In his highly significant book, Ethics: Inventing Right and Wrong, the late Oxford philosopher J.L. Mackie rejected moral objectivism and instead defended an error theory. Although Mackie admitted that ordinary moral language and first-level moral beliefs imply moral objectivism, he argued on empirical grounds that moral objectivism is false. Mackie called one of his anti-objectivist arguments the “argument from queerness.” Mackie viewed his argument as having “two parts, one metaphysical, the other epistemological. Since our focus here is on moral ontology, not epistemology, I shall discuss only the metaphysical part.
In the metaphysical part of the argument from queerness, Mackie argues that objective values, including objective moral values, do not exist because they are metaphysically anomalous. He writes, “If there were objective values, then they would be entities or qualities or relations of a very strange sort, utterly different from anything else in the universe.” As I read him, Mackie provides two reasons in support of that claim. (1) First, Mackie assumed that moral objectivism entails nonnaturalism, which Mackie considers ontologically queer. In his words, “Plato’s Forms give a dramatic picture of what objective values would have to be.” (2) Second, if moral objectivism were true, then internalism about moral motivation would also be true. As Mackie puts it, “if there were objective principles of right and wrong, any wrong (possible) course of action would have not-to-be-doneness somehow built into it.”
Mackie later imported his argument from queerness into the philosophy of religion. Given the queerness of objective values, Mackie argues, moral objectivism is more likely on the assumption that theism is true than on the assumption that metaphysical naturalism is true. In his landmark critique of theism, The Miracle of Theism, he states,
Moral properties constitute so odd a cluster of properties and relations that they are most unlikely to have arisen in the ordinary course of events without an all-powerful god to create them. … If … there … are objective values, they make the existence of a god more probable than it would have been without them. Thus, we have … a defensible argument from morality to the existence of a god.
Ironically, what began as an argument against moral objectivism became an increasingly popular argument among some moral objectivists—proponents of ontological versions of the moral argument for God’s existence, to be exact—as an argument for theism and against metaphysical naturalism. Given this popularity as well as Mackie’s own immense influence, this argument is worth a detailed look. Has Mackie shown that objective values are queer given metaphysical naturalism? Let’s consider Mackie’s two reasons in turn.
(1) begs the question against ethical naturalism by assuming without argument a nonnaturalist interpretation of objectivism. By itself, however, moral objectivism is metaphysically neutral since it does not specify whether moral facts and properties are natural, nonnatural, or supernatural. Moreover, nonnatural moral facts and properties are not queer even on the assumption that metaphysical naturalism is true. Metaphysical naturalism only rules out the existence of nonnatural entities that can affect nature; it does not rule out the existence of acausal objects, including abstract objects or irreducible, sui generis moral properties. Of course, ethical nonnaturalism does pose a problem for reductive physicalism, since reductive physicalism by definition rules out irreducible, sui generis moral facts and properties. Since Mackie accepted not only metaphysical naturalism but also reductive physicalism, it is not surprising that Mackie considered nonnatural facts and properties queer. But this biographical information is of little philosophical significance. As Quentin Smith writes, “nonreductive physicalism can allow for nonnatural moral values, as Post has plausibly argued, and most physicalists today accept a nonreductive version” of physicalism.
I conclude, therefore, that Mackie’s argument from queerness fails. On the assumption that metaphysical naturalism is true, it neither follows nor is probable that objective moral truths are “queer.” Thus, Mackie’s argument from queerness does not support the claim that, given metaphysical naturalism, objective moral truths cannot be truths about nonnatural facts or properties.
 J.L. Mackie, Ethics: Inventing Right and Wrong (New York: Penguin, 1977).
 Mackie 1977, 38.
 Ibid., 40. Italics are mine.
 J.L. Mackie, The Miracle of Theism (Oxford: Clarendon Press. 1982), 115-16.
 J.P. Moreland, “The Ethical Inadequacy of Naturalism” Promise (May/June 1996): 36-39, republished electronically at <URL:http://apollos.ws/against-naturalism/The%20Ethical%20Inadequacy%20of%20Naturalism.pdf>; Paul Copan, “Can Michael Martin Be A Moral Realist?: Sic et Non,” Philosophia Christi, Series 2, 1/2 (1999): 45-72. Cf. George I. Mavrodes, “Religion and the Queerness of Morality,” Rationality, Religious Belief, and Moral Commitment (ed. Robert Audi and William J. Wainwright, Ithaca: Cornell University Press, 1986).
 Smith 1998, 174; cf. John Post 1987.
 Mackie 1979, 40.
 David O. Brink, Moral Realism and the Foundations of Ethics (New York: Cambridge University Press, 1989), chapter 3.
 Larry Arnhart, Darwinian Natural Right: The Biological Ethics of Human Nature (State University of New York Press, 1998), 211-30.