The proofs of God

By the end of Meditation II, the cogito has given Descartes one thing he cannot doubt: that he exists as a res cogitans (a thinking thing). But that’s a lonely island. Every other belief — that there’s a world, that his memory is trustworthy, that clear reasoning tracks truth — is still hostage to the possibility of a deceiving God or evil demon. To get off the island, Descartes must first prove that the author of his mind is no deceiver. So Meditation III sets out to prove that God exists.

The causal (or “trademark”) argument, Meditation III. Descartes surveys the ideas in his mind and asks where each could have come from. Most ideas he could plausibly have produced himself. But one idea stands out: his idea of God — an infinite, eternal, independent, supremely perfect being. The argument is that he could not possibly be the source of that idea. Something else must have caused it.

The engine of the argument is a scholastic distinction. The formal reality of an idea is just its reality as a mental event (all ideas are on a par here — each is a mode of thinking). But an idea also has objective reality (realitas objectiva): the reality it has in virtue of what it represents. An idea of a mere mode has little objective reality; an idea of a finite substance has more; and the idea of an infinite substance — God — has the maximum objective reality. Objective reality measures the “representational content” or greatness of what an idea depicts.

Now the causal principle: there must be at least as much reality in the cause as in the effect — something cannot come from nothing, and the greater cannot come from the less (a stone can’t be produced by something that lacks whatever it takes to make a stone). Descartes extends this to objective reality: the cause of an idea must have at least as much formal reality as the idea has objective reality. A representation of infinite perfection needs a cause that actually possesses that much reality.

Put the pieces together. Descartes is a finite substance. His idea of God has infinite objective reality. By the causal principle, only a being with infinite formal reality — an actually existing infinite, supremely perfect being, i.e. God — could have caused this idea in him. He could no more have manufactured it than a finite thing could manufacture infinity. Therefore God exists, and has, so to speak, stamped this idea on His creature “like the mark of the craftsman stamped on his work” (Meditation III). Hence the nickname: the trademark argument.

The ontological argument, Meditation V. Having proved God once via causation, Descartes offers a second, entirely different proof — this time from God’s essence alone, with no appeal to His effects. He notes that when he examines the idea of a supremely perfect being, he finds that existence is inseparable from it, exactly as certain properties are inseparable from a geometrical figure.

The key premise is that existence is a perfection. A being that has every perfection including existence is greater than one that has every perfection except existence. Since God is by definition the being with all perfections, existence cannot be missing from Him. Therefore God — the supremely perfect being — necessarily exists. Unlike everything else, whose essence leaves it an open question whether it exists, God’s essence entails His existence.

The payoff: God is no deceiver. Both proofs converge on a being that is supremely perfect. Deception, Descartes argues, always springs from some defect — malice, weakness, or want — and a defect is an imperfection. A supremely perfect being can have no imperfection. Therefore God is not a deceiver (non fallax). And that is the conclusion Descartes actually needs: it licenses him to trust his clear and distinct perceptions and to start rebuilding the world beyond the cogito.