My aim is to challenge recent attempts at reconciling moral realism and naturalism by pushing ethical naturalists into a dilemma. According to one horn of the dilemma, ethical naturalists must either (a) build unique facts and properties about divergent social structures (or varying moral belief systems) into their subvenient sets of natural facts and properties, and so jeopardize the objectivity of moral truths, or (b) insist, in the face of all possible worlds in which people have different moral beliefs than ours, that they are all mistaken—this despite the fact that the belief-forming mechanism responsible for their moral beliefs was never concerned with the truth of those beliefs in the first place. This will bring me to suggest that moral properties might only weakly supervene upon natural phenomena. But, according to the other horn of the dilemma, weak supervenience is a defeater for moral knowledge.