Abū Yaʿqūb al-Sijistānī (Sijzī), Isḥāq b. Aḥmad, was a distinguished Ismaili dāʿī (summoner) who lived in the 3rd/9th and 4th/10th centuries. He was particularly active in Sīstān and Khurāsān and wrote numerous treatises on Ismaili ‘theosophy’ (al-ḥikma al-ilāhiyya) and in defence of Ismaili doctrine. Most of these works have been published during the last fifty years. Until the middle of the 14th/20th century, these works were preserved only in manuscript form …