Abstract

I argue against the halfer response to the Sleeping Beauty case by presenting a new problem for halfers. When the original Sleeping Beauty case is generalized, it follows from the halfer’s key premise that Beauty must update her credence in a fair coin’s landing heads in such a way that it becomes arbitrarily close to certainty. This result is clearly absurd. I go on to argue that the halfer’s key premise must be rejected on pain of absurdity, leaving the halfer response to the original Sleeping Beauty case unsupported. I consider two ways that halfers might avoid the absurdity without giving up their key premise. Neither way succeeds. My argument lends support to the thirder response, and, in particular, to the idea that agents may be rationally compelled to update their beliefs despite not having learned any new evidence.