This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Hasan al-Basri wrote a closely studied letter on free will and human responsibility for sin, a documented text later Mu'tazilite theologians built on directly when formalizing their own doctrine of human free agency. That later use is recorded plainly, but Hasan himself predates the Mu'tazilite school and its formal doctrines by a generation or more, since the school only organized into a distinct theological position after his own lifetime had already ended.
Scholars caution against reading his own recorded position in that letter as identical to the later, more fully worked out Mu'tazilite doctrine it helped found, a documented reservation distinct from denying the letter's influence altogether. The letter is treated as a genuine foundation text for the later argument, while the argument itself, once formalized by a school Hasan never belonged to, is recorded as having gone further than his own surviving words can be shown to have gone on their own.
That a preacher's own letter on free will became a foundation text for a theological school he predated and never joined, while scholars still caution against treating his recorded position as simply an early version of that school's later doctrine, marks Hasan al-Basri's free will letter as a documented case where influence and identity of position are recorded as two separate claims rather than one.