Forthcoming in Claus Kreß and Robert Lawless, eds., Necessity and Proportionality in International Peace and Security Law (New York: Oxford University Press) Necessity and Proportionality in Morality and Law Jeff McMahan * I. Introduction If an act would harm someone, there is a moral presumption against doing it. For the act to be permissible, there must be a justification for the harm it would cause. There are various types of moral justification for harming someone. Several of these are based on what the person has done. Most people believe, for example, that by culpably engaging in wrongdoing, a person can deserve to be harmed. If this is so, there can be a desert justification for harming that person. Or, if one person is responsible for making it unavoidable that someone will be harmed, there may be a liability justification for harming that person as a means or side effect of preventing an innocent or nonresponsible person from being harmed instead. Finally, there may be a consent justification for harming a person who has freely and rationally consented to be harmed as a means or side effect of the achievement of some good aim. There are other types of justification for harming a person that may apply independently of what that person has done. The most important of these applies when harming a person is necessary to prevent a greater harm. In the least controversial case of this sort, it can be permissible to harm a person when that is necessary to prevent that same person from suffering a greater harm. Certain forms of consequentialism also imply that it is permissible to harm a person when that is necessary to prevent a greater harm to another person or persons, even if the harm prevented is only slightly greater. The more common view, however, is that there is a moral constraint against harming people that is stronger than any corresponding constraint against allowing people to suffer equivalent harm. Thus, for there to be a justification for harming an innocent person as a necessary means or unavoidable side effect of preventing greater harm to others, the harm prevented must be substantially greater than that inflicted. Only then is the constraint overridden. In moral philosophy, this form of justification is often referred to as a lesser-evil justification. Sources of justification for harming people that are rather more controversial include duties derived from authoritative commands, permissions to give some degree of priority to one’s own interests over those of others, and permissions or duties to give some degree of priority to the interests of those to whom one is specially related. Although I will briefly discuss the last two of * This chapter provides a condensed explanation, intended primarily for legal theorists, of various claims about proportionality that I have defended in other publications that would normally be seen only by philosophers. Although there is a substantial amount of new material here, my main aim has been to collect together a unified set of views about proportionality that would otherwise remain scattered throughout a number of mainly philosophical publications. References to these publications are provided in the notes.