Full Idea
Lewis argues that there are natural properties, which makes various analyses possible, especially of similarity in intrinsic respects. Naturalness comes in degrees, with perfectly natural properties being the limiting case.
Gist of Idea
Natural properties figure in the analysis of similarity in intrinsic respects
Source
report of David Lewis (New work for a theory of universals [1983]) by Alex Oliver - The Metaphysics of Properties 4
Book Reference
-: 'Mind' [-], p.18
A Reaction
This sounds to be the wrong way round. We don't start with similarities and work back to natural properties. We encounter natural properties (through their causal action), and these give rise to the similarities.