Clear and distinct ideas

Having secured “I exist,” Descartes does something ambitious. He inspects the cogito to find out why it is certain, hoping the reason will generalise. He notices: in this first certainty there is nothing but “a clear and distinct perception of what I am asserting.” So he proposes a general rule: “whatever I perceive very clearly and distinctly is true” (Meditation III). The cogito becomes the sample from which the criterion of truth is drawn.

The two terms are technical, and they differ. A perception is clear (clara) when it is present and accessible to an attentive mind — vivid, unmistakably there, the way sharp pain is undeniable while you feel it. A perception is distinct (distincta) when it is also so sharply separated from everything else that it contains nothing but what is clear — no blur bleeding in from neighbouring ideas (Principles I.45). Every distinct perception is clear, but not every clear one is distinct.

Here is where the ground trembles. The criterion works while I am actually attending to something clear and distinct — mid-thought, I cannot doubt it. But memory is weaker. When I look away from a proof and merely recall having proved it, the old poison returns: the demon hypothesis is still live, and I can wonder whether my mind was rigged to feel certain of falsehoods. “How do I know I am not so constituted as to go wrong even in those matters which I think I see most clearly?” (Meditation III).

So the criterion, on its own, cannot stand fully secured while an all-powerful deceiver remains possible. To make clear and distinct perception reliable — trustworthy even in memory, even about the faculty itself — Descartes must prove two things: that a God exists, and that this God is no deceiver. A perfect being would not equip me with a nature that goes wrong when I use it rightly. Remove the deceiver, and the last shadow over my clearest perceptions lifts.

But readers spotted a trap immediately, and it is the most famous objection in the book — the Cartesian Circle. To trust his clear and distinct perceptions, Descartes must first prove God exists. Yet the proof of God is itself just a chain of reasoning he accepts because each step seems clear and distinct. So he seems to use clear-and-distinct perception to prove God, then use God to validate clear-and-distinct perception — arguing in a circle (raised by Arnauld in the Fourth Objections).