What's the alternative, if we continue along the suburban-sprawl route? Foster has a stark warning. "The end result is that, in a very few generations, you will be saying to your kids - we're going to the museum for the weekend. What's the museum, they'll ask? And you'll say - it's the museum of the countryside. You'll say, once upon a time there used to be a lot of this, and it wasn't a museum."

Sitting high in the Gherkin, looking out over London to the green fringes, it is a convincing prediction. Foster is certain that there is plenty of unexploited building space in London and other cities. But it all comes down to local authorities, he says. Some accept well-designed high-density housing schemes, others reject them out of hand and apply rigid bureaucratic limits. Those limits, says Foster, "are ludicrous in a city like London. They're absolutely mad."

The good news, he says, is that the planning debate is becoming more enlightened. Foster has always kept well away from party politics, unlike Rogers, who nailed his colours to the New Labour mast. But he believes in London's elected mayor, Ken Livingstone. Strong city mayors always make things happen, he points out. He should know, since he has received many a commission from them over the years. And Ken made his Trafalgar Square pedestrianisation scheme happen. Foster knows who he likes to deal with. "Strong mayors, strong regional leaders - accountable, electable - people who are prepared to take decisions, have some courage, some guts. Not people who run on the basis of doing nothing, who avoid criticism by maintaining the status quo. In the end, somebody has to care."

It seems a good point to close the discussion. Whatever else Foster has done in his extraordinarily productive career, nobody could ever accuse him of being satisfied with the status quo. He can be a bit of a dry, over-analytical, unemotional figure. Some of his buildings can be chilly, antiseptic. But with the Gherkin - the fat, friendly skyscraper everyone instantly recognizes - he has got the public firmly on his side. Popular architecture? It doesn't have to have a thatched roof.