Al-Muhasibi, born Harith bin Asad in Basra around 781 CE, was a significant Arab theologian, philosopher, and ascetic. He is recognized as the originator of the Baghdad School of Islamic thought, a synthesis of Kalam (rational theology) and Sufism (Islamic mysticism). His teachings profoundly influenced prominent Sufi masters like Junayd al-Baghdadi and Sirri Saqti, and he is also noted as an inspiration for later theologians such as al-Ghazali. Despite his father's wealth, al-Muhasibi embraced an ascetic life, partly due to theological differences regarding the nature of the Quran. He authored around 200 works, focusing on theology and Sufism, including titles like Kitab al-Khalwa and Kitab al-Ri'ayah li-huquq Allah.

His writings emphasized ethical self-discipline and spiritual introspection. Al-Muhasibi critically examined common Sufi practices of his time, such as ascetic clothing and devotional recitations, cautioning against their potential to foster hypocrisy or pride if not rooted in genuine inner transformation. He stressed the continuous practice of self-examination as a means to purify the soul and prepare for the Day of Judgment. Al-Muhasibi also studied jurisprudence under al-Shafi'i and engaged with theological debates of his era, opposing views that posited the Quran was created and contributing to discussions on the nature of divine speech.

Al-Muhasibi is credited with establishing the Baghdad School of Islamic philosophy, a significant intellectual movement that integrated the rationalist methods of Kalam with the spiritual practices of Sufism. This approach aimed to reconcile theological reasoning with mystical experience, creating a comprehensive framework for understanding faith and the divine. His role as a teacher was pivotal, directly shaping the development of Sufism through his influence on disciples like Junayd al-Baghdadi and Sirri Saqti, who became central figures in the tradition. This synthesis laid groundwork for future theological and mystical thought, impacting prominent figures like al-Ghazali.

Adopting an ascetic lifestyle, Al-Muhasibi focused extensively on the importance of ethical self-discipline and continuous spiritual introspection. He analyzed the outward manifestations of piety common among Sufis, such as specific attire or devotional practices, warning that these could become mere displays of virtue, leading to pride or hypocrisy, if not accompanied by genuine inner reform. His core teaching revolved around the necessity of constant self-examination as a path to purify the soul and prepare for divine accountability, emphasizing that true spiritual progress stems from internal purification rather than external performance.

Al-Muhasibi's intellectual journey involved engagement with major theological currents of his time. He studied jurisprudence under al-Shafi'i and later associated with a group of theologians led by Abdullah ibn Kullāb. This circle actively opposed prevailing doctrines, including those of the Jahmis, Mu'tazilis, and anthropomorphists. Notably, Ibn Kullāb, with whom Al-Muhasibi aligned, developed arguments distinguishing between God's eternal attributes and their temporal manifestations, particularly concerning the nature of the Quran, challenging the Mu'tazilite view of its createdness. This intellectual engagement occurred during a period of significant theological debate, including the formal end of the Mihna (inquisition) under Caliph al-Mutawakkil.

“Know that the first thing that corrects you and helps you correct others is renouncing this world. For renunciation is attained by realisation, and consideration is attained by reflection...”