ABU YAQUB AS-SIJISTANI
"Abu Yaqub Ishaq bin Ahmad as-Sijistani, nicknamed "cotton-seed" (Iranian, panba-dana, Arabic khayshafuj) was born in 271/883 in Bandan, a district in north of Sijistan and was trained in Yamen. He was a great philosopher and scholar and considered to be one of the major Ismaili thinkers whose share in the development of the Ismaili system of thought is considerable. Paul E. Walker writes in Abu Yaqub al-Sijistani: Intellectual Missionary (London, 1996, p. 13) that, "Yet, from the prominence of his books and the profoundly impressive intellectual contribution they (Ismailis) represent, we discover a truly significant mind and voice - one that deserves recognition as an outstanding figure in the Ismaili past and as a major force in Islamic thought in general."
The Abbasids took notice of the rapid conversion of the Ismailis in Khorasan, notably Nasr bin Ahmad, and insinuated Nuh bin Nasr (331-343/943-954), the son of Nasr bin Ahmad; against his father and the Ismailis. Nuh bin Nasr dethroned his father and conducted a barbarous massacre of the Ismailis in 331/942, known in the Ismaili history as al-mainat al-uzama (great calamity) in Khorasan and Transoxania. An-Nasafi and his chief associates were also executed in the wild operations at Bukhara in 332/943. It resulted a setback in Ismaili mission, but was resumed under an-Nasafi's son, Masud, surnamed Dihqan and Abu Yaqub as-Sijistani.
It is generally agreed upon by the scholars that as-Sijistani was not executed with an-Nasafi in 331/942. The mistake however arose from misreading of al-Baghdadi's statement in al-Firaq bayn al-Firaq, stating that both an-Nasafi and as-Sijistani were executed. In the introduction of both Risalat al-Mawazin and Risalat al-Mabda wal Ma'ad, he himself mentions the name of Imam al-Hakim, who acceded to the throne in 386/996. Thus, it implies that he was still alive in 386/996. His death, therefore, could be placed between 386/996 and 393/1003. He had managed to escape the widespread massacre, and continued the mission in Bukhara.
He was executed by Khalaf bin Ahmed (353-393/964-1003), the Saffarid ruler of Khorasan. The period of as-Sijistani saw many prominent Ismaili thinkers, such as Abul Haytham Ahmad bin Hasan al-Jurjani, an Ismaili philosopher-poet from Gurgan, who composed many poems on Ismaili doctrines. His Ismaili disciples were Muhammad bin Surkh al-Nishapuri, Hamiduddin Kirmani, etc.