Saadia is the first important rabbinic figure to write extensively in Judeo Arabic. Known for his works on Hebrew linguistics, Halakha, and Jewish philosophy, he was one of the more sophisticated practitioners of the philosophical school known as the "Jewish Kalam" . In this capacity, his philosophical work "The Book of Beliefs and Opinions" represents the first systematic attempt to integrate Jewish theology with components of Greek philosophy. Saadia was also very active in opposition to Karaism, in defense of rabbinic Judaism.