Abu Ya'qub al-Sijistani

Abu Ya'qub Ishaq ibn Ahmad al-Sijistani (also called al-Sijzi) was a tenth-century thinker and missionary of Ismaili Shia Islam, a branch of Shi'ism that organized its preaching through a network called the da'wa ("the summons/mission"). His birth year is unknown; his name (nisba) points to origins in Sistan, in the eastern Iranian world. He worked as a da'i (missionary-teacher) across Khurasan and Transoxiana, regions then under Sunni Samanid rule, where Ismaili preaching was risky.

He is reported to have succeeded an earlier missionary, Muhammad al-Nasafi, both as a leading da'i and as a developer of his ideas. In his own writing he mentions being in Baghdad/Iraq in 934 (322 AH), having returned from the pilgrimage (hajj) to Mecca. Early in his career his movement preached independently of the Fatimid caliphs in North Africa; later, traditionally during the reign of the Fatimid imam al-Mu'izz (953–975), he came to accept the Fatimid imams.

His lasting importance is intellectual: he absorbed Neoplatonic philosophy into Ismaili thought, describing God as beyond both being and non-being, knowable only through a "double negation," and placing a single universal Intellect at the summit of creation. Works ascribed to him include Kashf al-mahjub ("Unveiling of the Hidden," surviving in Persian paraphrase), Kitab al-Yanabi' ("Wellsprings"), and Kitab al-iftikhar (composed around 971). The date and circumstances of his death are genuinely disputed: a late source (Rashid al-Din) reports he was executed by the Saffarid emir Khalaf ibn Ahmad, but other scholarship, noting that his works mention the Fatimid caliph al-Hakim (who acceded in 996), places his death considerably later — perhaps between 996 and 1003. No firm death year can be given.