Responsa are where Jewish law actually happens. Someone asks; an authority answers in writing; the answer is copied, collected and cited. The genre runs unbroken from the Geonim of Sura and Pumbedita in eighth-century Iraq, answering questions sent from Spain and North Africa, through the Rif, the Rosh, the Rashba, the Maharam, and on into the modern responsa of Moshe Feinstein on artificial insemination, autopsy and electricity, and the Israeli poskim on military service and organ donation. There are hundreds of thousands of them, and they are simultaneously the case law and the finest social history the Jewish world possesses — what people ate, what they wore, what they fought about, whether a woman could refuse a husband whose trade made him stink, whether a community could tax a wealthy member who claimed residence elsewhere. Because there is no legislature and no synod, the responsum is the mechanism by which the law meets a new situation, which is why the tradition can absorb electricity, in vitro fertilisation and artificial intelligence without a constitutional crisis, and why it can also stall for decades when no authority wants to answer.