 His major scholarly pursuits were works on the Pharisees, a Jewish sect in second Temple times from which modern Jewish tradition developed, and the Sifra, the oldest rabbinic commentary on the book of Leviticus, which was completed in Palestine in the fifth century.Even in his retirement he continued writing, working at the dining room table of his Riverside Drive apartment to complete several annotated volumes of the Sifra.When he became frail in his later years and had trouble walking to the synagogue, his former students turned his home into a synagogue on Saturday mornings, assembling the quorum of 10 needed for prayer.