The principle established by Descartes: "I think, therefore I am" — the one thing that cannot be doubted because doubting it proves it.
From Latin cogito ergo sum (I think, therefore I am), from Descartes' Discourse on the Method (1637). Descartes tried to doubt everything — his senses, his body, the external world — and found one thing he could not doubt: the fact that he was doubting. Doubt proved the doubter.
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