  The essence of the argument can be put in more general terms. Both forms of moral naturalism entail that facts that are specifiable in non-moral terms determine the moral facts, where the determination involves either identity or else supervenience without identity. But which moral facts do they determine? Most philosophers will grant that natural facts (or properties) that are specifiable non-morally can be experienced. The problem is in making the step from the latter kind of fact to a moral fact just to establish the moral fact in even one example. Appeal to experience, it may seem, cannot completely account for this step without circularity.