Civilization and the pleasantness of everyday life depend on unwritten rules. Early in the 20th century, an English mathematician and government official, Lord Moulton, described complying with these rules as "obedience to the unenforceable"--the area of personal choice that falls between illegal acts and complete freedom. Listen as economist Michael Munger talks with EconTalk's Russ Roberts about the power and challenge of the unenforceable.