A scaffold went up on Portobello Road, then a tarpaulin. Unnoticed by streams of passers-by, the artist known as Banksy slipped into his temporary hide and painted furiously for most of a Sunday morning. Then the hide came down, and where there had been bricks, mortar and indifference, there was art.

He did not paint “Banksy woz ‘ere”. That would have been bafflingly unoriginal. But it was his autograph nonetheless: his nom de plume, or at least de spray can, and an artist in the process of spraying it. Last night the painting was sold at electronic auction for more than £200,000, on condition that the buyer work out what to do with it.

Vermeer and his admirers might have choked at this swift triumph of concept over craft, but unlike them Banksy understands walls. Walls work as barriers, but better as mirrors. The one that raked through Berlin for 30 years reflected murderous nihilism on one side and exuberant, multicoloured defiance on the other. Even in the nadir of the Troubles, the wire-topped sectarian frontline on Shankill Road did a better job of telling painted stories of strife than keeping people apart. In California, they go one better, turning drab sides of apartment blocks into giant murals of frolicking blue whales. And on the West Bank? The barrier dividing two worlds that once co-mingled is a challenge to guerrilla artists on both sides to turn it into the world's biggest two-way mirror and so learn about each other. In China, and all over the Lake District, walls have transcended function even by themselves. But it's the human touch that makes them live.