  Obviously there is a lot more that can be said by both parties to this debate.  For example, for useful more recent discussion of the more general issue of whether there can be necessary connections between distinct existences, see Wilson 2010.  In any event, it is good that non-naturalists in moral philosophy are not merely relying on the appeal to companions in guilt, but are also actually offering positive explanations of supervenience.  Shafer-Landau offers an ambitious explanation of supervenience in non-naturalist terms.  Even if his initial strategy fails for some of the reasons discussed above, it at least is a step in the right direction, and it might be possible to refine his strategy in a way to avoid some or all of the worries raised above.  It is early days for this approach to supervenience in the case of moral philosophy, and it will be interesting to see how this debate plays out.  For further discussion of Shafer-Landau’s strategy here, see also Mabrito 2005, Shafer-Landau 2005, Wedgwood 2009 and McPherson 2012. Useful discussion can also be found in Dreier 2019, Gert 2019, and Moberger 2019. 