world. A series of "What If?" books edited by Robert Cowley presented dozens of essays by historians or prominent writers about "how a slight turn of fate at a decisive moment could have changed the very annals of time." Some scholars argue that a counterfactual is not as much a matter of what happened in the past but it is the disagreement about which past events were most significant. For example, William Thompson employs a sequence of counterfactuals for eight lead economies that have driven globalization processes for almost a thousand years. From Song dynasty in China to Genoa, Venice, Portugal, the Netherlands, Britain, and the United States, and claims that each actor in succession played an unusually critical role in creating a structure of leadership that became increasingly global in scope across time. Differences from alternate history Counterfactual history is neither historical revisionism nor alternate history. Counterfactual history distinguishes itself through its interest in the very incident that is being negated by the counterfactual, thus seeking to evaluate the event's relative historical importance. Historians produce arguments subsequent changes in history, outlining each in broad terms only, since the main focus is on the importance and impact of the negated event. An alternate history writer, on the other hand, is interested precisely in the hypothetical scenarios that flow from the negated incident or event. A fiction writer is thus free to invent very specific events and characters in the imagined history. An example of a counterfactual question would be: "What if the Pearl Harbor attack did not happen?"; whereas an alternate history writer would focus on a possible series of events arising therefrom. The line is sometimes blurred as historians may invent more detailed timelines as illustrations of their ideas about the types of changes that might have occurred. But it is usually clear what general types of consequences the author thinks are reasonable to suppose would have been likely to occur, and what specific details are included in an imagined timeline only for illustrative purposes. The line is further blurred by novelists such as Kim Stanley Robinson, whose alternate-history novel The Years of Rice and Salt has a character talking of historians' use of counterfactuals, within the novel's alternate history. He dismisses this as "a useless exercise". Criticism Most historians regard counterfactual history as perhaps entertaining, but not meeting the standards of mainstream historical research due to its speculative