The Cartesian version of the ontological argument claims that God's existence follows necessarily from the very idea of God as a supremely perfect being. The argument's inferential structure moves from the concept of perfection to necessary existence: (1) I have a clear and distinct idea of God as a supremely perfect being; (2) a supremely perfect being possesses all perfections; (3) existence is a perfection; (4) therefore, God necessarily exists. Unlike Anselm's formulation, which focuses on conceivability and greatness, Descartes grounds his argument in the doctrine of clear and distinct ideas and the notion of perfection as including existence.
Descartes developed this argument primarily in the Fifth Meditation of his Meditations on First Philosophy (1641) and defended it in his Replies to Objections. The argument also appears in his Principles of Philosophy (1644) and Discourse on Method (1637). Key defenders include Nicolas Malebranche in The Search After Truth (1674-75), who integrated it with occasionalism, and Gottfried Wilhelm Leibniz, who modified it by adding a proof of God's possibility. In the contemporary period, philosophers like Norman Malcolm in "Anselm's Ontological Arguments" (1960) and Charles Hartshorne in The Logic of Perfection (1962) have defended versions influenced by Cartesian insights about necessary existence.
The strongest objection comes from Kant's critique in the Critique of Pure Reason (1781/1787) that existence is not a real predicate or perfection that can be included in a concept. Pierre Gassendi earlier raised similar concerns in the Fifth Objections (1641), arguing that existence adds nothing to the concept of a thing. David Hume in Dialogues Concerning Natural Religion (1779) contended that nothing can be demonstrated to exist a priori. Defenders respond that Kant misunderstands the argument: it doesn't treat existence as an ordinary predicate but as a unique mode of being for a necessary being. They argue that for God alone, essence entails existence, making the Kantian objection inapplicable to infinite perfection.
The Cartesian version differs from the Anselmian argument by focusing on perfection rather than greatness, and by explicitly invoking clear and distinct perception. Unlike Leibniz's version, it doesn't require a separate proof of God's possibility. It differs from modal versions like Plantinga's by remaining within classical metaphysics rather than possible worlds semantics. Unlike Gödel's mathematical proof, it relies on intuitive notions of perfection rather than formal logical axioms.