This essay explores one of the distinctive features of Western Christian thought in the Middle Ages: medieval canon lawyers’ highly abstract and legalistic treatment of sexual intercourse within marriage, a product of their efforts to give legal meaning to the biblical injunction that spouses “shall be one flesh.” It does so by describing the lawyers’ development of a ius in corpus, a right of each spouse to the body of the other for the purpose of sexual intercourse. The canon lawyers treat the ius in corpus as a property right: either as one spouse’s ownership of the other’s body or as a real servitude – a type of easement – that the body of one spouse holds over the body of the other. Moreover, a spouse can demand that “possession” of that right be restored to him or her by court order. The canon lawyers’ reasoning “purifies” marital sex of its concrete, physical features and instead transforms it into a legal act that reifies the fiction of the one marital flesh.