Sefer HaYashar The Book of Righteousness Rabinu Tam ספר הישר רבינו תם Sefer HaYashar [The Book of Righteousness]. One of the most popular ethical books in the Middle Ages. It was attributed to Rabinu Tam, the Tosafist. The style and language conform to contemporaneous philosophical and ethical writings and ideas; the author specially made use of Aristotelian terms and concepts. He includes several other beliefs which hint at possible Qabbalistic tendencies. The work is also similar in substance to ideas present amongst Ḥasidei Ashkenaz. Jacob ben Meir [1100 - 9 June 1171 4 Tammuz], best known as Rabinu Tam [Hebrew: רבינו תם], was one of the most renowned Ashkenazi Jewish rabbis and a leading French Tosafist, a leading halakhic authority in his generation, and a grandson of Rashi. Known as Rabinu [our teacher], he acquired the Hebrew suffix "Tam" meaning straightforward; it was originally used in the Book of Genesis to describe his biblical namesake, Jacob.