  Naturalism in this sense is opposed by those who reject a parsimonious naturalistic metaphysics and stand willing to allow a domain of nonnatural or supernatural facts to play an essential role in our understanding of morality. Naturalism is also opposed by “anti-realists”, including error theorists, constructivists, relativists, and expressivists. According to error theorists, there are no moral facts of any kind. And according to constructivists, relativists, and expressivists, there are moral facts, but these facts are subjective, rather than objective. Anti-realists hold that, if there are any moral facts, these facts are merely products of our contingent attitudes. Anti-realists are typically moral naturalists according to the more general definition of ‘moral naturalism’, as anti-realists typically attempt to understand morality in terms that are consistent with general philosophical naturalism. But for the purposes of this entry, anti-realism should be understood as opposed to moral naturalism.