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 Qur'an and Hadith. Seeking to affirm the imamate from a philosophical perspective, he uses numerous devices to bolster his thesis, ranging from the so-called ten categories of the philosophers to several biological metaphors from plants and trees to animals and minerals. His foremost device is the theory of 'degrees of excellence'. Accepting that there are differences and disparities within each genus and species, he gives paradigms of perfect examples in each variety, and demonstrates in a parallel manner that it is the Imam who stands at the apex of humanity.