  Because of this stress on figuring out what makes a difference in a context, medicine is a natural home to manipulationist accounts of causation, as Whitbeck also saw (1977, 627–28). The basic idea behind the manipulationist approach is that it puts us in a position to begin explaining a phenomenon “when we have identified factors or conditions such that manipulations or changes in those factors or conditions will produce changes in the outcome being explained” (Woodward 2003, 10). We may have a correlation—say that between smoking and heart disease. We get from there to a causal explanation if we can intervene so as to manipulate the value of the causal variable by itself—by stopping one group from smoking and letting an otherwise identical group smoke, for example. If smoking causes heart disease, we will see an effect on the cancer variable that reflects our meddling with the smokers. If smoking doesn’t cause heart disease then there will not be a correlation between the two variables. (This view is compatible with many proposals about the underlying metaphysics. There will be more on this account of causation in section 3.1. For the technical details see Spirtes et al 2000, Pearl 2009).