One cannot deny that it is more difficult to extract general principles from studying history than from studying planetary orbits. However, the difficulties seem to me not fatal. Similar ones apply to other historical subjects whose place among the natural sciences is nevertheless, secure, including astronomy, climatology, ecology, evolutionary biology, geology, and paleontology. People's image of science is unfortunately often based on physics and a few other fields with similar methodologies. Scientists in those fields tend to be ignorantly disdainful of those fields in which those methodologies are inappropriate and which must therefore seek other methodologies -- such as my own research areas of ecology and evolutionary biology. But recall that the word "science" means "knowledge" (from the Latin scire, "to know," and scientia, "knowledge"), to be obtained by whatever methods are most appropriate to the particular field. Hence I have much empathy with students of human history for the difficulties they face.
Historical Sciences in the broad sense (including astronomy and the like) share many features that set them apart from nonhistorical sciences such as physics, chemistry and molecular biology. I would single out four: methodology, causation, prediction and complexity.
In physics the chief method for gaining knowledge is the laboratory experiment, by which one manipulates the parameter whose effect is in question, executes parallel control experiments with that parameter held constant, holds other parameters constant throughout, replicates both the experimental manipulation and the control experiment, and obtains quantitative data. This strategy, which also works well in chemistry and molecular biology, is so identified with science in the minds of many people that experimentation is often held to be the essence of the scientific method. But laboratory experimentation can obviously play little or no role in many of the historical sciences. One cannot interrupt galaxy formation, start and stop hurricanes and ice ages, experimentally exterminate grizzly bears in a few national parks, or rerun the course of dinosaur evolution. Instead, one must gain knowledge in these historical science by other means, such as observation, comparison, and so called natural experiments (to which I will return in a moment).
Historical sciences are concerned with chains of proximate and ultimate causes. In most of physics and chemistry the concepts of "ultimate cause," "purpose," and "function" are meaningless, yet they are essential to understanding living systems in general and human activities in particular. For instance, an evolutionary biologist studying Arctic hares whose color turns from brown in summer to white in winter is not satisfied with identifying the mundane proximate causes in terms of the fur pigments' molecular structure and biosynthetic pathways. The more important questions involve function (camouflage against predators?) and ultimate cause (natural selection starting with ancestral hare populations with unchanging fur color?). Similarly, a European historian is not satisfied with the condition of Europe in both 1815 and 1918 as having achieved a just peace after a costly pan-European war. Understanding the contrasting chains of events leading up to the two peace treaties is essential to understanding why an even more costly pan-European war broke out again within a few decades of 1918 but not of 1815. But chemists do not assign a purpose or function to a collision of two gas molecules, nor do they seek the ultimate cause for the collision.
Still another difference between historical and non-historical sciences involves prediction. In chemistry and physics the acid test of one's understanding of a system is whether one can successfully predict its future behavior. Again, physicists tend to look down on evolutionary biology and history, because those fields appear to fail this test. In historical sciences, one can provide a posteriori explanations (e.g., why an asteroid impact on earth 66 million years ago may have driven dinosaurs but not many other species to extinction), but a priori predictions are more difficult (we would be uncertain which species would be driven to extinction if we did not have the actual past to guide us). However, historians and historical scientists do make and test predictions what future discoveries of data will show us about past events. The properties of historical systems that complicate attempts at prediction can be described in several alternative ways. One can point out that human societies and dinosaurs are extremely complex, being characterized by an enormous number of independent variables that feed back on each other. As a result, small changes at a lower level of organization can lead to emergent changes at a higher level. A typical example is the effect of one braking driver's braking response, in Hitler's nearly fatal accident of 1930, on the lives of a hundred million people who were killed or wounded in World War II. Although most biologists agree that biological systems are in the end wholly governed by their physical properties, and obey the laws of quantum mechanics, the systems' complexity means, for practical purposes, that that deterministic causation does not translate into predictability. Knowledge of quantum mechanics does not help one understand why introduced placental predators have exterminated so many Australian marsupial species, or why the Allied Powers rather than the Central Powers won World War I.
Each glacier, nebula, hurricane, human society, and biological species, and even each individual and cell of a sexually reproducing species, is unique, because it is influenced by so many variables and made up of so many variable parts. In contrast, for any of the physicist's elementary particles and isotopes, and for the chemist's molecules, all individuals of the entity are identical to each other. Hence physicists and chemists can formulate universal deterministic laws at the macroscopic level, but biologists and historians formulate only statistical trends. With a high probability of being correct, I can predict that, of the next 100 babies born at the University of California Medical Center, where I work, not fewer than 480 or more than 520 will be boys. But I had no means of knowing in advance that my own two children would be boys. Similarly, historians note that tribal societies may have been more likely to develop into chiefdoms if the local population was sufficiently large and dense and if there was potential for surplus food production than if that was not the case. But each such local population has its own unique features, with the result that chiefdoms did emerge in the highlands of Mexico, Guatemala, Peru, and Madagascar, but not in those of New Guinea or Guadalcanal.
Still another way of describing the complexity and unpredictability of historical systems, despite their ultimate determinacy, is to note that long chains of causation may separate final effects from ultimate causes lying outside the domain of that field of science. For example, the dinosaurs may have been exterminated by the impact of an asteroid whose orbit was completely determined by the laws of classical mechanics. But if there had been any paleontologists living 67 million years ago, they could not have predicted the dinosaurs' imminent demise, because asteroids belong to a field of science otherwise remote form dinosaur biology. Similarly, the Little Ice Age of A.D. 1300-1500 contributed to the extinction of the Greenland Norse, but no historian, and probably not a modern climatologist, could have predicted the Little Ice Age,
Thus, the difficulties historians face in establishing cause-and-effect relations in the history of human societies are broadly similar to the difficulties facing astronomers, climatologists, ecologists, evolutionary biologists, geologists, and paleontologists. To varying degrees each of these fields is plagued by the impossibility of performing replicated, controlled interventions, the complexity arising from enormous numbers of variables, the resulting uniqueness of each system, the consequent impossibility of formulating universal laws, and the difficulties of predicting emergent properties and future behavior. Prediction in history, as in most historical sciences, is most feasible on large spatial scales and over long times, when the unique features of millions of small-scale brief events become averaged out. Just as I could predict the sex ratio the next 1000 newborns but not the sexes of my own two children, the historian can recognize factors that made inevitable the broad outcome of the clash between American and Eurasian societies after 13000 years of separate development, but not the outcome of the 1960 U.S. presidential election. The details of which candidate said what in October 1960 could have given the electoral victory Nixon instead of Kennedy, but no details of who said what could have blocked the Europeans conquest of Native Americans.
How can students of human history profit from the experience of scientists in other historical sciences? A methodology that has proved useful involves the comparative method and so called natural experiments. While neither astronomers studying galaxy formation nor human historians can manipulate their systems in controlled laboratory experiments, they both can take advantage of natural experiments, by comparing systems differing in the presence or absence (or in the strong or weak effect) of some putative causative factor. for example, epidemiologists, forbidden to feed large amounts of salt to people experimentally, have been able to identify effects of high salt intake by comparing groups of humans who already differ greatly in their salt intake: and cultural anthropologists, unable to provide human groups experimentally with varying resource abundances for many centuries, still study long term effects of resource abundance on human societies by comparing recent Polynesian populations living on islands differing naturally in resource abundance. The students of human history can draw on many more natural experiments than just comparisons between the five inhabited continents. Comparisons can also utilize large islands that have developed complex civilizations in a considerable degree of isolation (such as Japan, Madagascar, Native American Hispaniola, New Guinea, Hawaii and many others), as well as societies on hundreds of smaller islands and many regional societies within each of the continents.
Natural experiments in any field, whether in ecology or human history are inherently open to potential methodological criticisms. These include confounding effects of natural variation in additional variables beside the one of interest,as well as inferring chains of causation form observed correlations between variables. Such methodological problems have been discussed in great detail i some of the historical sciences. On particular epidemiology, the science of drawing inferences about human diseases by comparing groups of people (often by retrospective historical studies), has for a long time successfully employed formalized procedures for dealing with problems similar to those facing historians of human societies. Ecologists have also devoted much attention to the problems of natural experiments, a methodology to which they must resort in many cases where direct experimental interventions to manipulate relevant ecological variables would be immoral, illegal, or impossible. Evolutionary biologists have recently been developing ever more sophisticated methods for drawing conclusions from comparisons of different plants and animals of known evolutionary histories.
In short, I acknowledge that it is much more difficult to understand human history than to understand problems in fields of science where history is unimportant and where fewer individual variables operate. Nevertheless, successful methodologies for analyzing historical problems have been worked out in several fields. A a result, th histories of dinosaurs, nebulas, and glaciers are generally acknowledged to belong to the fields of science rather than to the humanities. but introspection gives us far more insight into the ways of other humans than into those of dinosaurs. I am thus optimistic that historical studies of human societies can be pursued as scientifically as studies of dinosaurs--and with profit to our own society to day, by teaching us what shaped the modern world, and what might shape our future.