The Lion, the 'Which?' and the Wardrobe -- Reading Lewis as a Closet One-boxer

Newcomb problems turn on a tension between two principles of choice: roughly, a principle sensitive to the causal features of the relevant situation, and a principle sensitive only to evidential factors. Two-boxers give priority to causal beliefs, and one-boxers to evidential beliefs. A similar issue can arise when the modality in question is chance, rather than causation. In this case, the conflict is between decision rules based on credences guided solely by chances, and rules based on credences guided by other sorts of probabilistic evidence. Far from excluding cases of the latter kind, Lewis’s Principal Principle explicitly allows for them, in the form of the caveat that credences should only follow beliefs about chances in the absence of "inadmissible evidence". In this paper I exhibit a tension in Lewis’s views on these two matters. I present a class of decision problems –- actually, I argue, a species of Newcomb problem –- in which Lewis’s view of the relevance of inadmissible evidence seems to recommend one-boxing, while his causal decision theory recommends two-boxing. I propose a diagnosis for this dilemma, and suggest a remedy, based on an extension of a proposal due to Ned Hall and others from the case of chance to that of causation. The remedy dissolves many apparent Newcomb problems, and makes one-boxing non-controversial in those that remain