 Martin argues that the screening-off stage of the argument is only problematic if we accept a positive, non-derivative account of causally matching hallucinations. On such an account, hallucinations have a positive nature which doesn’t derive from that of veridical perception: a nature that can be specified independently of any reference to veridical perception. For instance, hallucinations are direct presentations of sense-data, or representations of ordinary objects. Instead, Martin suggests, the disjunctivist should conceive of causally matching hallucinations in a purely negative epistemic way: such a hallucination as of an F is a state of mind which is not introspectively knowably not a veridical perception of an F. What makes it the case that your hallucinatory experience is as of a snow-covered churchyard, with a certain sort of phenomenal character, is just that it is an occurrence which cannot be discriminated, by introspection alone, from a veridical perception of a snow-covered churchyard. The particular subjective perspective that a hallucinator has in a causally matching hallucination as of a snow-covered churchyard is explained just by the obtaining of this negative epistemic condition, not by anything more positive such as a relation to a white sense-datum or the representation of white snow (c.f., Dancy (1995: 425)). On such a view, causally matching hallucinations are derivative: specifying their nature requires essential reference to the basic case of veridical perception.