The survival of the personal copy of Commentary on the Mishnah by Maimonides, which he revised throughout his life, provides an unparalleled window into the ways he continually reconsidered legal and conceptual questions. This manuscript covers five of the six orders of the Mishnah and contains countless corrections and emendations, the vast majority in the author’s own hand. This article argues that Maimonides’s intense interest in solving problems related to the enumeration of the commandments, which he addresses at length in (...) Book of the Commandments and, to a lesser extent, Mishneh Torah, led him to make a number of emendations to his Commentary and to rethink other aspects of his work. That is, when writing Book of the Commandments and Mishneh Torah in the decade after completing his Commentary, Maimonides tackled, and sometimes even concocted, questions that he had no reason to consider in the latter work. This study traces two ways that Maimonides’s later works diverge from his earlier ones, in the meaning of Hebrew and Arabic technical terms and in increased attention to scripture in determing Jewish law, revealing a great medieval mind “in perpetual motion.”. (shrink)