 But if that is so [if H screens off the explanatory role of N], then the property of being a veridical perception of a tree [i.e. N] never has an explanatory role, since it is never instantiated without the property of being indiscriminable from such a perception being instantiated as well. But if the property of being a veridical perception lacks any explanatory role, then we can no longer show that being indiscriminable from a veridical perception has the explanatory properties which would screen off the property of being a veridical perception (2004: 69).