Case Studies and Theory Development in the Social Sciences - Alexander L. George [118]
We have found Clayton Roberts’ book, The Logic of Historical Explanation, particularly useful.448 Roberts offers a detailed statement of his own position that is, on the whole, remarkably consistent with our concept of process-tracing. Roberts rejects, as do we, the view advanced in the past by some commentators that historical explanation is no more than—and requires no more than—a description of a sequence of events. In principle, he holds, each step or link of a causal process should be supported by an appropriate “law,” defined for historical explanation by Carl Hempel as a statement of a regularity between a set of events. Roberts distinguishes, however, between universalistic and probabilistic laws. While the Hempelian “covering law” model is deductive in form, it is clear that no explanation using probabilistic laws can be strictly deductive. Moreover, the covering law model cannot explain, Ernest Nagel observed, “collective events that are appreciably complex.”449 Given this problem, Roberts observes, “historians rarely seek to explain the occurrence of a complex event by subsuming it solely under a covering law,” a process that he calls ″macrocorrelation.″ Attempts to rely on macrocorrelation to explain complex events have failed: “The vast majority of historians do not use macrocorrelation to explain the occurrence of events they are studying, and those who do have met with little success.”450
How, then, Roberts asks, do historians explain the occurrence of complex historical events if not by subsuming them under covering laws? Roberts argues that they do so “by tracing the sequence of events that brought them about.” The similarity to what we call “process-tracing” is clear. Roberts notes that a number of earlier writers have made the same point, referring to process-tracing variously as “a genetic explanation” (Ernest Nagel), “a sequential explanation” (Louis Mink), “the model of the continuous series” (William Dray), “a chain of causal explanations” (Michael Scriven), “narrative explanations” (R. F. Atkinson), and “the structure of a narrative explanation” (Arthur Danto). Roberts chooses to call this explanatory process “colligation,” drawing on earlier usages of this term and clarifying its meaning.451
Roberts’ contribution is to explicate better than earlier writers the logic of such historical explanations. Laws that embody but are no more than “regularities” and “correlations,” he argues, are not adequate explanations. A mere statement of a correlation, such as that between smoking and cancer, may have some explanatory power, but it is incomplete and unsatisfactory unless the causal relation or connection between the two terms is specified. He notes that historians and philosophers have given many names to such causal connections. (Later, Roberts refers approvingly to the recent philosophy of scientific realism and its emphasis on the need to identify causal mechanisms.)
Given that a correlation is not a substitute for investigating causation, how then can one determine whether some correlations are causal and others are not? Roberts asserts (as others, including ourselves, do) that it is only through colligation (process-tracing) that this can be done. He notes that historians, like geologists, often rely on process explanations to answer the question, “What has happened [to bring this about]?”
Roberts regards efforts to explain complex events solely by invoking a covering law insupportable for two reasons: it is rarely possible to formulate general covering laws for this purpose, and reliance solely on them foregoes the necessary process-tracing of the sequence in the causal chain. Each step in such