Written in Arabic in the first half of the ninth century, Twenty Chapters is the first medieval Jewish philosophical work, and had a major influence on the development of Jewish theological and philosophical thought. Dawūd al-Muqammaṣ, a Jewish convert to Christianity, who wrote this theological Summa after his return to Judaism, also authored the first Judeo-Arabic Bible commentaries, and some of the earliest Judeo-Arabic polemical works. The annotated Hebrew translation of this pioneering work, published here for the first time, presents al-Muqammaṣ in his intellectual and historical context and throws light on his Jewish, Muslim and Christian background.