Full Idea
Later Lewis said we must choose between the intersection of the axioms of the tied best systems. He chose for laws the axioms that are in all the tied systems (but then there may be few or no axioms in the intersection).
Gist of Idea
Lewis later proposed the axioms at the intersection of the best theories (which may be few)
Source
comment on David Lewis (Subjectivist's Guide to Objective Chance [1980], p.124) by Stephen Mumford - Laws in Nature
Book Reference
Mumford,Stephen: 'Laws in Nature' [Routledge 2006], p.46