Eight hundred years ago next month, on a reedy stretch of riverbank in southern England, the most important bargain in the history of the human race was struck. I realize that’s a big claim, but in this case, only superlatives will do. As Lord Denning, the most celebrated modern British jurist put it, Magna Carta was “the greatest constitutional document of all time, the foundation of the freedom of the individual against the arbitrary authority of the despot.”

It was at Runnymede, on June 15, 1215, that the idea of the law standing above the government first took contractual form. King John accepted that he would no longer get to make the rules up as he went along. From that acceptance flowed, ultimately, all the rights and freedoms that we now take for granted: uncensored newspapers, security of property, equality before the law, habeas corpus, regular elections, sanctity of contract, jury trials.

Magna Carta is Latin for “Great Charter.” It was so named not because the men who drafted it foresaw its epochal power but because it was long. Yet, almost immediately, the document began to take on a political significance that justified the adjective in every sense.

The bishops and barons who had brought King John to the negotiating table understood that rights required an enforcement mechanism. The potency of a charter is not in its parchment but in the authority of its interpretation. The constitution of the U.S.S.R., to pluck an example more or less at random, promised all sorts of entitlements: free speech, free worship, free association. But as Soviet citizens learned, paper rights are worthless in the absence of mechanisms to hold rulers to account.

Magna Carta instituted a form of conciliar rule that was to develop directly into the Parliament that meets at Westminster today. As the great Victorian historian William Stubbs put it, “the whole constitutional history of England is little more than a commentary on Magna Carta.”