Every two months or so, each member of the Monetary Policy Committee is encouraged to give a speech outside London. It’s part of an effort to erase the Bank of England’s reputation for seeming uninterested in the concerns of average citizens, or at least those who live far away from the city’s bankers and financiers. And so on the afternoon of Posen’s final speech as an M.P.C. member, it was hard to miss the paradox. He was in a small town on the southeast coast of England delivering an argument directed at a handful of decision makers in the capital.

It was a rainy day in Chatham, the grim place where Charles Dickens spent much of his childhood and where the kitschy indoor amusement park, Dickens World, is now the main attraction. (The park features a flume ride through an ersatz centuries-old London, replete with the actual smell of rotting cabbage.) Posen spoke down the road, at the Universities at Medway, a consortium of local schools open to students “without traditional qualifications.” The audience was made up of a few dozen pensioners, some students, local businesspeople and three wire-service reporters who would pass on whatever Posen had to say to the insiders back in the city. “I’m one of their beats,” he said. “Once I leave the job, who knows if anybody is going to show up to any of my speeches.”

Posen gave the speech, which ran over an hour, with only minimal notes. Much as he did in the bank’s monthly committee meetings, he projected economic data on a screen and explained the narrative that he thought best accounted for what was happening in the British economy. He quickly focused on the core economic riddle — the data showing that worker productivity has fallen while the unemployment rate has stayed the same. It seemed impossible, he said. How do more people make less? “If you’re a worker, you didn’t wake up one day in October 2008 and find your left arm missing,” Posen told the crowd. “You didn’t find yourself lobotomized and forget how to run whatever machine or do whatever it is you do. In November 2008, a business owner did not wake up one day and find that their factory had been bombed in the blitz like their grandparents had. It’s all still there.”

Then Posen’s defense became more technical. He said the problem wasn’t a sudden collapse in the capacity of workers and factories. The problem, more simply, was that there wasn’t enough demand to support full production. The private sector, in other words, was no longer able or willing to spend enough money for the economy to function at its peak levels. If the government or Bank of England could do more to stimulate the economy through any number of tools at their disposal — like more quantitative easing and infrastructure spending — demand would rise, and everyone’s productivity would shoot back to normal.

It was a subtly (but crucially) different way of looking at the problem: the country was not truly less capable. The problem was not one of supply, Posen suggested, but rather of demand. Workers were not less productive than they used to be. They merely needed some help from the government and the central bank — rather than budget cuts — to close the output gap.

To those who follow the economic debates in the Bank of England and Parliament, Posen’s speech was a barely concealed attack on Cameron’s policies. In Chatham, though, the crowd didn’t seem to understand that the polite American enthusiastically pointing to graphs on a projector was declaring that their prime minister was a financial philistine. Posen followed central-bank custom and declined to mention Cameron by name or explicitly critique the government’s policy. (His sharpest and most overt statement — “When your government is cutting its budget deficit, it tends to drag down consumption” — wouldn’t exactly start a bar fight.) Months later, Posen told me that his tenure in England was often enormously frustrating. Next month he begins a new job as the president of the Peterson Institute. He thinks that he can do more to revise the economic narrative from Washington than he ever could from within the Bank of England.

The economics profession advances by one confusing financial disaster at a time. A series of 19th-century banking crises in England and the United States inspired policy makers to create the modern central bank. Then came the Great Depression, a period of economic misery that existing ideas could not explain. John Maynard Keynes and the far more libertarian Friedrich Hayek spent the 1930s trying to make sense of the inexplicable economic data. In the process, they developed the two schools of thought that still dominate economic arguments. The Keynesians overwhelmed public discussion in the United States and Britain until the 1970s, when Hayek, along with Milton Friedman and his Chicago school, became more popular.