 The best-system approach was pioneered by Lewis (1994b). His analysis of chance is based on his account of laws of nature (1973), which in turn refines an account due to Ramsey (1928/1990). According to Lewis, the laws of nature are the theorems of the best systematization of the universe—the true theory that best combines the theoretical virtues of simplicity and strength. These virtues trade off. It is easy for a theory to be simple but not strong, by saying very little; it is easy for a theory to be strong but not simple, by conjoining lots of disparate facts. The best theory balances simplicity and strength optimally—in short, it is the most economical true theory.