A philosopher doubted literally everything to find one thing he couldn't doubt

René Descartes imagined an all-powerful demon deceiving him about everything: the outside world, his own body, even basic arithmetic. Stripping away every belief that could possibly be false, he found one thing survived — the very act of doubting proved a doubter existed. 'I think, therefore I am' became the foundation he tried to rebuild all knowledge on.

— René Descartes, Meditations on First Philosophy — 1641 — public domain
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