From Alan Jacobs:

Long ago Thomas Kuhn introduced into the history of science the concept of incommensurability: theories whose premises are so radically divergent that adherents of one theory simply cannot speak coherently and usefully with adherents of another. Alasdair MacIntyre would later, in After Virtue, apply this concept to debates in moral philosophy: “Every one of the arguments is logically valid or can be easily expanded so as to be made so; the conclusions do indeed follow from the premises. But the rival premises are such that we possess no rational way of weighing the claims of one as against another…. It is precisely because there is in our society no established way of deciding between these claims that moral argument appears to be necessarily interminable.”