A law requires black bus passengers to sit in the back of the bus. The U.S. Food and Drug Administration approves a drug for use by black heart failure patients. A state refuses to license drivers under age 16. A company avoids hiring women between the ages of 20 and 40. We routinely draw distinctions among people on the basis of characteristics that they possess or lack. While some distinctions are benign, many are morally troubling.

In this boldly conceived book, Deborah Hellman develops a much-needed general theory of discrimination. She demonstrates that many familiar ideas...