The advent of Islam in the seventh century brought profound economic changes to the Jews living in the Middle East, and Talmudic law, compiled in and for an agrarian society, was ill equipped to address an increasingly mercantile world. In response, and over the course of the seventh through eleventh centuries, the heads of the Jewish yeshivot of Iraq sought precedence in custom to adapt Jewish law to the new economic and social reality.

In Maimonides and the Merchants, Mark R. Cohen reveals the extent of even further pragmatic revisions to the halakha, or body of Jewish law,...