Nasir al-Din al-Tusi, the 13th-century Shi'i philosopher and scholar, is one of the most controversial Muslim personalities of his time. His contributions to different fields of learning, including astronomy, theology and philosophy, won for him the honorific titles of scholar ('Muhaqiq') and the 'Eleventh Intellect'. In Isma'ili circles he was held in the highest esteem and addressed as 'Da'i al-du'at wa khwaja-i ka'inat', chief propagandist and master of creation. This is al-Tusi's spiritual autobiography. Although it is not traditionally listed in his writings, contemporary scholars have no doubts about its authenticity. In it, al-Tusi recounts his spiritual attraction and conversion to Isma'ilism. The work of and inquisitive and highly intelligent mind, the importance of this document lies not only in al-Tusi's account of his spiritual journey but also in his clear and vivid elaboration of the Shi'i Isma'ili doctrine of 'ta'lim', or the need for authoritative teaching by the Isma'ili imam.