  This is convincing when “duty” means “moral duty.” It is less convincing when, as Prichard also thinks, the issue is simply what one ought to do.  He takes there to be only one sense of “ought,” which he treats as “morally ought.” Any other “ought” is treated as really making the non-normative claim that a certain means is efficient for attaining a certain end.  But ethical egoism can be seen as making categorical ought-claims.  And the historical popularity of ethical egoism, which Prichard so often notes, indicates that self-interest is not obviously irrelevant to what one ought to do (in a not specifically moral sense).