The fixity of reasons

I consider backtracking reasoning: that is, reasoning from backtracking counterfactuals such as if Hitler had won the war, he would have invaded Russia six weeks earlier. Backtracking counterfactuals often strike us as true. Despite that, reasoning from them just as often strikes us as illegitimate. A number of diagnoses have been offered of the illegitimacy of such backtracking reasoning which invoke the fixity of the past, or the direction of causation. I argue against such diagnoses, and in favor of one that invokes a principle I call the fixity of reasons. Backtracking reasoning violates the fixity of reasons. But, the fixity of reasons is a principle that must be observed in order to engage in practical reasoning at all.