Iḥyāʾ ʿUlūm al-Dīn · The Revival of the Religious Sciences

Forty books in four quarters — worship, custom, the things that destroy a person, the things that save them — written after the crisis and the eleven years away. It takes the whole of Islamic law and shows what each part is for inwardly, so that the outward act and the inner state stop being two subjects.

The book that made Sufism orthodox, and therefore the book that made the fence low enough to trade across. Its structural argument — that the deed and the intention are one subject — is also the argument of half the Jewish devotional literature that came after it, including Baḥya's.