Purification of the soul is a principle that is central to understanding Islamic spirituality but despite this, relatively little has been written explicitly in the Islamic tradition regarding this discrete method of spiritual purification. This book examines the work of a scholar of this discipline, al-Harith al-Muhasibi, who lived and worked during the classical Islamic period under the Abbasids. Although al-Muhasibi was well known for his skills in many disciplines, including the Qur'an, Prophetic narration and scholastic theology, it is his mastery in the field of Islamic spirituality and moral psychology for which he is best remembered. Assessing the extent to which the political, social and economic factors played a part in his life and work, Gavin Picken provides a comprehensive overview of his work and its great significance in the development of Islamic spirituality. Reconstructing his life in chronological order and providing the most comprehensive appraisal of his works to date, it explores a facet of al-Muhasibi's teaching which as yet has not been studied, namely his understanding, concept and methodology regarding the purification of the soul within the Islamic paradigm. As such, it will be of great interest not only to researchers and students of Sufism but also to scholars of comparative spirituality and mysticism.