This essay discusses how medieval authors from the thirteenth and fourteenth centuries dealt with a philosophical problem that social institutions pose for the Aristotelian dichotomy between natural and artificial entities. It is argued that marriage, political community, and language provided a particular challenge for the conception that things which are designed by human beings are artefacts. Medieval philosophers based their arguments for the naturalness of social institutions on the anthropological view that human beings are political animals by nature, but this (...) strategy required rethinking the borderline between nature and art. The limits of nature were extended, as social institutions were considered to be natural even though they are in many ways similar to artificial products. (shrink)