Eight hundred years ago, King John of England made peace with his rebellious nobles in a field not far from Windsor Castle called Runnymede. He did so by affixing his seal to Latin text inscribed on parchment in which he made a great number of promises he had no intention of keeping.

Most related to money. John had been inventive in finding ways to extract funds from the nobility, and the nobles demanded that he stop. They also demanded that he promise not to imprison anyone or take their property, except by “the law of the land.”

As the nobles held the military advantage, John promised. Once the immediate threat had passed, however, John got the pope to declare the document void, and he repudiated it. Conflict resumed, and England still was embroiled in civil strife when John died the following year, and his 9-year-old son ascended to the throne as Henry III.

The Charter of Runnymede seems an unlikely object of veneration, but its octocentennial has been marked by events around the English-speaking world. At Runnymede, the anniversary was formally celebrated by Queen Elizabeth, British Prime Minister David Cameron and U.S. Attorney General Loretta Lynch.

In celebrating the anniversary of King John’s humiliation, however, these dignitaries were not so much honoring the charter he was forced to issue on June 15, 1215, as the idea for which that charter subsequently came to stand — the idea of constitutional government, which is something that is very much worth celebrating.

But to celebrate that properly, we must remember that written charters, bills of rights and constitutions have no inherent power of their own to guarantee their own enforcement.

King John’s charter contained two nascent ideas that would later inspire England’s Glorious Revolution of 1688 and the American Revolution of 1776, that there should be no taxation without consent and that liberty requires freedom from arbitrary power.

But this charter, which later came to be known as Magna Carta (“the great charter”), was not the true foundation of English liberty.

The true foundation of liberty lies in the minds and hearts of the people, in their love of and respect for the rule of law. It consists in a hard-to-master combination of assertiveness and modesty — assertiveness in insisting that others respect one’s own rights and modesty in acknowledging that others also have rights.

The English proved remarkably successful in performing this difficult balancing act because, as Edmund Burke observed, they took a basically conservative attitude toward the law. They supposed that whatever rights the king traditionally held were the ones he should have and that whatever rights the people had were the ones they should have. Any changes should be made gradually, for good and pressing reasons, and by consent.

In rebelling against King John, the English nobles were in large part asserting rights they had enjoyed under his father, King Henry II. They were outraged because John had become excessively assertive, unilaterally inventing new taxes and powers, at their expense.

When Henry III took the throne as a boy, he re-issued John’s charter (with some alterations) as a way of making peace with the nobles. He was promising to recognize their traditional rights, so long as they would pay taxes and acknowledge his rights as king.

Over the centuries, the balance of power in Britain gradually shifted in favor of the nobles, and still later, in favor of the people. But the core idea — that we should assertively protect our rights but not demand any radical transformation of society — endured.

While the French sought to re-make their society from the ground up in the Revolution of 1789 but ended up ruled by the military dictator Napoleon, the English gradually secured their liberty in an evolutionary process that was substantially peaceful.

That same spirit infused the American legal system, as well. Though we made a revolutionary break with the English crown, our new republic continued to observe the common law we inherited from England, something that drew favorable comment from Alexis de Tocqueville in his 1835 book, “Democracy in America.”

That difficult-to-achieve combination of tempered spiritedness is under threat today, from radicals of the left, who are impatient for socialism, and radicals of the right, impatient for laissez-faire.

But as we celebrate 800 years of liberty under the common law, we would do better to cherish and cultivate the moderate and conservative virtues that transformed the dead letter of King John’s parchment into a living charter of freedom.

Joseph R. Reisert is associate professor of American constitutional law and chairman of the department of government at Colby College in Waterville.

Editor’s note: Beginning this week, Joseph Reisert’s column will run every other Saturday in this space.