Meditation III-V - God and the Guarantee of Reason

At the end of Meditation II, Descartes knows exactly one thing with certainty: he exists as a thinking thing. Everything else — the external world, the reliability of his own reasoning, the validity of mathematics — is still in doubt. The Evil Demon hypothesis has not been defeated. It has only been held at arm’s length by the cogito.

This creates a serious problem. Descartes’ entire project depends on using reason to rebuild knowledge from the ground up. But if a powerful deceiver might be corrupting his reasoning faculty, then every conclusion he draws — including the ones he draws in Meditations III through VI — might be systematically false. He needs something that rules out the possibility of a deceiving demon before he can trust his own mind.

That something is God. If Descartes can prove that a perfectly good, all-powerful God exists and is the source of his cognitive faculties, then those faculties cannot be designed to deceive him. A good God would not create a rational creature whose clearest and most careful reasoning was unreliable. God, in other words, is the guarantee that clear and distinct perception tracks the truth. Without that guarantee, the whole project of the Meditations collapses.

Descartes’ first argument for God’s existence begins from a fact he is certain of: he has an idea of God in his mind. This is an idea of a being that is infinite, eternal, omniscient, omnipotent, and supremely good — the most perfect being conceivable.

Now Descartes applies a causal principle: a cause must have at least as much reality as its effect. You cannot get more out of a cause than was put in. A fire heats a room because the fire has more heat than the air; a sculptor produces a statue because the sculptor has more reality (as an existing agent) than the stone arrangement that results. This principle, Descartes thinks, is something he perceives clearly and distinctly, and so he can rely on it.

Applied to his idea of God: that idea has a specific content — infinite perfection. What could cause an idea of infinite perfection? Not Descartes himself, who is finite and imperfect. Not the external world, which is also finite. Only something with infinite perfection could produce an idea of infinite perfection. Therefore, a being of infinite perfection — God — must exist as the cause of the idea in his mind.

This is a difficult argument, and it has attracted fierce criticism. The causal principle Descartes uses — that a cause must contain as much reality as its effect — is not obviously true. Can you think of a counterexample? A finite being imagining infinite space? A child conceiving of a number larger than any she has ever encountered? Whether those examples defeat the argument is worth thinking through carefully.

Meditation IV: the problem of error

Having argued that God exists and is not a deceiver, Descartes faces an immediate objection: if God is not a deceiver, why do humans make errors? We form false beliefs constantly. How is that compatible with a perfectly good creator?

Descartes’ answer distinguishes between the intellect and the will. The intellect presents ideas; the will judges them to be true or false. God gave Descartes a finite intellect — one that does not perceive everything clearly and distinctly — but a will that is unlimited in scope. Error occurs when Descartes extends his will beyond his intellect: when he affirms or denies something that he does not yet perceive clearly and distinctly. The fault lies not in God’s design but in the misuse of his own freedom. If he restricts his judgments to what he perceives clearly and distinctly, he will never err.

Meditation V: the ontological argument

Meditation V offers a second, independent proof of God’s existence — one that does not depend on causal reasoning at all. This is a version of the ontological argument, originally formulated by Anselm of Canterbury in the eleventh century.

The argument runs roughly as follows. God is defined as the most perfect being conceivable. Existence is a perfection — a being that exists is greater than an otherwise identical being that does not. Therefore, if we conceive of the most perfect being, that being must exist: a God who lacked existence would not be the most perfect being conceivable, and would contradict our own definition.

Descartes frames this in terms of essence and existence. Just as the essence of a triangle includes having interior angles that sum to 180 degrees — you cannot coherently conceive of a triangle without that property — the essence of God includes existence. God’s existence follows from his nature the way mathematical truths follow from definitions.

One of the most serious objections to Descartes’ entire project is known as the Cartesian Circle, first pressed by the theologian Antoine Arnauld in the set of objections Descartes published alongside the Meditations. The problem is this:

In other words, the argument seems to go in a circle: reason validates God, God validates reason. Each step depends on the other. This is a textbook case of circular reasoning — one of the most fundamental logical errors.

Descartes scholars have offered various responses to the Circle. One influential reply is that Descartes only needs God to validate remembered clear and distinct perceptions — things he is not currently attending to directly. While he is actively perceiving something clearly and distinctly, he does not need God’s guarantee; the perception is self-validating in the moment. God’s role is to assure him that those past perceptions were also reliable. Whether this reply fully escapes the Circle is debated. It is worth deciding for yourself.

With God established as the guarantor of reason, Descartes is finally in a position to do what Meditation I stripped away: affirm the existence of the external world, and settle the question of what the relationship between mind and body actually is.

Meditation VI is in many ways the payoff of the entire project. But it is also where the most difficult and unresolved questions surface. The mind/body relationship Descartes defends — substance dualism — raises an immediate problem he cannot fully solve: if mind and body are genuinely distinct substances, how do they interact? How does a thought cause an arm to move?