Indeed, how would any system of ethics argue with the Underground Man? If you tell him that God wants him to care for himself and other people, he could just as easily reply that he does not care what God wants and why should he? If you respond that God will send him to hell if he does not do what God wants, you are merely threatening him, not engaging him in ethical debate. Ethical naturalists could threaten too, though we lack the sanction of eternal punishment. Really, if there are no categorical imperatives, all norms in any ethical system will be hypothetical imperatives, and can be rationally rejected with the rejection of the value named in the antecedent clause. The upshot is that EN seems to be no worse off when it comes to formulating and inculcating ethical norms than any other ethical system.