Descartes doubted everything — then found one thing he couldn't

René Descartes began his Meditations on First Philosophy (1641) by systematically doubting everything he believed: his senses could deceive him, the external world might be an illusion, a malicious demon might be feeding him false experiences. After stripping away every belief he could not prove, he arrived at a single undeniable fact: that the very act of doubting requires a thinker. 'Cogito, ergo sum' — 'I think, therefore I am' — became the foundation on which Descartes rebuilt certain knowledge. It remains the most famous sentence in the history of Western philosophy.

Source: Descartes, Meditations on First Philosophy (1641)