determined. The morally significant differences between the states can be classified under three headings. First, there is length of life: at least one person lives longer in one state than in the other. Second, there is wellbeing: since life-saving uses up resources, on the whole people will be worse off in the state where lives are saved. But there will usually be exceptions. Saving a life usually benefits several people, including the person who is saved and his dependants. The person may also contribute to the economy's production during his extra years of life, and so benefit the rest of us, but this has to be set against the demands he makes on the world's resources for his own consumption. The third heading is population: the two states may have different populations. In one sense they certainly must have: there is a time when their populations must be different. Immediately after a person's life is saved the world's population is one more than it would have been. But in this paper when I speak of population I mean timeless population: all the people who live at some time or other. In this sense our alternative states do not necessarily have different populations, but often they will have. Saving a life often affects timeless population. The person who is saved may later have children, who may start a whole line of descendants. Or if somebody is not saved and dies his or her spouse may remarry and have more children. Or if a baby dies its parents may have another child instead. And so on. Sometimes the effect may be to change not the numbers of people who live but only their identities; the states may contain different people but the same number altogether. I call this too a difference in population.