Among the neighbours of the Khārijites in Basra were devotees of rationalizing theology (kalām) known as Mu‘tazilites. They are said to have appeared in 720s, and at least one of their doctrines (regarding the status of the sinner) plainly has its roots in the Umayyad period. But they remain shadowy down to about 800, when they emerge as a loose association of diverse people and principles in Basra and Baghdad. Their school was systematized from the late ninth century onwards and flourished, above all in Iran, down to the mid-eleventh century. It suffered in the so-called ‘Sunni revival’ and disappeared altogether as a school in its own right after the Mongol invasion.

Unlike the Khārijites, the Mu‘tazilites had neither a communal genealogy nor a law of their own, meaning that they did not form a complete saving vehicle. Some Mu‘tazilites were ‘Uthmānīs, as one would expect of Basrans who were not Khārijites; but most of them were fond of ‘Alī, and in Baghdad they were often Zaydīs (Shi‘ites of the type described below, ch. 9). All eventually accepted either the four-caliphs thesis or Shi‘ite affiliation, probably in the course of the ninth century. This finalized their status as a mere school of thought rather than a sect of their own.

As Basrans by origin, if not always domicile, the early Mu‘tazilites generally shared the libertarian outlook of the Khārijites.