  One is easily misled by the analogy between the ontological argument and a geometric demonstration, and by the language of “proof” in this passage and others like it. Descartes does not conceive of the ontological argument on the model of an Euclidean or axiomatic proof, in which theorems are derived from epistemically prior axioms and definitions.  On the contrary, he is drawing our attention to another method of establishing truths that informs our ordinary practices and is non-discursive. This method employs intuition or, what is the same for Descartes, clear and distinct perception. It consists in unveiling the contents of our clear and distinct ideas. The basis for this method is the rule for truth, which was previously established in the Fourth Meditation. According to the version of this rule invoked in the Fifth Meditation, whatever I clearly and distinctly perceive to be contained in the idea of something is true of that thing. So if I clearly and distinctly perceive that necessary existence pertains to the idea of a supremely perfect being, then such a being truly exists.