 So how does Causal Principle 2 help the arguer from hallucination? We have to run an argument in “the reverse direction, from what must be true of cases of causally matching hallucinations, to what must thereby be true of the veridical perceptions they match” (Martin 2006: 368). That is, take a hallucination as of a snow-covered churchyard h, and suppose that h is of some fundamental kind H. Now we can apply Causal Principle 2 to show that H is present in a causally matching veridical experience of a snow-covered churchyard, v. For now v is produced by the same kind of proximal cause in circumstances where there is no difference in the non-causal conditions necessary for the occurrence of an event of kind H. This is because all that is necessary for an occurrence of H is some brain condition, which is present in the circumstances in which v is brought about. This reverse causal argument does not show that v is not of fundamental kind N. What it does show, however, is that whatever fundamental kind is present in a hallucinatory case will also be present in a causally matching veridical case. So even if v is fundamentally N it is also H. That is, we have the Reverse Common Kind Assumption: