing Between Types of Ontological Arguments

Norman Malcolm (1960) distinguished between two arguments in Anselm:
- The first argument (in Proslogion chapter 2): treats existence as an attribute—and Kant's criticism succeeds against this
- The second argument (in Proslogion chapter 3): speaks of "necessary existence" as a mode of existence, not as an attribute

2. Development of Modal Logic

Charles Hartshorne (1962) and Alvin Plantinga (1974) developed formulations using modal logic. The argument does not say "existence is an attribute of perfection," but says:
- If God's existence is possible, then it is necessary
- God's existence is possible (not self-contradictory)
- Therefore: God exists necessarily

This formulation does not treat existence as a predicate, but speaks of modes of existence (possible/necessary/impossible).

3. Criticism of Kant's Criticism Itself

Contemporary philosophers (like Jerome Shaffer) questioned: Is Kant correct that existence is not a predicate? In contemporary logic and analytic metaphysics, the discussion about the nature of existence remains open. Some philosophers (Meinongians) accept that things can have properties without existing.

Does Kant's Criticism Succeed in Definitively Destroying the Argument?

The answer: It succeeds against classical formulations, but does not touch contemporary formulations.

What Kant's criticism succeeds in:
- Ends formulations that treat existence as an ordinary attribute
- Shows that mere analysis of concepts is insufficient to prove existence
- Forces defenders of the argument to be more logically precise

What Kant's criticism does not touch:
- Arguments that speak of modes of existence (necessary/possible) rather than existence as an attribute
- Arguments that use contemporary modal logic
- The discussion of whether "necessary existence" is a coherent concept

The Contemporary State of the Discussion

Today, the ontological argument has returned to serious philosophical discussion, but in a different form:

Contemporary Defenders: Plantinga, Robert Maydole, Joshua Rasmussen, Alexander Pruss. They use modal logic and avoid falling into Kant's trap.

Contemporary Opponents: Graham Oppy, Michael Tooley, J. L. Mackie. They do not content themselves with repeating Kant's criticism, but develop new criticism targeting the concept of "necessary existence" itself or reject the premise that God's existence is possible.

Deep Philosophical Point: Today's discussion has transcended the question "Is existence a predicate?" to deeper questions:
- What does metaphysical "necessity" mean?
- Can something exist necessarily?
- What is the relationship between conceptual possibility and metaphysical possibility?

Analytical Summary

Kant's criticism remains a great philosophical achievement that ended one stage and opened another. It did not "destroy" the ontological argument definitively, but forced it to evolve and deepen. Contemporary formulations transcend his criticism technically, but they face new challenges. Today's discussion is more technically sophisticated, and less confident in resolution from both sides.

For Advanced Reading

- Advanced level: The debate over S5 in modal logic and its relation to the ontological argument
- Kant, Critique of Pure Reason (A592/B620-A602/B630)
- Malcolm, "Anselm's Ontological Arguments" (1960)
- Plantinga, The Nature of Necessity (1974), ch. 10
- Oppy, Ontological Arguments and Belief in God (1995)
- "Ontological Arguments" page in the Stanford Encyclopedia of Philosophy