(Attributed to Rabbeinu Tam)

Sepher HaYashar, probably written in the 13th-century, was one of the most popular ethical works of the Middle Ages. It has often been erroneously ascribed to the Tosaphist Jacob (Rabbeinu) Tam, since he authored a Halachic work with the same title. Others have attributed Sepher HaYashar to R. Jonah Gerondi, or to Zecharia HaYevani. The style and language conformed to the contemporary philosophies of the time with references to Aristotelian terms and concepts, yet some of the main ideas conflicted with conventional philosophy to the extent that scholars have also attributed the authorship to a kabbalist who wished not to reveal the full scope of his mystical beliefs. Others find similarities between the work and the ethical concepts of the Aschkenazic Hasidic movement which peaked in the 13th-century. As of yet, no critical edition of this work has been published and until the correct text is ascertained, the questions regarding it's authorship and underlying philosophical thought are still in doubt. See EJ, Vol. XIV col. 1099. For a full discussion of the stucture and contents of Sepher HaYashar, see M. Waxman, Vol. II, pp. 276-8