Goodness and Moral Twin Earth

Terry Horgan and Mark Timmons's "Moral Twin Earth" thought experiment allegedly undercuts virtually any form of naturalist moral realism. I argue that a neo-Aristotelian conception of moral properties defeats Moral Twin Earth. Developing themes in the work of Peter Geach, Philippa Foot, and Rosalind Hursthouse, I sketch an Aristotelian moral semantics that is unique in construing terms like 'right' and 'good' exclusively as attributive adjectives that denote relational properties. On this view, moral goodness is a relational property predicated of those human beings that satisfy kind-relative criteria of goodness; i.e., morally good human beings are good qua human being. The reference of moral terms is therefore fixed by the natural properties of human beings. This account ensures that moral facts cannot differ across worlds in which the relevant natural facts do not differ and thus defeats Moral Twin Earth.