Even setting aside the rich interplay between naturalism, nonnaturalism, and noncognitivism that is one of the hallmarks of twentieth-century moral philosophy, a very rich history remains, one that is impossible to even fully summarize here. Much of the story may be found elsewhere in the present volume in discussions devoted to particular moral theories and philosophers. The present entry examines just a few of the many themes that have occupied the attention of moral philosophers working within a diversity of traditions and that have thus exercised substantial influence on the shape of moral philosophy in the twentieth century.

Moral Principles

Whether the number of principles governing right conduct is one, or several, or even indefinitely many is a question that has animated the development of moral philosophy over the past century. Of course, this...