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Huw Price
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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 begin by exhibiting a tension in Lewis’s views on these two matters. I present a class of decision problems – some of them themselves Newcomb problems – in which Lewis’s view of the relevance of inadmissible evidence seems in tension with his causal decision theory. I offer a diagnosis for this dilemma, and propose 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 suggests a new view of the relation between causal decision theory and evidential decision theory, namely, that they stand to each other much a chance stands to credence, as objective and subjective faces of the same practical coin.

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