The secretiveness of the family before c. 890 and the differing genealogies subsequently published by the dynasty itself further make it difficult for modern scholars to assess the exact origin of the dynasty.[12] Whatever their true origin, the Fatimids were the leaders of the Isma'ili sect of Shi'ism, and they headed a movement which, in the words of the historian Marius Canard, "was at the same time political and religious, philosophical and social, and whose adherents expected the appearance of a Mahdi descended from the Prophet through Ali and Fatima".[11] As such, they regarded the Sunni Abbasid Caliphate (and the Umayyads of the Caliphate of Córdoba) as usurpers and were determined to overthrow them and take their place at the head of the Islamic world. Their pretensions were not only ecumenical, but also universal: according to their doctrine, the Fatimid imam was no less than the incarnation of the 'world soul'.[13][14]