The Cartesian Circle

This is the single most famous objection to the Meditations, and it looks devastating. It was pressed hardest by Antoine Arnauld in the Fourth Set of Objections (published with the Meditations in 1641), and it has haunted Descartes ever since under the name the Cartesian Circle.

The objection, step by step. In Meditation III Descartes proves that God exists and is no deceiver. Why does he need that proof? Because only a non-deceiving God can guarantee the general rule that whatever I perceive clearly and distinctly is true. Until God is in place, an evil demon might be rigging even my clearest reasonings. So: clear and distinct perception is validated by the proof of God.

But now look at the proof of God itself. Every step of it — the causal principle, the idea of the infinite, the inference to a perfect cause — is accepted precisely because Descartes perceives it clearly and distinctly. So the proof of God relies on clear and distinct perception. Arnauld’s trap snaps shut:

Descartes’ reply — the standard reconstruction. The usual defence (drawn from his answer to Arnauld and from the Second Set of Replies) is that the objection trades on an ambiguity. There are two different things God is supposed to guarantee, and only one of them is used to prove God — so nothing is proved twice over.

First: a current, actually-attended clear and distinct perception. While I am presently grasping something clearly and distinctly — the cogito, or “2 + 3 = 5,” or a single step of the proof — I cannot doubt it. Descartes holds these are self-guaranteeing: so long as I hold the perception vividly in view, doubt is psychologically and rationally impossible. The light of clear perception compels assent while it shines. This needs no divine backing.

Second: the general rule that clear and distinct perception is always reliable — including for conclusions I once proved but am no longer attending to and now merely remember. Here a gap opens. When I only recall having proved something, the vivid perception is gone; a sceptic can now suggest that maybe my mind is built to go wrong even in its clearest moments. This is the doubt that God removes.

So the reply distinguishes two levels: (1) particular, present intuitions, which are self-certifying and do the proving; and (2) the general reliability of reason across time and memory, which is what the proof of God establishes. The proof uses only level-(1) perceptions — each luminous while held in view — and its conclusion secures level (2). No perception is both premise and product of the same guarantee. Hence, Descartes claims, no vicious circle.