29 CAUSATION Christopher Hitchcock Introduction In a paper read before the Aristotelian Society, Bertrand Russell (1913: 1) claimed: All philosophers, of every school, imagine that causation is one of the fundamental axioms or postulates of science, yet, oddly enough, in advanced sciences such as gravitational astronomy, the word “cause” never appears. To me, it seems that . the reason why physics has ceased to look for causes is that, in fact, there are no such things. The law of causality, I believe, like much that passes muster among philosophers, is a relic of a bygone age, surviving, like the monarchy, only because it is erroneously supposed to do no harm. Russell was hardly alone in that opinion. Other writers of the period, such as Ernst Mach, karl Pearson, and Pierre Duhem, also rejected as unscientific the notion of causation. Their view was shared also by most of the logical positivists. Indeed, the concept of causation was regarded with suspicion by philosophers, as well as by many statisticians and social scientists, throughout much of the twentieth century. Contrary to Russell’s claim, however, the most casual perusal of the leading scien- tific journals reveals that causal locutions are commonplace in science. The 2006 volume of Physical Review Letters contains articles with titles like “Inverse Anderson Transition Caused by Flatbands” (by Masaki Goda, Shinya Nishino, and Hiroki Matsuda) and “Softening Caused by Profuse Shear Banding in a Bulk Metallic Glass” (by H. Bei, S. Xie, and E. P. George). Indeed, physicists refer to a variety of phenomena as “effects”: the “Hall effect,” the “kondo effect,” the “Lamb-shift effect,” the “zeeman effect,” and so on. Presumably where there are effects, there are causes as well. Causal claims are even more common in the medical sciences: for example, a 2005 editorial by E. K. Mulholland and R. A. Adegbola in the New England Journal of Medicine bore the title “Bacterial Infections – a Major Cause of Death among Children in Africa.” Given the ubiquity of causal claims in the sciences, causation deserves to be a concept of great interest to philosophers of science. CHRISTOPHER HITCHCOCk Analyses of causation Diverse attempts have been made to analyze causation, and many of the debates that surround the concept of causation stem from fundamental disagreements about the best way to go about the project. Proposed analyses of causation can be divided into two broad categories: reductive and non-reductive. Reductive analyses of causation aim to provide truth-conditions for causal claims in non-causal terms. Non-reductive analyses of causation aim to establish systematic relationships between causation and other concepts of interest to philosophers; those relationships can then be used to derive interesting non-causal consequences from causal claims, even when the causal claims cannot themselves be paraphrased without causal remainder. Pressure to provide a reductive analysis of causation comes from at least two sources: epistemology and metaphysics. Epistemological pressure stems from the unobserv- ability of causal relations: we may observe the hot sun and the soft wax, but we do not observe the sun’s causing the wax to soften. Thus, it seems that in order to assess the truth-value of a causal claim, it must be possible to translate that claim into one that does admit of direct epistemic access. Metaphysical pressure stems from Ockham’s razor: in metaphysical system-building, it is preferable to analyze causal relations away rather than posit them as additional ingredients of the world. Both of these pressures are capable of being resisted. Epistemologically, causal claims may be treated as akin to claims about theoretical entities such as electrons. We do not expect to be able to translate a claim such as that “every hydrogen atom contains one electron” into purely observational terms. All that a reasonable episte- mology can demand of us is that such claims be susceptible to empirical confirmation or disconfirmation, for example, by entailing various observational consequences or by rendering some observations more probable than others. Causal claims are regularly subjected to empirical test in the sciences. In the medical sciences, for example, causal claims are often tested using controlled clinical trials. Such tests are capable of providing strong evidence in support of causal claims without the need to reduce those claims to non-causal claims. Metaphysically, systems that include causation as a basic feature of our world need not be unnecessarily complex: causal relations may well be the sorts of basic constituents of our world into which other relations are analyzed. Challenges There are a number of challenges that an adequate account of causation must meet. First, an account of causation must be able to distinguish between genuinely causal relationships and merely accidental relationships. Suppose, for example, that only a small handful of human beings eat a particular kind of fruit before the species of plant that bears it becomes extinct. By sheer coincidence, all of these people die shortly after eating the fruit. A theory of causation should not then rule that consumption of this particular fruit causes the death: the relationship between eating the fruit and death is merely accidental. In other words, an adequate theory of causation should entail that post hoc ergo propter hoc is, at least sometimes, a fallacy. 318 CAUSATION A second challenge is to distinguish causes from effects. Typically, perhaps even universally, when one event C causes another event E, it is not also the case that E causes C. In such typical cases, an adequate theory of causation must correctly rule that C causes E, but not vice versa. Some philosophers have attempted to address this problem by stipulating that, by definition, causes occur earlier in time than their effects. Thus if we have two events C and E that are related as cause and effect, we can identify the cause as the one that occurs earlier, and the effect as the one that occurs later. This solution to the problem has the disadvantage that it renders claims of backward-in-time causation false by definition. For example, there are solutions to the general field equations of general relativity that permit closed causal curves: time-like trajectories along which an object could travel from spatio-temporal region A to the distant spatio-temporal region B, and then back to A. Along such a trajectory, it may happen that the state of the object at A causes the state of the object at B, and the state of the object at B causes the state of the object at A. While such models may not describe the actual universe, that would seem to be an empirical matter, and not one to be settled a priori by our definitions of “cause” and “effect.” Thus it would be desirable for a theory of causation to provide an independent account of the direction- ality of causation. A third challenge is to distinguish causes and effects from effects of a common cause. It may be, for example, that smoking causes both stained teeth and lung cancer, with the former occurring before the latter. If so, then it may be common for individuals with stained teeth to develop lung cancer later in life. But stained teeth do not cause lung cancer; rather, stained teeth and lung cancer are effects of a common cause. An adequate theory of causation had better be able to mark the distinction. Finally, an account of causation ought to be able to distinguish between genuine causes and pre-empted backups. Suppose, for example, that a building receives its electricity from the city’s main power grid. In addition, the building has a backup generator that will kick in if there is a power failure. When the city’s power grid is functioning properly, it is that power source, and not the backup generator, that causes the lights in the building to be on. A successful theory of causation must be able to mark the difference. Regularity theories of causation Perhaps the best-known attempt to analyze causal relations is that of David Hume: “we may define a cause to bean object, followed by another, and where all the objects similar to the first, are followed by objects similar to the second” (Hume 1977 [1748]: 76; italics in original). Hume, then, analyzes causation in terms of constant conjunction: a cause is always conjoined with its effect. According to Hume, our experience of such a constant conjunction produces in us a customary transition in the mind. Thus “[w]e may . form another definition of cause; and call it, an object followed by another, and whose appearance always conveys the thought to that other” (ibid.: 77; italics in original). It is our impression of that mental operation from which our idea of causation is derived. In the nineteenth century, John Stuart Mill pointed out that simple causes will not invariably be followed by their effects. Thus, for example, smoking will not always be 319 CHRISTOPHER HITCHCOCk accompanied by lung cancer: some smokers may not be susceptible, or may die of other causes before cancer develops. In order to account for this sort of case, John Mackie (1974) developed his theory of INUS conditions. An INUS condition is an insuffi- cient but non-redundant part of an unnecessary but sufficient condition. Thus C will be an INUS condition for E if there is a conjunction of factors ABCD . such that whenever these factors occur together, they are followed by E, but where the factors ABD. without C are not invariably followed by E. This account allows that C may sometimes occur without E and vice versa. One problem with this account is that it may be an accident that all conjunctions of ABCD .