Four years ago the Cato Institute published a handsome, pocket-sized edition of the Declaration of Independence and U.S. Constitution, slightly smaller than a U.S. passport. Included was a brief preface that related the two documents through their underlying principles.

We had no idea what the demand for such a pocket Constitution might be. So at first we sent it free to justices, judges, members of Congress and the executive branch, and assorted state officials. Then we posted it at Cato’s Web site, and over time we made it available for sale at stores like Restoration Hardware, Borders Books, and Ama​zon​.com.

To date, we have sold and distributed over 3 million copies. Now the stores are telling us, as the holiday season approaches, “We need more.” That’s an encouraging sign for liberty.

The attacks of 9/11 brought a spike in flag sales. Yet the flag, for all its importance, is still just a symbol. It symbolizes the principles the Declaration sets forth and the Constitution secures in law. If flag and Constitution sales are any sign, it seems that Americans are coming to realize how fragile our way of life is, and how important it is to understand and protect the principles of government that preserve it.

Let’s review those principles. In the Declaration, Thomas Jefferson set out certain “self-evident truths,” the foundations of freedom: “that all Men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the Pursuit of Happiness.” What that means, quite simply, is that we’re all born free to pursue happiness as we wish, by our own lights, provided we respect the equal rights of others to do the same. In a nutshell, America’s basic moral principle is “Live, and let live.”

Our basic political principle, Jefferson went on to say, is equally simple: “That to secure these Rights, Governments are instituted among Men, deriving their just Powers from the Consent of the Governed.” Government is thus twice limited: by its end, to secure our rights; and by its means, to which we must consent if government is to be legitimate.

Those two principles, liberty and limited government, have inspired countless millions around the world for over two centuries, people clamoring to be free and to live under free governments. They have seen especially what the American experiment has taught, that the principles of liberty must be secured in law, in a constitution that protects them through the rule of law.

It was James Madison, under the leadership of George Washington, who brought that about. When the Founders met in Philadelphia in 1787 to draft the Constitution, they understood that government is both necessary and dangerous: necessary to secure our rights, but dangerous too, because government unrestrained could easily trample rights in the name of securing them. Thus, they drafted a document that both empowered and limited government, incorporating the subtle system of checks and balances that Madison first conceived.

The Constitution was not perfect, to be sure. It took a civil war and the Civil War Amendments to end its oblique recognition of slavery. Over the years, moreover, we’ve often ignored its principles in practice. Thus, today we have far more government than the Constitution authorizes. (Madison wrote that the new government’s powers would be “few and defined.”) And we struggle still to apply those principles correctly: At this moment, for example, courts are trying to determine whether the war against terrorism is invading the rights that make the war worth fighting.

By and large, however, the Constitution has served us well, enabling us to flourish under the blessings of liberty and law. As we gather with family and friends over the holidays, we would do well to count those blessings and give thanks that we continue to live under a constitution of liberty.