The advent of Islam in the seventh century brought profound economic changes to the Middle East and to the Jews living there. The Talmud, written in and for an agrarian society, was in many ways ill-equipped for the new economy. In the early Islamic period, the Babylonian Geonim made accommodations through their responsa, through occasional taqqanot, and especially by applying the concept that custom can be a source of law. Not previously noticed, in the Mishneh Torah Maimonides made his own efforts to update the halakha through codification. Mark R. Cohen's new book Maimonides and the Merchants suggests that, like the Geonim before him, Maimonides wished to provide Jewish merchants an alternative and comparable forum to the Islamic legal system and thereby shore up an important cornerstone of communal autonomy.