Full Idea
Perhaps we could call a property 'perfectly' natural if its members are all and only those things that share some one universal, ...where the natural properties would be the ones whose sharing makes for resemblance, and the ones relevant to causal powers.
Gist of Idea
Natural properties fix resemblance and powers, and are picked out by universals
Source
David Lewis (New work for a theory of universals [1983], 'Un and Prop')
Book Reference
'Properties', ed/tr. Mellor,D.H. /Oliver,A [OUP 1997], p.192
A Reaction
This is Lewis fishing for an account of properties that does a bit better than the mere recourse to set theory (which he intuitively favours) seems to do. He remains neutral about the ontological status of a universal (though he prefers nominalism).