Abu Hamid Muhammad al-Ghazali (1058–1111) was the most brilliant academic of his time. He took the chair at the Nizamiyya Madrasa in Baghdad at thirty-five, under the patronage of the Seljuk vizier Nizam al-Mulk — the leading university of the age. A few years later he suffered a major intellectual crisis, gave up his post, and lived for ten years as a wandering Sufi across Damascus, Jerusalem, the Hijaz, and Iran.

One fruit of that crisis was the Tahafut al-Falasifa (Incoherence of the Philosophers, c. 1095). The work questions, across twenty topics, the system of those philosophers in the contemporary Muslim world known as 'falasifa' — essentially the Aristotelians, al-Farabi and Ibn Sina. Three theses he treats as outright unbelief: (1) the eternity of the world, (2) that God knows only universals and not individual events, and (3) the absence of bodily resurrection. On the other seventeen topics he calls the philosophers merely 'incoherent', not heretical.

Ghazali's critique was especially decisive for the concept of causation. Fire does not burn cotton through cause and effect; each event is a fresh creation by God in the instant ('occasionalism') — a view that anticipates Hume's modern scepticism about necessary connection. The work did not kill Islamic philosophy but redirected it: the Mu'tazila and falasifa traditions weakened, the Ash'ari kalam strengthened. A century later Averroes would answer with the Tahafut al-Tahafut (Incoherence of the Incoherence) — but that answer carried in Latin Europe, not in the Islamic world.