deeply pious and ascetic Muslim who was one of the most important relgious figures in early Islām.

Ḥasan was born nine years after the death of the Prophet Muḥammad. One year after the Battle of Ṣiffīn (657), he moved to Basra, a military camp town situated 50 miles (80 km) northwest of the Persian Gulf. From this base, military expeditions to the east disembarked, and, as a young man (670–673), Ḥasan participated in some of the expeditions that led to the conquest of eastern Iran.

After his return to Basra, Ḥasan became a central figure in the religious, social, and political upheavals brought about by internal conflicts with the Muslim community. The years 684–704 marked the period of his great preaching activity. From the few remaining fragments of his sermons, which are among the best examples of early Arabic prose, there emerges the portrait of a deeply sensitive, religious Muslim. For Ḥasan, the true Muslim must not only refrain from committing sin but he must live in a state of lasting anxiety, brought about by the certainty of death and the uncertainty of his destiny in the hereafter. Ḥasan said that the world is treacherous, “for it is like to a snake, smooth to the touch, but its venom is deadly.” The practice of religious self-examination (muḥāsabah), which led to the activity of avoiding evil and doing good, coupled with a wariness of the world, marked Ḥasan's piety and influenced later ascetic and mystical attitudes in Islām.

The enemy of Islām, for Ḥasan, was not the infidel but the hypocrite (munāfiq), who took his religion lightly and “is here with us in the rooms and streets and markets.” In the important freedom- (free will) determinism debate, he took the position that man is totally responsible for his actions, and he systematically argued this position in an important letter written to the Umayyad caliph ʿAbd al-Malik. His letter, which is the earliest extant theological treatise in Islām, attacks the widely held view that God is the sole creator of man's actions. The document bears political overtones and shows that in early Islām theological disputes emerged from the politico-religious controversies of the day. His political opinions, which were extensions of his religious views, often placed him in precarious situations. During the years 705–714, Ḥasan was forced into hiding because of the stance he took regarding the policies of the powerful governor of Iraq, al-Ḥajjāj. After the governor's death, Ḥasan came out of hiding and continued to live in Basra until he died. It is said that the people of Basra were so involved with the observance of his funeral that no afternoon prayer was said in the mosque because no one was there to pray.

Al-Ḥasan al-Baṣrī was known to his own generation as an eloquent preacher, a paragon of the truly pious Muslim, and an outspoken critic of the political rulers of the Umayyad dynasty (AD 661–750). Among later generations of Muslims, he has been remembered for his piety and religious asceticism. Muslim mystics have counted him as one of their first and most notable spiritual masters. Both the Muʿtazilah (philosophical theologians) and the Ashʿarīyah (followers of the theologian al-Ashʿarī), the two most important theological schools in early Sunnite (traditionalist) Islām, consider Ḥasan one of their founders.