Muslim theologian considered the founder of the Muʿtazilah sect.

As a young man Wāṣil went to Basra, Iraq, where he studied under the celebrated ascetic Ḥasan al-Baṣrī and met other influential religious figures who lived there. In Wāṣil's time there began the discussions that led to the development of Islāmic speculative theology. At first theological controversies among Muslims were closely tied to political events, the principal issue being the legitimacy of the rule of the Umayyad house, which seized power after the death of the fourth caliph, ʿAlī.

Wāṣil's doctrinal formulations gave the Muʿtazilah faction coherence as a religious sect. At the same time, both Wāṣil and the Muʿtazilah became involved in a revolutionary movement led by the ʿAbbāsids that was to result in the overthrow of the Umayyads. Wāṣil gathered around himself many devoted believers and ascetics, whom he often sent out as emissaries to spread his doctrines in distant provinces.