In M5, Descartes offers the ontological argument for God’s existence. Unlike his cosmological proof in M3, this argument claims that existence belongs to God’s infinite perfection as necessarily as geometrical properties belong to shapes. Descartes begins with the general epistemic rule that whatever he perceives clearly and distinctly must be true, and claims that existence is contained in the essence of God.
Descartes first turns to the idea of God, laying the foremost premise: that I perceive the idea of God clearly and distinctly. This is due to two reasons. One, the idea of God is innate and inherent, “is one which I find within me just as surely as the idea of any shape or number.” (7:65) Two, proof of God in previous meditations prove that a God exists and cannot be a deceiver, so the clear and distinct perceptions God enables in Descartes’ mind are reliable.
He subsequently engages a first objection: “find it easy to persuade myself that existence can also be separated from the essence.” (7:66) He makes analogy of how “the idea of a mountain can be separated from the idea of a valley,” (7:66), suggesting how perfection and existence are similarly inseparable characteristics from the idea of God.
This is met with a second objection. Extending on the above analogy, one can reasonable argue that thinking about a mountain with a valley does not necessarily mean that an actual mountain exists, for “my thought does not impose any necessity on things” (7:66); similarly, even if the concept of God necessarily involves existence, it doesn’t necessarily mean that God exists. This argument fails, however, because its essence contains relationship, but not existence, whereas in the God argument, essence contains existence as an essential property. It is incorrect to assume that the thought is imposed, because “it is the necessity of the thing itself, namely the existence of God, which determines my thinking in this respect.” (7:67) Even though one “may imagine a winged horse even though no horse has wings,” one is not free to think of God without existence.
After addressing the counterarguments, Descartes draws on the earlier model of mathematical truth. If one clearly and distinctly perceive that the angle-sum belongs to triangle-essence, then that is a true feature of triangles. The truth does not depend on whether triangles exist physically; it depends on the objective structure of the essence. Descartes insists that clear and distinct intellectual perception is not subjective fantasy but a mode of accessing genuine essences. We discover, rather than invent, these natures. Therefore, if I clearly and distinctly perceive existence as belonging to God’s essence, that too must be true.
Descartes entertains a final objection questioning the truthfulness of perception. Perhaps Descartes is simply mistaken in thinking he perceives God’s existence clearly and distinctly. Error arises only when the will outruns the intellect, when we affirm without clarity, as already proven in M4. But in the present case, he claims to be attending directly to the idea of God, grasping it in full clarity, and seeing existence contained in it. Moreover, because God exists and is not a deceiver, the faculty of clear and distinct perception is trustworthy. Descartes thus treats this objection as a retreat into general skepticism that his system has already addressed.
In conclusion, Descartes’ ontological argument is simple in form but ambitious in scope. It rests on two central commitments. First, essences are accessible to reason in a way that yields necessary truths, as geometry demonstrates. Second, the idea of God is one such essence, and when reason inspects it, it finds existence inseparable from perfection. He arrives at the final claim that God’s existence is as self-evident to reason as the truths of mathematics, and that the very attempt to think God without existence collapses into contradiction.