Dreier on the supervenience argument against robust realism

Blackburn has put forward a very influential argument against moral realism, which turns around the supervenience relation. Dreier's version of the supervenience argument has a narrower target. It should be effective against non-reductive, robust moral realism, by revealing an explanatory cost that non-robust, naturalistic forms of moral realism do not have. The present paper argues that naturalist realism can explain the necessity involved in the supervenience relation only by assuming an unrestricted application of Boolean closure principles. Pending an independent argument to the effect that even an indefinite and perhaps infinite combination of natural properties is itself a natural property, robust realists are entitled to reject Dreier's attempt to build the Boolean closure clause into the very formulation of strong supervenience. The conclusion is that moral naturalists cannot claim a clear advantage over robust realists with regard to the challenge posed by the supervenience argument. A final section makes some remarks on how moral expressivism could answer the supervenience argument.