Political authorities claim to be able to impose moral duties on citizens by the mere expedient of legislating. This claim is problematic - in fact, among theorists, it is widely denied that political authorities have such powers. I argue that the legitimacy of political authority is not contingent upon the truth of its claim to be able to impose moral duties by mere legislation. Such claims are better seen as exercises of semiotic techniques to alter social meanings. These alterations serve to facilitate desirable social change that may not have been antecedently obligatory because of the nonfulfillment of a compliance condition, which normally attaches to any fair-play duty. Where political authority uses the semiotic technique of announcing a legal - and by implication moral - duty, thereby altering social meaning as a means of bringing about the satisfaction of a compliance condition, it makes a claim whose literal falsehood (if false it be) does not derogate from the authority's legitimacy.