Can Robert Adams Survive Moral Twin Earth?

Richard Boyd and Robert Adams have both developed semantic accounts of moral terms based on Hilary Putnam's causal regulation theory for natural kind terms, according to which the terms in question refer to the properties which predominantly causally regulated the terms. However, Terence Horgan and Mark Timmons have mounted an objection to Boyd's semantics—their Moral Twin Earth argument. If this argument is successful against Boyd then it might be thought that it should also be successful against Adams, given the similarity between their semantic accounts. I will argue in this essay that Adams's semantics is sufficiently different from Boyd's to enable him to survive Moral Twin Earth, but that he is vulnerable to a modified version of Moral Twin Earth that I describe.