During the formative period of Islam, in the first centuries after Muhammad's death, different ideas and beliefs abounded. It was during this period of roughly three centuries that two particular intellectual traditions emerged, Sunnism and Shi'ism. Sunni Muslims endorsed the historical caliphate, while Shi'i Muslims, supporters of 'Ali, cousin of the Prophet and the fourth caliph, articulated their own distinctive doctrines. The Sunni-Shi'i schism...