James Purnell, former Work & Pensions Secretary is going to become the BBC's "director of strategy and digital" on a a salary of ''295,000 - and no doubt worth every penny. This suggests we should change the way we think of Westminster politics.

Purnell is continuing a trend for quite young people to leave politics for more lucrative work: Ruth Kelly, John Hutton, David Miliband (perhaps temporarily) and even Tony Blair. Whereas politics used to be something one undertook after gaining experience in other careers, it is now a stepping stone, something youngish people do as a means to bigger money*.

Such a path is understandable. Politics offers someone of average-to-decent ability a chance to manage multi-billion pound budgets at a younger age than they'd often get in ordinary business. Sure, they might not accomplish much whilst they are in government. But as Marko Tervio has shown, employers often prefer the mediocrity with a track record to the potential star without, so "Secretary of State for Work & Pensions" looks better on the CV than a string of competently-done junior management jobs.

Once we we regard politics as a means whereby 30- and 40-somethings gain experience before going onto better things, then several things follow:

1. We should be more forgiving of mistakes in both policy and execution. These guys are only learning their craft, so mistakes are inevitable. Politics is youth team football, not the Premier League. Sure, they're making mistakes with our money, but hey-ho.

2.Political reporting - in the sense of who's up, who's down - is not important, any more than is the matter of who becomes deputy assistant marketing director at Amalgamated Aerosoles. Of course, policy matters, as do questions of values about how we are governed. But individual ministers don't matter; they're just passing through.

3. We should expect politics to be biased towards crony capitalism and managerialist ideology not because these are goods way of running the country, but because they are ways in which ministers increase their chances of getting lucrative work later.

* Actually, the distinction isn't so sharp. It was quite common in the 50s for men to become MPs in their early 30s, and ministers have often taken directorships after leaving politics - although many of these were sinecures to subsidize retirement rather than, as now, a career advancement.