One component of a moral theory consists of principles that assign moral status to individual actions-principles that evaluate acts as right or wrong, prohibited or obligatory, permissible or supererogatory. Consideration of such principles suggests that they play several different roles. The first such role may be viewed as theoretical. In this role, moral principles specify the characteristics in virtue of which acts possess their moral status. For example, they tell us that an act is right because it maximizes happiness, or is wrong because it violates a promise. Understood in this way, such principles are (roughly) comparable to a scientific theory which specifies the factors governing the state of a given physical system. Thus the gas laws tell us that the volume of a body of gas depends on its pressure and temperature. Such a scientific law can be illuminating even though it may be impossible, at least in particular cases, to ascertain what the pressure and temperature of a given body of gas are. Similarly (or so it appears) moral principles may be illuminating as theories even though it may be impossible, at least in particular cases, to ascertain whether a given action has one of the characteristics (such as maximizing happiness) that determine its moral status. The second function for moral principles may be thought of as practical. This encompasses two subfunctions. On the one hand, there is the second person practical use of moral principles-their use as standards by reference to which an observer can guide another person's behavior by recommending or advising against various courses of action. On the other hand, there is the first person practical use of moral principles-their use as a standard by reference to which a person can guide his or her own behavior: a standard to help the