  We began with an objection to moral naturalism that applies as well to moral non-naturalism, noting that the former is in a better position to meet it. Let us now direct our attention to an important objection that applies specifically to moral naturalism. Kant argues (Groundwork of the Metaphysic of Morals, Chapter 2, paragraph five) that moral knowledge cannot be based on experience of the natural world. We might interpret one of his arguments as having the following structure. (a) If we have moral knowledge at all, we must know a general moral truth from which we can deduce specific conclusions. But (b) we could know this general truth on the basis of experience only by generalizing from examples of right and wrong that we encounter in experience. Consider then, as a specific example, an act of deliberate cruelty done for fun. How would we know that this is a wrong act or a right act? We must infer it from a general moral truth, according to (a), but to explain how we know a general moral truth, we must, given (b), experience specific examples of right and wrong, and now we are back where we began. To avoid this circle, we must assume that we know a priori that some act or kind of act is wrong. In general, it follows that no moral knowledge is based solely on experience. Moral naturalism cannot, therefore, provide an appropriate ontology for moral knowledge.