Can Moral Realists Deflect Defeat Due to Evolutionary Explanations of Morality?

I address Andrew Moon's recent discussion (2016, this journal) of the question whether third-factor accounts are valid responses to debunking arguments against moral realism. Moon argues that third-factor responses are valid under certain conditions but leaves open whether moral realists can use his interpretation of the third-factor response to defuse the evolutionary debunking challenge. I rebut Moon's claim and answer his question. Moon's third-factor reply is valid only if we accept externalism about epistemic defeaters. However, even if we do, I argue, the conditions Moon identifies for a valid third-factor response are not met in the case of moral realism.