In 1971, Simon Blackburn worked out an argument against moral realism appealing to the supervenience of the moral realm on the natural realm.1 He has since revised the argument, in part to take account of objections,2 but the basic structure remains intact. While commentators3 seem to agree that the argument is not successful, they have not agreed upon what goes wrong. I believe this is because no attempt has been made to see what happens when Blackburn's argument is addressed to (...) particular varieties of moral realism. As I see it, we must look to these various brands if we want to understand just where the concept of supervenience can be usefully employed. (shrink)