Ahmad b. Ibrahim al-Naysaburi (996-1021) lived at the pinnacle of Fatimid rule in Egypt. One of the most distinguished scholars of his time, several important works are accredited to him. However, it is his major treatise - the Kitab Ithbat al-imama - which is of seminal importance in the philosophical development of medieval Muslim thought. It is in this work - which is here presented in a completely fresh edition and a new translation - that al-Naysaburi daringly uses rational tools to explain and expound his theology, rather than base his assertions exclusively on the authority of the...