 Once we have the notion of qualitative indiscernibility, we can use it to analyse the notion of a qualitative property. Intuitively, a qualitative property is one that is not “about” any individual: admiring a famous philosopher is qualitative, while admiring David Lewis is not. For Lewis, a monadic property \(F\) is qualitative if and only if whenever \(x\) is qualitatively indiscernible from \(y\), \(x\) has \(F\) just in case \(y\) has \(F\). As in the case of duplication, this analysis is tenable only given a super-abundant ontology like Lewis’s. Non-modal realists may, however, adapt the idea by using a world-relativised relation of qualitative indiscernibility, whose definition stands to  Indiscernibility  as  Cross-World Duplication  stands to  Duplication.[26] 