We Turing Machines Can’t Even Be Locally Ideal Bayesians

Abstract

Vann McGee has argued that, given certain background assumptions and an ought-implies-can thesis about norms of rationality, Bayesianism conflicts globally with computationalism due to the fact that Robinson arithmetic is essentially undecidable. I show how to sharpen McGee’s result using an additional fact from recursion theory—the existence of a computable sequence of computable realswith an uncomputable limit (a Specker sequence). In conjunction with the countable additivity requirement on probabilities, such a sequence can be used to construct a specific proposition to which Bayesianism requires an agent to assign uncomputable credence—yielding a local conflict with computationalism.