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...