 Lewis’s reductionist project starts with  laws of nature.  Building on some scattered remarks by Ramsey and Mill, Lewis proposed a version of the ‘best-system’ theory of laws of nature. There is no paper devoted to this view, but it is discussed in section 3.3 of Counterfactuals, in “New Work For a Theory of Universals”, extensively in Postscript C to the reprint of “A Subjectivist’s Guide to Objective Chance” in (1986b), and in “Humean Supervenience Debugged” (1994a).