Let the Guidance Constraint be the following norm for evaluating ethical theories: Other things being at least roughly equal, ethical theories are better to the extent that they provide adequate moral guidance. I offer an account of why ethical theories are subject to the Guidance Constraint, if indeed they are. We can explain central facts about adequate moral guidance, and their relevance to ethical theory, by appealing to certain forms of autonomy and fairness. This explanation is better than explanations that feature versions of the principle that ‘ought’ implies ‘can’. In closing, I address the objection that my account is questionable because it makes ethical theories subject not merely to purely theoretical but also to morally substantive norms.