A national economy is an unimaginably complex system. And yet we compress all its complexity into a single measure, and then focus obsessively on that. If you want a metaphor for this, think of King Kong spending most of his time staring at a pinhead, worrying about whether it is moving or not. That pinhead is GDP or, to give it its full moniker, gross domestic product.
It's defined as "the sum of all goods and services produced in a country over time, without double counting products used in other output. It is a comprehensive measure, covering the production of consumer goods and services, even government services, and investment goods." From the movement of this single number over time we get an idea of whether the economy is contracting or expanding, which is why governments all over the world at the moment, and especially in Britain, devote every waking hour to monitoring it. If it's up by 0.3 percent, then it's trebles all round in Whitehall; if down by 0.3 percent, then Cameron and Osborne start thinking of life after government.
That this is absurd goes without saying. In fact, it's been absurd ever since GDP was invented in 1934 by the economist Simon Kuznets. The measure has been subjected to ridicule and criticism for as long as I can remember. If a parent chooses to stay home to look after his or her children, then the "work" involved (producing stable and happy children?) doesn't get counted in the GDP. But if the same parent employs a nanny, then it does. People have pointed out that increasing GDP may simply be an indicator of how quickly we are boosting global warming rather than increasing social welfare: a gas-guzzling, high-emission SUV contributes the same amount to GDP as much as a thousand bicycles. And so on.