Third factor explanations and disagreement in metaethics

Several moral objectivists try to explain the reliability of moral beliefs by appealing to a third factor, a substantive moral claim that explains, first, why we have the moral beliefs that we have and, second, why these beliefs are true. Folke Tersman has recently suggested that moral disagreement constrains the epistemic legitimacy of third-factor explanations. Apart from constraining third-factor explanations, Tersman’s challenge could support the view that the epistemic significance of debunking explanations depends on the epistemic significance of disagreement. This paper aims to show that disagreement does not constrain the epistemic legitimacy of third-factor explanations in metaethics, and it suggests a way forward in addressing the view that debunking depends on disagreement. First, Tersman’s constraints are impossible to violate, given the assumption that the justification relation exhibits monotonicity. Second, some disagreements are irrelevant, given that they cannot be about beliefs whose reliability the objectivist seeks to defend. Third, actual disagreement about moral beliefs is implausible, given recent ethnographic findings. In light of this discussion, the paper shows that the prospects of the disagreement view depend on which moral beliefs objectivists need to defend and the criteria we use to assess epistemically relevant moral disagreement.