The most important Jewish source for Hermann Cohen’s rational theology of Judaism is Maimonides’ Guide of the Perplexed. Indeed, the Guide is of such importance that Cohen bases his entire idealistic interpretation of the Jewish religion on it. In particular, Cohen derives his discussion of the continued authority of Mosaic law from the Guide. What follows focuses on Cohen’s discussion of the “Law” in his Religion of Reason out of the Sources of Judaism, and attempts to fill a gap in recent Cohen research by dealing with questions of halakhah and the interpretation of rabbinical sources. Cohen’s original reading of, inter alia, Guide III.31‐32 led him to formulate a theory wherein Mosaic law—and by extension Judaism—guarantees the highest end of human morality. In identifying God with this end, Cohen eventually finds the ultimate criterion for the decision of how much of traditional Jewish law must still be observed in the need for the preservation of the purest monotheism—another central point in Maimonides’ philosophy.