Simon Blackburn’s supervenience argument—focusing on the mysterious “ban on mixed worlds”—is still subject to a variety of conflicting interpretations. In this paper, I hope to provide a defense of the argument that clarifies both the argument and the objections it must overcome. Many of the extant objections, I will argue, fail to engage the argument in its true form. And to counter the more compelling objections, it will be necessary to bring in additional argumentation that Blackburn himself does not clearly (...) provide. One important upshot of this discussion is that moral realism is not the only target of the argument: it raises trouble for a variety of metaethical views wed to realist-friendly construals of moral evaluation. Rightly understood, this argument not only survives the prominent responses, but turns out to be more powerful than his critics and Blackburn himself have recognized. (shrink)