Full Idea
A statement is a law of nature if it is true in all those worlds which differ only as to their initial conditions, that is in which the underlying mechanisms of nature are the same.
Gist of Idea
Laws of nature remain the same through any conditions, if the underlying mechanisms are unchanged
Source
Rom Harré (Laws of Nature [1993], 4)
Book Reference
Harré,Rom: 'Laws of Nature' [Duckworth 1993], p.87
A Reaction
Harré takes it that laws of nature have to be necessary, by definition. I like this way of expressing natural necessity, in terms of 'mechanisms' rather than of 'laws'. Where do the mechanisms get their necessity?