  Blackburn’s supervenience argument is a second argument against realism. Blackburn formulated the argument in terms of moral properties, as follows. If there are moral properties, then they supervene on non-moral properties as a matter of conceptual necessity. That is, in every conceptually possible world, if two things share all non-moral properties, they share all moral properties. But if there are moral properties, the pattern of supervenience is not itself conceptually necessary. So, even if all Ps are Ms in fact, there is some conceptually possible world in which there is a P which isn’t an M. Blackburn’s question is this: if moral properties can come apart from non-moral properties across worlds, why can’t they come apart from them within worlds?  That is: what explains the “ban on mixed worlds”? A similar problem can be formulated for truth as a feature of moral propositions. What explains the ban on conceptually possible worlds in which one moral proposition &lt;x is M&gt; is true, while another moral proposition &lt;y is M&gt; is false, but in which all relevant non-moral propositions &lt;x is P&gt; and &lt;y is P&gt; are alike in truth-value? What is not immediately clear is, first, how deflationary moral propositions will prove useful to the expressivist in answering this question, and second, how, supposing they do prove useful, why they won’t prove equally useful to the realist.