The task of breaking the past into manageable, labeled, chunks of time raises several sorts of problems. We can classify them as theoretical, organizational, and ethical.
Periodization poses theoretical problems because any chronological scheme highlights some aspects of the past and obscures others. While a historian of gender might look for eras in which the relative status and power of women and men changed (the granting of suffrage to women, perhaps, or the emergence of patriarchal social relations in early agrarian societies), a historian of war might be more interested in technological changes that transformed military conflict (such as the use of gunpowder or the appearance of the first organized armies), while a historian of religion might look to the appearance of the so-called universal religions in the first millennium BCE. Different questions highlight different aspects of the past and generate different periodizations. To choose a periodization is to make some critical judgments about what is and what is not most important in human history. By focusing on a particular region, era, or topic, historians can avoid some of these challenges, but in world history, periodization requires judgments as to the most important changes across all societies on earth. Is there sufficient consensus among historians as to what those changes are? At present, the answer is probably no.
Periodization also poses severe organizational challenges. How can we find labels that can do justice to many different regions and societies, each with its own distinctive historical trajectory? The problem is peculiarly acute in world history because while neighboring regions or states may evolve in closely related ways, societies separated by large distances may often seem to have little in common. The modern history profession emerged in Europe, and many well-established schemes of periodization were designed to make sense of European history. This is true, for example, of the traditional division into ancient, medieval, and modern periods. Such labels make little sense outside of Europe, but they are so well established that they sometimes get used nevertheless. Similarly, Chinese historians have long used dynastic labels to provide a framework for historical writing, but these, too, are labels that mean little elsewhere. Is it possible to find labels that make sense for Africa as well as for the whole of Eurasia, the Americas, and the Pacific? On this question, too, there is currently no consensus among historians.
Periodization also poses ethical problems because it can so easily imply value judgments. School texts on European history have commonly used such labels as “ Dark Ages,” “Middle Ages,” “Renaissance,” “Scientific Revolution,” and “Age of the Democratic Revolution.” When used of entire historical periods, such labels were by no means neutral. They were generally used with the clear understanding that the Dark Ages were backward, that the Middle Ages were transitional, and that real progress towards modernity began with the Renaissance. Such schemes carry value judgments about different regions as well as different eras, because they implicitly compare the differing levels of “progress” of different regions. Until recently, it was commonly argued that, while Western societies had modernized, many other societies were stuck in earlier historical eras or stages and needed to catch up. Is it possible to construct a system of periodization that avoids imposing the values of one period or region on another?
No system of periodization can satisfy all these different demands. Like historical writing in general, schemes of periodization reflect the biases and judgments of the era that produced them. They also reflect the questions being asked and the scale on which those questions are posed. This means that no single scheme will be appropriate for the many different scales on which historians can and do write about the past.