Abstract
My goal here is to explain why it is harder than one might expect to find a satisfying package that combining the Best System Account of chance and the Principal Principle. One can show that for a certain prima facie attractive version of the Best System Account of chance, the only priors that satisfy the Principal Principle are non-computable. So fans of the Lewisian package must either find a more suitable version of the Best System Account, weaken the Principal Principle, or maintain that rationality requires us to perform tasks beyond the capability of any Turing machine.
