Street presents the moral realist with a dilemma posed by the question as to how our human evaluative beliefs are related to human evolution. It is clear, she believes, that evolution has strongly shaped our evaluative attitudes. The question concerns how those attitudes are related to the objective evaluative truths accepted by the realist. If the realist holds that there is no relation between such truths and our evaluative attitudes, then this implies that “most of our evaluative judgments are off track due to the distorting influence of Darwinian processes.” The other alternative for the realist is to claim that there is a relationship, and thus that is not an accident or miracle that our evaluative beliefs track the objective truths. However, this view, Street claims, is scientifically implausible. Street argues therefore that an evolutionary story about how we came to make the moral judgments we make undermines confidence in the objective truth of those judgments. Street's argument is of course controversial and thinkers such as Erik Wielenberg (2014) have argued against evolutionary debunking arguments. Still, many regard such arguments as problematic for morality, particularly when developed as a “global” argument (Kahane, 2010).
Moral realists such as David Enoch (2011) have attempted to respond to Street's argument, though Enoch acknowledges its force and evidently has some worries about the strength of his reply. However, it is not hard to see that a good deal of the force of Street's argument stems from the assumption that naturalism is true, and therefore that the evolutionary process is one that is unguided. It does appear that in a naturalistic universe we would expect a process of Darwinian evolution to select for a propensity for moral judgments that track survival and not objective moral truths. Mark Linville (2009, 391–446) has developed a detailed argument for the claim that it is difficult for metaphysical naturalists to develop a plausible evolutionary story as to how our moral judgments could have epistemological warrant.
Michael Martin (2002), for example, has tried to suggest that moral judgments can be analyzed as the feelings of approval or disapproval of a perfectly impartial and informed observer. Linville (2009) objects that it is not clear how the feelings of such an observer could constitute the intrinsic worth of a person, since one would think that intrinsic properties would be non-relational and mind-independent. In any case, Linville notes that a “Euthyphro” problem lurks for such an ideal observer theory, since one would think that such an observer would judge a person to be intrinsically valuable because the person has intrinsic value.