As we know since Ludwig Mitteis, Egypt's incorporation to the Roman Empire brought little change to the legal practice of the local population. Since the province lacked autonomous courts, this would not have been possible without the consistent endorsement of this 'peregrine' law by the Roman jurisdiction - an endorsement fully confirmed by the abundant available evidence. The political rationale behind this Roman attitude is clear enough, and entirely consistent with the general imperial policy of minimum intervention. And yet, within the Greco-Roman political tradition, that required a civitas for the existence of a ius civile, the legal status of this peregrine law - in a province that lacked civitates proper until 200 ce, and whose inhabitants were mostly peregrini nullius civitatis - is a theoretical puzzle. Hans Julius Wolff has maintained that it was, strictly speaking, no law at all: the fall of the Ptolemies deprived it of all its binding force, as the occasional instances of Roman rejection (notably in P. Oxy. n 237, the famous petition of Dionysia) would corroborate; from the point of view of the Roman jurisdiction, there was in Egypt, Wolff argued, a legal vacuum to be filled at discretion. Joseph Meleze Modrzejewski, instead, has insisted on the pertinence here of the notion of customary law: the local legal traditions, as well as the Ptolemaic legislation, were reduced to customary law, persisted as a merely tolerated mos regionis. These ideas are here reassessed, taking into account the discretionary nature of the Roman jurisdiction, the roots of the doctrine of customary law, and of the idea itself of the law as a binding system of rules. © For the book by Fundacja im. Rafata Taubenschlaga.