The Rule
Descartes reflects on why the cogito is certain. The answer is that he perceives it clearly and distinctly — he cannot doubt it while attending to it. So clear and distinct perception becomes his general criterion of truth: the guarantee that a judgment is not merely believed but known.
Whatever I Clearly and Distinctly perceive is certainly True.
The Cartesian Circle
There is an apparent circularity here. Clear and distinct ideas are certain only if God — a non-deceiving creator — guarantees them. But Descartes uses clear and distinct ideas in his very proof of God's existence. Arnauld, one of the original critics, named this the Cartesian Circle.
Descartes's response distinguishes between current clear perception — always reliable, even without God — and remembered clear perception, which only stays reliable once God's existence and goodness are known. Whether this resolves the circularity remains debated.
What Qualifies
Mathematical truths are the paradigm of clear and distinct ideas. So is the cogito. God's existence, Descartes argues, becomes clear and distinct once properly examined. Sensory knowledge — the feel of heat, the look of colour — does not meet this standard. It is obscure and confused, and reliable only for practical navigation, not for knowledge.
Leibniz argued that Descartes's criterion was too vague: seemingly clear ideas could still harbour hidden contradictions. Only complete analysis into primitive, undefinable concepts could give genuine distinctness. Spinoza pushed further, arguing that adequate ideas — not merely clear ones — were the proper standard.
