MUʿTAZILAH. A religious movement in early Islam, the Muʿtazilah turned into a theological school that become dominant in the third and fourth centuries AH (ninth to tenth century CE) and persisted in certain areas until the Mongol invasion at the beginning of the thirteenth century CE. The history of the movement is comprised of three different phases: (1) an incubation period that lasted roughly through the eighth century; (2) a short period of less than half a century (c. 815–850) when the Muʿtazilī school, after having defined its identity, developed an astonishing variety of individual, sometimes contradictory ideas and permeated the intellectual life at the Abbasid court; and finally (3) several centuries of scholastic systematization channeled into two branches or schools that were named after the towns of Basra and Baghdad respectively.

Each of these phases presents its own problems for the researcher...