al-Ghaz(z)ālī, Abū Hāmid Muḥammad (d. 1111 (AH 505)). The ‘Proof of Islam’ (hujjat al-Islam), often considered the greatest religious authority after the Prophet Muḥammad. As a result of the esteem accorded to him by his contemporaries, al-Ghaz(z)ālī deeply influenced the direction of Islamic thought, in particular Islamic jurisprudence (sharīʿa), dialectical theology (kalām), philosophy, and mysticism (taṣawwuf).

He was born at Tūs and was educated there and at Nishapur. He rose to be a distinguished professor at the Baghdād Nizamiya, a formidable scholar in Islamic law and theology. However, in 1095, he underwent a crisis brought on by a search for inner conviction, and by an awareness that although he was lecturing about God, he did not know God. He therefore abandoned his high position for the life of a Sūfī, seeking to know the reality of which, hitherto, he had only spoken. After ten years, he returned to Nishapur and wrote his magnum opus, ʾIhyā ʿulūm al-dīn (The Revival of the Religious Sciences) and other key works, such as Mishkāt al anwār (Niche of the Lamp), al-Qistās al-mustaqīm (The Just Balance), Kīmiyaʾ al-Saʿāda (The Alchemy of Happiness), and Tahāfut al-Falāsifah (The Incoherence of the Philosophers) in which the inadequacy of reason outside its appropriate spheres points to the necessity for revelation and mystical knowledge. It was his achievement that he successfully harmonized Sufism into the field of orthodoxy and gave it acceptance as an inner dimension of Islam. At the same time, his emphasis on the limits of reason in relation to faith led eventually to a withdrawal of Islam from the leadership it had given to the world in science and philosophy.