Most philosophers dream of having one big, deeply influential idea. Peter Singer has had three. His 1975 book Animal Liberation served as the founding text of the animal rights movement, and provided a powerful ethical rationale for vegetarianism. In Practical Ethics and Rethinking Life and Death, he challenged traditional ideas about personhood and offered a vigorous defense of abortion and, in some cases, infanticide. His 1972 essay “Famine, Affluence, and Morality” used a simple thought experiment to back up a powerful conclusion: people in rich countries, he thinks, are morally obligated give away a big portion of their income to poor people in the developing world.