William David Ross (1877–1971) made contributions to ancient philosophy and to moral philosophy. The focus of this entry is his work in the latter area. Ross’s The Right and the Good is one of the most important contributions to moral philosophy published in the twentieth century. In it and other works, Ross develops a novel (pluralistic) deontological ethical theory rivalling Kantianism and utilitarianism. The most innovative element of this theory is the notion of a prima facie duty. Although...