Introduction

Considered in schematic form, the Cartesian a priori proof of the existence of God—defined by Kant as an ontological argument—contains two main stages: (a) the first derives knowledge of the reality of God's essence from the innate, clear, and distinct idea of God; while (b) the second derives knowledge of God's existence from knowledge of His essence.

In the second stage of the proof, the predicative proposition “God is existent” is deduced from the proposition “God is a supremely perfect being” and the supposition that existence is a perfection or a real predicate (a supposition explicitly admitted by Descartes in the Fifth Set of Replies), in the same way that the predicative proposition “God is omnipotent” could be deduced. But since the proposition “God is existent” means that God exists, the conclusion of the ontological argument is therefore an existential proposition.

Kant's refutation of Descartes' version of the ontological argument is based on the thesis that existence is not a real predicate. This thesis in turn leads to the argument that existential judgments cannot be assimilated to categorical judgments (predicative judgments), since categorical judgments represent an object with its properties, connecting different conceptual representations by means of the copula, while existential judgments signify that objects are actual instances of concepts.