Troubles on moral twin earth: Moral queerness revived

J. L. Mackie argued that if there were objective moral properties or facts, then the supervenience relation linking the nonmoral to the moral would be metaphysically queer. Moral realists reply that objective supervenience relations are ubiquitous according to contemporary versions of metaphysical naturalism and, hence, that there is nothing especially queer about moral supervenience. In this paper we revive Mackie's challenge to moral realism. We argue: (i) that objective supervenience relations of any kind, moral or otherwise, should be explainable rather than sui generis; (ii) that this explanatory burden can be successfully met vis-à-vis the supervenience of the mental upon the physical, and in other related cases; and (iii) that the burden cannot be met for (putative) objective moral supervenience relations.This paper is thoroughly collaborative. The order of authorship is alphabetical. During its evolution from early drafts to its present form, we have written several related papers; see Horgan and Timmons (1991 and forthcoming), and Timmons (1990).