The periodization of history, like chocolate cake, can have some bad effects on us, but it is hard to resist. We realize, of course, that Julius Caesar didn’t think of himself as “Classical” and Richard the Lionhearted didn’t regard the time in which he lived as the Middle Ages. Placing historical figures in subsequently defined periods separates us from them and impairs our ability to understand them on their own terms. But it is difficult to understand anything about them at all if we try to envision history as continuous and undifferentiated. We need periodization to organize events that are numerous, remote, and unfamiliar, and to create stable images of cultures that are dramatically different from our own. One of the greatest services...