Descartes tried to doubt everything, and found one thing he couldn't

Trying to build knowledge on unshakeable foundations, Descartes doubted his senses, his body, even mathematics, imagining an evil demon deceiving him about all of it. But the very act of doubting proved something had to exist to do the doubting. 'Cogito, ergo sum' — I think, therefore I am — became the one certainty Descartes would build his entire philosophy back up from.

— René Descartes, Meditations on First Philosophy — 1641
Go deeper: get the book →