Overall similarity lies at the foundation of David Lewis’s metaphysics (Lewis 1968, 1973a, 1973b). He wrote little about how similarities and differences in various respects might go together to yield overall similarities, though (Lewis 1979) gives some idea of what he had in mind. The Arrow-Sen framework lends itself to studying this aggregation problem as well; and an impossibility theorem, if it applies, limits the possibilities for arriving at overall similarities of the sort that Lewis presupposes. Morreau (2010) presents the case that a variant of Arrow’s theorem does apply. Kroedel and Huber (2013) take a more optimistic view of overall similarity.