St. Paul says that same-sex sexual acts are “contrary to nature.” Plainly this is intended as a condemnation, but beyond that its meaning is obscure. In particular, we are given no general account of what it means to be contrary to nature, including what other acts might fit this description. This article attempts to provide such an account. It relies for this purpose on the biblical and classical sources of this idiom as well as its subsequent use within the Greek (...) patristic tradition. It argues that the core meaning of “contrary to nature” is that of violating the integrity of structure and function of the human body. The article further contrasts this biblical and Greek patristic understanding with that of the “sin against nature” developed in the West during the Middle Ages. It argues that they are different in important ways and that a recovery of the biblical and Greek patristic view can help restore the concept of the unnatural to its proper place within Christian ethics. (shrink)