Full Idea
For Lewis natural properties are important for their role in making language and thought determinate: principles of charity or humanity tell us to attribute natural properties to predicates wherever possible, break underdetermination of their reference.
Gist of Idea
Lewisian natural properties fix reference of predicates, through a principle of charity
Source
report of David Lewis (New work for a theory of universals [1983]) by Katherine Hawley - How Things Persist 3.8
Book Reference
Hawley,Katherine: 'How Things Persist' [OUP 2004], p.98
A Reaction
Lewis always seems to find reasons in semantics or logic for his metaphysics, instead of in the science. Lewis ends up with 'folk' natural properties, instead of accurate ones.