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Why did Immanuel Kant reject the ontological argument for God’s existence?

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Immanuel Kant German philosopher Immanuel Kant surrounded by Ouroboros, an emblematic serpent of ancient Egypt and Greece, engraving by John Chapman, published in London, 1812.

Immanuel Kant rejected the ontological argument for the existence of God primarily because he identified it as resting on a logical fallacy regarding the nature of existence.12

The argument, famously championed by thinkers like St. Anselm and René Descartes, posits that because God is the greatest conceivable being, he must possess the attribute of existence, as a being that exists is greater than one that does not. Kant countered this by arguing that existence is not a real predicate or attribute—it does not add to the concept of a thing in the way that properties like color or shape do.34

By treating existence as a property that can be added to a definition to make it "greater," proponents of the ontological argument commit a category error. For Kant, to say something exists is not to describe its qualities, but rather to posit the object in relation to our experience. Consequently, he maintained that one cannot define an object into existence through pure reason alone.125