Cosmopolitan Local Law in the Medieval Latin West

Medieval lay law, especially customary law, is known as a quintessential form of 'local knowledge'. It serves in legal history as the inward-looking and parochial foil to the 'common laws' (ius commune) of the medieval Latin West: the Roman law of universities and the canon law of the church. These conventional notions of common and particular have overdetermined how we think of medieval law. This article argues that John of Ibelin's Assises de Jerusalem (c.1264), a lawbook written for the Kingdom of Jerusalem and Cyprus, shows that the jurisdiction and authority of lay customary law did not have to be based on changelessness or on the cohesive identity of a homogenous community but could also be based on constant updating and cosmopolitanism. John imagined this, not accidentally, in the context of conquest and Christian mission. His vision offers crucial insight into the universalizing potential of 'local' law and, as such, reconfigures our understanding of medieval law broadly.