Full Idea
A contingent generalization is a law of nature if and only if it appears as a theorem (or axiom) in each of the true deductive systems that achieves a best combination of simplicity and strength.
Gist of Idea
A law of nature is a general axiom of the deductive system that is best for simplicity and strength
Source
David Lewis (Counterfactuals [1973], 3.3)
Book Reference
Lewis,David: 'Counterfactuals' [Blackwell 2001], p.73