Conceivability, possibility and rationality

Chalmers argues that ideal conceivability (conceivability on ideal rational reflection) entails possibility and on this basis assumes that zombies are possible and, therefore, that materialism is false. I argue that the paradigm cases of conceivability intuitions that Chalmers takes to be reliable guides to possibility are not only conceptually coherent, even on ideal rational reflection, but in addition have some rational explanation. The conceivability of zombies, however, has no rational explanation. So it is not ad hoc to deny that the conceivability of zombies entails possibility.