Since 1973, with the simultaneous publication of his essay “Animal Liberation” in the New York Review of Books and the National Observer, Peter Singer has been the leading advocate of animal rights from a rigorously argued and philosophically defined ethical perspective. In that essay, Singer popularized Richard Ruder’s concept of “speciesism.” Singer defined speciesism as “the belief that we are entitled to treat members of other species in a way in which it would be wrong to treat members of our own.” Singer’s use of the term “animal liberation” tied the liberation of animals to the other liberation movements of the early 1970’s, such as the movements for the liberation of blacks and women. Singer’s work can be understood as a philosophical examination of how far the human moral horizon and human ethical obligations extend. Once humans extend that horizon to include animals, then practices that were once regarded as natural and inevitable, such as using animals for food, clothing, or medical research, can no longer be simply assumed to be ethical. According to Singer, some of these practices will “now be seen as intolerable.”