Full Idea
The proposal is that if F is a universal appearing in some natural law, then Fs form a natural kind.
Gist of Idea
If F is a universal appearing in a natural law, then Fs form a natural kind
Source
Alexander Bird (Philosophy of Science [1998], Ch.3)
Book Reference
Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.113
A Reaction
Such proposals always invite the question 'What is it about F that enables it to be a universal in a natural law?' Nothing can be ultimately defined simply by its role. The character (essence, even) of the thing makes the role possible.