Al-Mu'ayyad bi-llah al-Haruni (d. 411/1020) was a representative of the intellectual center of the Zaydiyya in Northern Iran and a student of the leading Muʿtazilite theologians of the time. In his Kitab Ithbat nubuwwat al-nabi he presents a proof of prophecy of Muḥammad and a refutation of the Isma'ilyya.The present volume explores the historical and intellectual context of the oeuvre and includes a partial critical edition of the text.