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When Descartes set about the reconstruction of philosophy, his first step was to (theoretically) permit scepticism and to discard the practice of the schoolmen of looking to authority as the ultimate source of truth.  That done, he sought a more natural fountain of true principles, and thought he found it in the human mind;  ...

When Descartes set about the reconstruction of philosophy, his first step was to (theoretically) permit scepticism and to discard the practice of the schoolmen of looking to authority as the ultimate source of truth.  That done, he sought a more natural fountain of true principles, and thought he found it in the human mind;  ...

Self consciousness was to furnish us with our fundamental truths, and to decide what was agreeable to reason.  But since, evidently, not all ideas are true, he was led to note, as the first condition of infallibility, that they must be clear.  The distinction between an idea seeming clear and really being so, never occurred to him.

Self-consciousness was to furnish us with our fundamental truths, and to decide what was agreeable to reason.  But since, evidently, not all ideas are true, he was led to note, as the first condition of infallibility, that they must be clear.  The distinction between an idea seeming clear and really being so, never occurred to him.

(Peirce, CP 5.391).

(Peirce, CP 5.391).