eminent Muslim mystic (Ṣūfī) and theologian renowned for his psychological refinement of pietistic devotion and his role as a precursor of the doctrine of later Muslim orthodoxy. His main work was ar-Ri ʿāyah li-ḥūqūq Allah, in which he acknowledges asceticism to be valuable as an act of supererogation but always to be tempered by inner and outer duties toward God.

There is little historical information about al-Muḥāsibī's life. His parents apparently left for Baghdad shortly after his birth, perhaps attracted by the many opportunities afforded by the newly founded capital. His father had acquired some wealth, but al-Muḥāsibī is said to have refused it because of doctrinal differences. He led a normal life, owned a beautiful house, and liked sumptuous clothes. This image of the ordinary bourgeois, however, is qualified by a trait that al-Muḥāsibī imported from Basra: the otherworldly spirituality propagated by the famous Ṣūfī theologian al-Ḥasan al-Baṣrī (died 728).

Muslim asceticism had developed some specific features: nightly recitals of the Qurʾān (the Muslim sacred scriptures), restrictions concerning the kind and quantity of food one should eat, and a special attire consisting of woollen clothing. These habits had been adapted from the life-style of Christian monks. But whereas Christian monks used to live in seclusion, a Muslim ascetic felt obliged to remain an active member of his community.

Thus, al-Muḥāsibī came to realize that, in his urban society with its inevitable public exposure, the practice of outward asceticism was open to ambiguity: although it could serve to suppress the normal sins of passion, it could also become a deceptive vehicle for inner vices like hypocrisy and pride. As soon as outward piety becomes a part of a person's image, it can act as a screen for the hidden intentions of the ego. Man has to recognize that sinful actions are frequently defined not by their objective reality but by the subjective attitude of the sinner. Outside the realm of the commandments and prohibitions of the Qurʾān, nothing should be labelled good or bad without restriction. The most commendable attitude is scrupulosity, although even this can be ambiguous, because it might result in spiritual paralysis. Asceticism is valuable as something additional, an act of supererogation, but it must always be tempered by attention paid to the inner and outer duties toward God (ar-Ri ʿāyah li-ḥūqūq Allāh, the title of al-Muḥāsibī's main work). The proper instrument for this is reason, the importance of which al-Muḥāsibī stressed far beyond the normal practice of mystics, who often tended to emphasize irrationality and spiritual intoxication. The method he proposed was muḥāsabah, the anticipation of the Last Judgment through constant self-examination. This seems to have been an impediment to real mystical experiences; the ruthlessness of this psychological technique buried every attempt at ecstatic exaltation under an enormous inferiority complex.

Al-Muḥāsibī propagated his ideas in didactic conversations, which he would record immediately afterward; his books still preserve this dialogical structure. His influence on posterity was immense, especially through his pupil Junayd. During his lifetime, however, he was regarded with suspicion, and his last years were embittered by persecution. He had joined a group of theologians who, led by ʿAbd Allāh ibn Kullāb (died 855), criticized the doctrines of the rationalist Muʿtazilī school dominant at that time.

The discussion was focussed on the problem of the essence of God and the nature of his attributes. The Muʿtazilī, in stressing the unity of God, tended to reduce the attributes to mere nominal aspects; al-Muḥāsibī, in order to preserve their individual value, accentuated much more their independent status. And whereas the Muʿtazilī held the attribute of God's speech to be created, realized in temporal revelations like that of the Qurʾān, al-Muḥāsibī believed that it was also uncreated if seen under the aspect of the eternal Word of God. He did not go so far as to support the popular belief that the Qurʾān, too, was uncreated; he avoided this shibboleth used in the inquisition initiated in favour of the Muʿtazilī by the caliph al-Maʾmūn in 833.

This diplomatic attitude became precarious when, in 850–851, a later caliph, al-Mutawakkil, put an end to the pro-Mu ʾtazilī policy of his predecessors and, two years later, prohibited rationalist theology altogether. Al-Muḥāsibī's theological position was now viewed as treasonous by the former victims of the inquisition, precisely because he had been closest to them in their dogmatic outlook, for they considered the use of any rational theological method as heresy, regardless of the doctrine it supported. He was consequently forced to give up his public teaching and appears to have emigrated to Kūfah. Later on he was allowed to return to Baghdad, perhaps at the price of abandoning his theological convictions. Yet the boycott persisted: when he died there in 857, only four people attended his funeral.