The feature of moral judgements that whilst a moral judgement may concern a particular subject in a particular situation, it must supervene upon general features of the situation, that can in principle occur in other cases. Someone making a judgement thus incurs an obligation to treat those other cases alike. The principle is purely formal, since its application will depend upon a selection of what it is that makes other cases alike: this may be a highly abstract feature (‘telling a lie’) or a highly specific concurrence of circumstances (see particularism, situation ethics). Although there is widespread agreement that the idea points to one essential feature of moral thought, there is less certainty about its definition, the source of its authority, and its significance in constraining possible moral positions. Hume sees adoption of the general or common point of view as a necessary feature of evaluation; Adam Smith thinks its authority derives from our dislike of being unable to justify ourselves to others; Kant sees it as a requirement of reason. Hare was the most resolute recent champion of the substantive ethical significance of universalizability. See also categorical imperative, ideal observer.