Frank Jackson endorses epiphenomenalism because he thinks that his knowledge argument undermines physicalism. One of the most interesting criticisms of Jackson's position is what I call the 'inconsistency objection'. The inconsistency objection says that Jackson's position is untenable because epiphenomenalism undermines the knowledge argument. The inconsistency objection has been defended by various philosophers independently, including Michael Watkins, Fredrik Stjernberg, and Neil Campbell. Surprisingly enough, while Jackson himself admits explicitly that the inconsistency objection is 'the most powerful reply to the knowledge argument' he knows of, it has never been discussed critically. The aim of this paper is to evaluate the objection and to identify and consider its implications. The objection is alleged to be based on a causal theory of knowledge. I argue that the objection fails by showing that any causal theory of knowledge is such that it is either false or does not support the inconsistency objection. In order to defend my argument, I offer a hypothesis concerning phenomenal knowledge.