 Armstrong  (1978)  developed an influential theory of “universals” (a category he subdivides into properties and relations) which was distinctive for its emphasis on the idea that universals are “sparse” by comparison with predicates: for example, Armstrong refuses to posit disjunctions of universals. Lewis  (1999,  1) credits Armstrong with convincing him that the concept of naturalness is “commonsensical and serviceable”. Lewis is open to a view that posits Armstrong-style universals in addition to properties and relations conceived of as “abundant”, and analyses ‘perfectly natural property’ as ‘property corresponding to some universal’, i.e. as ‘property that is the property of instantiating \(u\) for some universal \(u\)’. 