A philosopher who thinks substantive necessities obtain in re, this paper argues, need not believe in non-actual worlds, or maximal consistent sets of propositions, but merely in properties. For most properties, on even the sparsest property realism, are flanked by contraries with which they cannot be co-instantiated. True, Armstrong has shown that the impossibility that a property bearer should bear each of two contraries is sometimes just the impossibility that the bearer should be identical with its own proper part-hence is no substantive impossibility. But for many genuine contraries Armstrong's analysis fails; their incompatibility cannot be reduced to facts of identity. The main examples are dispositional properties, so the paper also argues that being dispositional is no bar to a property's being real in its own right.
Elder, Crawford, "Ontology and Realism about Modality" (1999). Philosophy Articles. 7.