Abū Jaʿfar Naṣīr al-Dīn Muḥammad b. Muḥammad b. al-Ḥasan al-Ṭūsī, also known as al-Muḥaqqiq al-Ṭūsī, or Khwāja Ṭūsī, or Khwāja Naṣīr al-Dīn, was a major Islamic philosopher and affiliate of the Ilkhānids in Iran. Born in 597/1201 in the district of Ṭūs, he first began studying the Islamic sciences under the tutelage of his father. He then moved to nearby Nīshāpur between 610/1213 and 618/1221 to continue his studies, reportedly with the pupils of Fakhr al-Dīn al-Rāzī (d. 1210). He traveled to Iraq, where he studied jurisprudence with the Shiʿī scholar Muʿīn al-Dīn Sālim b. Badrān al-Māzinī. Later in Mosul, he studied with the reputed mathematician and astronomer, Kamāl al-Dīn Yūnus (d. 639/1242). A twelver Shiite, Ṭūsī found patrons among various Ismāʿīlī rulers. In Sartakht, a province of Qūhistān, he was patronized by the Ismāʿīlī governor Muḥtasham Naṣīr al-Dīn ʿAbd al-Raḥīm b. Abū Manṣūr, to whom he dedicated his work on ethics, the Akhlāq-i Nāṣirī (in dedication to Naṣīr al-Dīn), which was completed in 633/1245. By 644/1246, Ṭūsī had reached Alamut, where he found his residence for at least twenty years. The library at Alamut proved to be crucial for Ṭūsī’s intellectual development as a philosopher and scholar. In 653/1255, Ṭūsī was sent as an intermediary between the Ismāʿīlīs and the Ilkhānids. At the behest of Ṭūsī, the Ismāʿīlī ruler at Alamut submitted to the Ilkhānid ruler Hülegü. By 645/1256, Alamut had fallen to the Mongols. Ṭūsī was then to become a close affiliate of the Ilkhānids, and he accompanied Hülegü on his conquests westward, most importantly the conquest and fall of Baghdad in 656/1258, which was the capital of the Abbasid caliphate. By 1259, Ṭūsī had secured funds from his Ilkhānid patrons to establish an observatory at Marāgha in Iranian Azerbaijan, a major undertaking which would bring together the leading scholars of the age. Ṭūsī reportedly died in Baghdad in 672/1274. As a scholar and philosopher, Ṭūsī subscribed to a form of critical Avicennism. His contributions to Islamic thought were beyond a doubt crucial to later developments in kalām and Islamic philosophy writ large.