The principles of classical liberalism are an important reason why the American founders believed so strongly in having a written constitution. It is a myth that we were the first country with a written constitution; several existed at the time, and the Greek philosopher Aristotle had a large collection of constitutions from the city-states of the ancient Greek world.
Why is a constitution so important? A Constitution is a nation's most basic law. It sets out how the country’s other laws are going to be made and how the country will govern itself. That basic process and structure gives life to everything else in the government.
Constitutions do several things. The most important is that they create political institutions. Things like the House of Representatives don’t exist by nature; the Senate doesn’t sprout from the ground like a new tree. When the Constitution says, “All legislative powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives,” it is actually creating those institutions.
Constitutions also assign powers in government to the various institutions that they create. Which branch will make laws, and which will enforce them? How will authorities at the local level exercise their powers? Constitutions can’t just say that there will be institutions; they have to say which institutions do what, even if by simply saying one institution shall do everything: “All authority whatsoever shall be vested in the Grand-High Mucketey-muck, and all shall obey him whatever he commands.” Saying “Let there be a Grand-High Mucketey-muck” just doesn’t do very much by itself.
Finally, Constitutions usually provide certain guarantees to citizens. They state that certain things must be done for them, that they will be allowed to do certain things, and that they will be obligated to do certain other things. Not all constitutions do this, but certainly most provide some statement of rights, liberties, and obligations for those who must live under the government that a constitution creates.