[ethics]. 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 is often erroneously ascribed to the Tosaphist Jacob (Rabbenu) Tam as he authored an Halachic work under the same title. 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 some scholars attributed the authorship to a Kabbalist who did not wish to reveal the full scope of his mystical beliefs. Others find similarities between the work and the ethical concepts of the Ashkenazic Hasidic movement which peaked in the 13th century.