In the ancient world, the idea that killing animals for food is wrong arose mainly from belief in a deep continuity between the animal and human psyche. The underlying thought is that the victimization of an animal is sinful and dehumanizing. Among the Greeks, orphic ritual and mysticism mixed with philosophy prescribe a vegetarian diet as a condition of self-purification. Perhaps the major extant work on vegetarianism dating from classical antiquity is On Abstinence from Animal Flesh by the neo-Platonist Porphyry, the student and biographer of Plotinus, himself a vegetarian. Peter Singer's immensely popular book Animal Liberation (1975) almost immediately generated a new movement for animal rights as distinct from a program limited to animal welfare, animal protection, and prevention of cruelty. This article explores the link between animal rights and political theory, focusing on the views of such thinkers as John Wesley, Bernard Mandeville, Francis Hutcheson, David Hume, Jean-Jacques Rousseau, Jeremy Bentham, Tom Regan, Immanuel Kant, Christine M. Korsgaard, and Charles Hartshorne.