While much of twentieth-century philosophy busied itself with abstract questions of language, logic, and consciousness, the Australian philosopher Peter Singer (b. 1946) turned philosophy directly toward everyday moral decisions. His 1975 "Animal Liberation" became one of the founding texts of modern applied ethics.

Singer's argument derives from Bentham's utilitarianism and is startlingly simple. The only thing that matters morally is not whether a being can reason or speak but whether it can feel pain and pleasure. If suffering is bad, it does not matter whose suffering it is — a human's pain does not deserve to "count" more than an animal's. To ignore a being's interests merely because it is not of our own species is, for Singer, "speciesism," an arbitrary discrimination just like racism or sexism.

From this principle Singer presented the suffering that industrial animal farming and animal experimentation inflict on billions of sentient beings as a moral scandal. The book was not an abstract thesis but a call meant to change behaviour; it became the philosophical basis of the modern animal rights movement.

Singer applied the same method — the impartial weighing of interests — to other hard questions: global poverty (the moral obligation of rich countries toward distant hunger), life-and-death decisions, the effective altruism movement. Some of his conclusions are highly contested and draw sharp criticism. But his method is clear and his influence large: it lifts philosophy from the armchair and carries it to the clinic, the farm, the donation decision. Singer posed anew, in the most practical form of our age, the question of whom our moral circle includes — that circle which has been steadily widening since antiquity.