Few works of 20th-century art have such a distinguished list of past owners

A POSTWAR BABY BOOMER AND A LATE SIXTIES student, in my adult life I naturally grew up optimistic. I believed in progress.

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I witnessed the civil rights movement, feminism, decolonisation, the beginning of 'Reform and Opening Up' in China at the end of the seventies, and the break-up of the communist tyranny in Europe a decade later.

Not that there weren't bad things in the world: horrific wars in Vietnam and Biafra (Nigeria), the ravages of international capitalism, the first manifest intimations of climate disaster. But, despite terrors in far-off places, it seemed that - to paraphrase Martin Luther King - the long arc of the moral universe was indeed bending to justice. But now?

On the wall in my student room I had a black-and-white copy of a monoprint by the artist Paul Klee. He served as a soldier in the German army in the First World War - a conflict in which a number of his friends, including fellow artists such as August Macke and Franz Marc, were killed. When the war was over, Klee returned to his art, and in 1920 created the picture that I put on my wall.

He called it