The aim of this paper is to show how Islamic legal institutions (e.g. hajj, wudu, hadd penalties) and Islamic legal principles (e.g. that the burden of proof falls on the party not currently in possession of the contested good) can become so embedded in the mind of a jurist that he imagines other religious legal systems must simply be variations on these institutions and principles. The Kitab al-Bad' wa-l-Ta'rikh of al-Maqdisi, a 10th-century historiographer about whom we know practically nothing, contains a short but surprisingly rich description of Jewish religious praxis. Previously translated by Camilla Adang but never thoroughly investigated, this description is remarkable for a number of reasons. First, it is one of only two or three medieval Islamic texts which purport to describe Jewish practice both neutrally and systematically; by contrast, polemical accounts of Jewish activities and discussions of heretical Jewish beliefs are significantly more common. Second, al-Maqdisi seems to know not just what Jews do, but why they do it; that is, he provides us with Jewish legal concepts and not just ethnographic jottings. What is most striking about the text, however, is that al-Maqdisi’s perception of halakhah is heavily affected by his Islamic legal training. Not only does he highlight practices that would be of interest to Muslim jurists, but the description itself is ordered like a medieval Islamic code— an order foreign to Jewish legal codes. On several occasions, al-Maqdisi projects onto Judaism concepts which exist only in Islamic law, leading him to make several false claims. Furthermore, al-Maqdisi, whose knowledge of Hebrew was most likely limited, frequently uses quotations from hadith as substitutes for imagined Hebrew counterparts, some of which did exist and some of which did not. The result, marvelously, is a decent description of Jewish practice as seen through a Muslim jurist’s eyes. Legal institutions which Judaism and Islam share have the potential to be over-correlated, while concepts or rituals unique to one or the other are sometimes incorrectly correlated — or ignored all together. This description, along with a similar but shorter description in al-Ya’qubi’s universal history, highlights some of the ways in which medieval Islamic jurists understood all religious legal systems to be structured.