Two Men Doubted Everything
Al-Ghazali in 1095 and Descartes in 1641 doubted the same three things in the same order and had the same crisis. There is no smoking gun, and it is not the way to bet.
Two men, five and a half centuries apart, set out to doubt everything they knew.
The first is Abu Hamid al-Ghazali, around 1095. He is the most celebrated scholar of his generation, a professor at the Nizamiyya, the most prestigious madrasa in the Islamic world, at the summit of intellectual life. And he has realised that he does not actually know anything: that his beliefs rest on assumptions he has never examined, accepted on the authority of teachers who accepted them on the authority of theirs.
Authority is not knowledge. He decides to strip it all out and rebuild.
The second is René Descartes, in 1641, troubled by the same problem and reaching for the same instrument.
Al-Ghazali sets out his crisis in the Munqidh min al-Dalal, Deliverance from Error, and the structure of it is worth following.
He doubts the senses first: they deceive, a stick appears bent in water, the sun looks the size of a coin. Then he doubts reason, on the grounds that rational systems generate contradictions and he has no non-circular way to choose between them. Then he doubts consensus, because the agreement of one age is routinely overturned by the next.