 In the nonreductive physicalism case, the issue is about whether all nonidentity relations are on a par as far as the exclusion argument is concerned. Nonreductive physicalists think that mental properties supervene with metaphysical necessity upon physical properties (whether they are also entailed by physical properties depends upon what property-forming operations the set of physical properties is taken to be closed under; see  Section 3.2).  The interesting question is not whether this enables them to truly say that mental properties are “nothing over and above” physical ones, but rather whether it enables them to solve the exclusion problem. (For views that more or less say that it does, see Yablo 1987, Shoemaker 2001, Bennett 2003, Melnyk 2003). Note that this issue also arises in the case of mereological fusions; Merricks (2001) uses a version of the exclusion problem to argue against the existence of nonliving composites.