I’ve been studying Plantinga’s very interesting paper, “Naturalism, Theism, Obligation, and Supervenience.” (See here for Ex-Apologist’s very brief post about it.) Plantinga’s stated goal is to show that metaphysical naturalism cannot accommodate realism about moral obligation by "displaying the failure of the most natural way of arguing" that metaphysical naturalism can accommodate moral realism, viz., supervenience.
There are many things about this paper which I find interesting.
First, unlike William Lane Craig, Plantinga directly interacts with some of the strongest cases for naturalistic moral realism, such as those provided by David Brink and Peter Railton.
Second, Plantinga concedes what many critics of moral arguments (ranging from Richard Swinburne to Michael Martin) have argued, viz., that moral properties do supervene on nonmoral properties. Indeed, Plantinga argues that moral properties strongly supervene on nonmoral (or descriptive) properties and so the two are, in some sense, equivalent.
Third, (in footnote 14) Plantinga makes the interesting (and, IMO, correct) observation that theism does not entail that there be such a thing as moral obligation, since theism does not entail that there be created rational agents. Rather, he says, “what is necessary is that if there are rational agents, there is such a thing as moral obligation.”
This is arguably the most important (and most interesting) part of the entire paper. In his words, “To show that obligation is naturalistic, one must find a naturalistic property that is much more tightly connected with obligation; mere equivalence isn’t sufficient.” Plantinga thinks SMPNMP is an insoluble problem for metaphysical naturalism conjoined with moral realism: in his words, “one can’t show that rightness is naturalistically acceptable by finding a naturalistic property to which it is equivalent.”
If, like me, you find these things interesting, then you’ll want to read his paper for yourself.