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.
Descartes doubts the senses first, on the same grounds. Then reason. Then received authority. He describes his own period of vertigo. He arrives at one indubitable proposition, cogito ergo sum, and rebuilds outward from it.
So: did Descartes read al-Ghazali?
I want to be careful here, because there is a weak version of this argument that gets made loudly and it is worth not making.
There is no letter in which Descartes cites the Munqidh. Nobody has produced a smoking gun and I am not going to pretend otherwise. It is also true that radical doubt is not an exotic idea; the ancient sceptics had versions of it, and two people can reach the same method independently.
But consider what Descartes demonstrably had access to.
Al-Ghazali's Tahafut al-Falasifa was well known in medieval Europe, largely through the reply it provoked from Ibn Rushd, whose Tahafut al-Tahafut was a foundational text of Scholasticism. Descartes was educated by Jesuits at La Flèche, on a Scholastic curriculum built around Aristotle as transmitted and commented on by exactly that tradition. The Munqidh itself existed in Latin.
The proposition required for independence is therefore not that Descartes never read one specific book. It is that the most famous philosophical argument in Islamic history, by its most famous philosopher, reached seventeenth-century Europe through the tradition Descartes was trained in, and he arrived at a structurally identical method without ever encountering it.
That is possible. It is not the way to bet.
Descartes is a genuinely great philosopher. The cogito is not al-Ghazali's answer, and the uses he puts it to are his own.
The claim is narrower and harder to dismiss: he is a link, not an origin, and he is taught as an origin.
Descartes is the father of modern philosophy. Al-Ghazali is a footnote.
The method is the same. The credit is not.