One of the most fundamental questions of political theory is deceptively simple: why have a constitution at all? The answer to this question should be easy, given the widespread adoption of written constitutions in recent years, even if their substantive provisions vary widely from country to country.

Regrettably, however, the success of constitutionalism is far from guaranteed if social conditions do not support limited government, which is why so many constitutions have very short half-lives. The most notable exception to this unhappy fate is the American Constitution of 1787—up and running, with many changes along the way, for 235 years. Its duration is not just happenstance, for it rests on a sound intuitive assessment of how governments should generally work. To understand why some constitutions work, consider the arguments against constitutional precommitments.