One of the putative founders of the Mutazili school of theology. Studied with theologian Hasan al-Basri. Declared that grave sinners were neither believers nor unbelievers but occupied “a position between these two” (manzilah bayna al-manzilatayn). This story was often used to explain Mutazilis' refusal to take a position on which of those involved in the first civil war (656–61) were right or wrong. He died in 749.