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.
