This book presents veteran scholar Sir William Muir’s history of the Caliphate. A Caliph is the spiritual head and temporal ruler of the Islamic state. In principle, Islam is theocratic: when Muhammad the Prophet died, a caliph (successor) was chosen to rule in his place. The caliph had temporal and spiritual authority but was not permitted prophetic power; this was reserved for Muhammad. The caliph could not, therefore, exercise authority in matters of religious doctrine.