Kant called “ontological” the argument that, deducing God's existence from his definition, aims to turn the sentence “God exists” into a logical truth. Anselm of Canterbury first devised this kind of argument in the eleventh century; yet, Kant referred to Descartes’ formulation of it, which appears in the Discourse on Method, Meditations on First Philosophy, and Principles of Philosophy. In Descartes’ time, the argument that was intended to prove God's existence from his definition was classified among a priori arguments, that is, among arguments that assume as a premise the essence of the being whose properties they attempt to deduce. Kant called it “ontological” precisely because it assumes as a premise what a being is. The a priori proof is placed in different positions among the proofs of God's existence in Descartes’ works: it is the third proof in the Discourse and Meditations; it is the first one in the Principles.

Descartes often compares the a priori proof of God's existence to a mathematical one and, in the Fifth Meditation, adds a discussion of the essences of material things, including geometrical objects. There is a double affinity between the a priori proof of God's existence and a mathematical demonstration. First, the conclusion of each such argument cannot be denied without contradiction. Second, each argument takes as its major premise a definition that, according to Descartes, picks out a “true and immutable nature,” which is represented by an innate idea.

The innateness of the idea of God is crucial to Descartes’ attempts to validate the argument. Imposing their content on the mind, innate ideas cannot be modified by the will. This feature shows that innate ideas individuate essences that have a real being out of the mind, that is, what an innate idea represents is never “a mere nothing” (AT VII 65, CSM II 45). Thus, if the idea of God can be proved to be innate, it represents an essence that is fully real and not just mental. The standard charge against the proof – that it makes an illicit logical leap from the mental world of concepts to the extramental realm of things – would not be defensible, and the proof would correctly conclude from the existence contained in the idea of God to God's existence.