Full Idea
To class nominalism we can add a primitive distinction between natural and unnatural classes.
Gist of Idea
We can add a primitive natural/unnatural distinction to class nominalism
Source
David Lewis (Against Structural Universals [1986], 'Why believe')
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.86
A Reaction
Lewis explores this elsewhere, but this looks like a very complex concept to play the role of a 'primitive'. Human conventions seem to be parts of nature.