Michel de Montaigne, the great Renaissance philosopher and essayist, inscribed above the door of his study que sais-je?—what do I know? And thus Descartes set himself the task of doubting everything, up to and including his own existence .

These thinkers weren’t nihilists, nor even skeptics. They believed in truth, and they wanted to discover it. But they were chastened by the still-palpable possibility of drastic error, and they understood that, from a sufficiently distant vantage point, even their most cherished convictions might come to look like mistakes.