Full Idea
We need natural properties, so that the principle of charity will impute a bias towards believing that things are green rather than grue, and towards a basic desire for long life, rather than long-life-unless-one-was-born-on-a-Monday....
Gist of Idea
We need natural properties in order to motivate the principle of charity
Source
David Lewis (New work for a theory of universals [1983], 'Cont of L')
Book Reference
'Properties', ed/tr. Mellor,D.H. /Oliver,A [OUP 1997], p.224
A Reaction
Lewis always seems to be approaching things from the wrong end. We don't need properties so that we can attribute charity, so that we can interpret. We interpret, because we can be charitable, because we all experience natural properties.