Ula Zarosa · CC BY 2.0

The philosopher most responsible for putting animal suffering and global poverty on the ethical agenda of ordinary people, not just academics. Animal Liberation (1975) gave the modern animal-rights movement its founding argument; his essay "Famine, Affluence, and Morality" made the case that distance doesn't diminish a moral obligation to help. He founded The Life You Can Save to turn that argument into a habit of giving, and spent decades as Princeton's chair of bioethics pressing utilitarian reasoning into debates most philosophers preferred to leave alone.