 The challenge is to explain the nature of, and the grounding of, these categorical requirements. If we try to explain this in terms of what one could rationally will to serve as requirements incumbent on all agents, or in terms of what one could rationally will everyone to do, we run up against the same problem as afflicted the account that invokes the idea of self-legislation, which we just discussed. A normative naturalist might propose a naturalistic explanation of categorical requirements but would need to take a very different approach (see Copp 2015)