A debate has recently appeared regarding whether non-naturalism is better than other metaethical views at explaining moral progress. I shall take the occasion of this debate to present a novel debunking dilemma for moral non-naturalists, extending Sharon Street's Darwinian one. I will argue that moral progress indicates that our moral attitudes tend to reflect contingent sociocultural and psychological factors. For non-naturalists, there is then either a relation between these factors and the moral facts, non-naturalistically construed, or there is not. If (...) there is no relation, the contingent factors are unlikely to lead to moral knowledge. If there is a relation, they must be likely to lead to non-naturalist-style moral knowledge, but no theoretically virtuous explanation of moral progress is likely to accommodate non-naturalist commitments. It follows that non-naturalist moral realism cannot explain our moral knowledge. I call this a contingentist challenge to non-naturalism. (shrink)