  There is another version of the causal argument that can be used to put pressure on the disjunctivist to accept that there is a common element to veridical perception and causally matching hallucination. (See Johnston 2004 and Martin 2004.) The proponent of this argument may allow for the possibility that there are non-causal constitutive conditions necessary for the occurrence of certain kinds of mental events, where such constitutive conditions may include the presence in the environment of appropriate candidate objects of perception. So this version of the causal argument can accommodate the disjunctivist’s claim that there are significant non-causal conditions necessary for the occurrence of the kind of experience that occurs when one veridically perceives the world. What this version of the causal argument relies upon is, rather, the claim that there are no such significant non-causal conditions necessary for the occurrence of the kind experience that occurs when one hallucinates. The proponent of this modified causal argument claims that there are no non-causal conditions necessary for the occurrence of the kind of experience that occurs when one hallucinates that cannot also obtain when one veridically perceives the world. So whatever kind of experience occurs when you have a causally matching hallucination of an F will also be produced when you have a veridical perception of an F.  So there is, after all, a common element to veridical perception and causally matching hallucination.