Define moral realism as the claim there are objective moral truths and that we know some of them. Consider the argument:
- If no religious beliefs are true, moral realism is false.
- Moral realism is true.
- So, some religious beliefs are true.
I won’t argue for (2), but only for (1). For my argument I will assume a form of reliabilism. (I think the arguments here work well to establish (1) on non-reliabilist epistemologies. The present argument plugs the weakness of that argument.)
Here is the line of thought. Start with this plausible observation:
- If no religious beliefs are true, the correct explanation of our moral beliefs is that moral beliefs were beliefs about unobservable realities that evolved to help prevent defection in prisoner’s dilemmas in cognitively sophisticated hominids.
Now, if reliabilism is true, the question is whether the process, P, of evolutionarily forming beliefs about unobservable realities to help prevent defection in prisoner’s dilemmas is reliable: is likely to produce true belief. But now observe that if no religious beliefs are true, very likely our religious beliefs also arose out of P . Positing supernatural judges who can see if one is sneakily defecting in prisoner’s dilemmas is obviously quite helpful. Thus, we have two families of beliefs produced by P: moral and religious. If the religious ones are all false, the process is unreliable. If the process is unreliable, then its outputs are not knowledge. And so if no religious beliefs are true, we have no moral knowledge, and hence moral realism is false.
Of course, we have the usual tricky thing with reliabilism: What is the relevant level of description of the belief-forming process? Is it: “evolutionarily forming beliefs about unobservable realities to help prevent defection in prisoner’s dilemmas”, or is it something narrower that is special to the moral case, and not present in the religious case? I think it would be difficult, however, to formulate a description narrowed to the moral case without being completely ad hoc.