The Mu‘tazili world view and rational theology was facing increasing competition and criticism from philosophy of Greek origin, which claimed to provide the only scientific world view based on cogent logical demonstration independent of religious beliefs. Study of the philosophical sciences was mostly shunned in religious scholarship, but was an integral part of the education of the medical profession. Among Qadi ‘Abd al-Jabbar’s disciples in Rayy was for some time a young physician trained in the philosophical sciences, Abu l-Husayn al-Basri (d. 1044), who challenged some of his teaching in his lectures and went on to compose a massive critical review of the arguments and proofs used in kalam. His theological works were generally ignored among the Mu‘tazila and handed down among students of medicine. Only a century later his teaching was revived and espoused by the Mu‘tazili scholar Mahmud b. al-Malahimi in Khorezm in Central Asia and gained recognition as a school of Mu‘tazili theology.
The present study presents evidence that Abu l-Husayn’s theology was immediately registered and controversially debated in the Karaite community under the Fatimid caliphate. The study is based on source material preserved in Genizahs and now dispersed in libraries around the world.