Cogito
Descartes's foundational principle: the existence of the thinking self is indubitable, since to doubt is already to think.
The cogito is the foundational claim of René Descartes's First Philosophy. In the Discourse on the Method (1637) and the Meditations (1641), Descartes attempts to find a single proposition that resists every form of doubt — even the systematic doubt of an evil demon who might deceive him about the entire content of his experience. He arrives at the cogito: even if everything else is illusion, the very act of doubting requires a doubter, and the existence of the thinking self is therefore indubitable.
From the cogito Descartes derives the rest of his philosophical system: the real distinction between mind and body, the existence of God, and ultimately the reliability of clear and distinct ideas. Subsequent philosophers — from Hume and Kant to Husserl and Wittgenstein — have contested almost every step of the inference while remaining shaped by it. The cogito remains the canonical first move of modern philosophy.