A nice summary of a general problem in the social sciences, from Eric Hilt:
The historians of capitalism would also have benefitted from incorporating counterfactuals into their analysis. These are, of course, thought experiments in which some condition is changed, contrary to fact, and the consequences considered. Many historians apparently have a strong distaste for counterfactual histories … Yet the reason economic historians think about counterfactuals is not due to an interest in specifying alternative histories. Rather, it is because all statements about causal relationships contain counterfactuals. To say that the gold standard caused the Great Depression is to say that absent the gold standard, the Great Depression would not have happened; these two statements are equivalent. There is of course no way to know exactly what the international monetary system of the late 1920s would have looked like in the absence of the gold standard without making strong assumptions, but thinking about that world helps identify forces unrelated to the gold standard that may have contributed to the Great Depression. Economic historians typically investigate counterfactuals not by specifying counterfactual histories, but by comparing cases where a condition is present to cases where it is absent.
For it is written, Thou shalt do homage to the counterfactual, at least a little bit, and a little bit explicitly (not poetically).