In “Morality and Mathematics: The Evolutionary Challenge” (Ethics 2012), Justin Clarke-Doane raises fascinating and important issues about evolutionary debunking arguments. He argues that insofar as our knowledge of the evolutionary origins of morality poses a challenge for moral realism, exactly similar difficulties will arise for mathematical realism. Clarke-Doane concentrates on the claim that we were not selected to have true moral beliefs, which he interprets to mean that we would have evolved the very same moral beliefs even if the moral facts were radically different from what we take them to be. He argues that an analogous claim holds with respect to our mathematical beliefs: we would have evolved the same mathematical beliefs even if the mathematical facts were radically different from what mathematical realists take them to be. However, even if Clarke-Doane is correct in this, we suspect that his points miss two other kinds of evolutionary debunking arguments, which look to pose a special problem for moral realism.
First, Clarke-Doane twice quotes this claim by Sharon Street: “to explain why human beings tend to make the normative judgments that we do, we do not need to suppose that these judgments are true” (Street, “Reply to Copp”, 208). We take Street’s point to be that one can give a complete explanation of why humans tend to make certain moral judgments rather than others without ever saying anything that implies that any moral beliefs are true. This claim is only about what needs to be said in a complete explanation. It does not assume that moral truths or facts could be different than they are now. Moreover, this claim has no parallel regarding mathematics, because arguably a complete explanation of why humans tend to make certain mathematical judgments (e.g. 1+1=2) rather than others (e.g. 1+1=0) would need to say or imply that 1+1=2 and 1+1≠0. Hence, an evolutionary debunking argument based on this claim by Street understood in this way is not affected by Clarke-Doane’s points.