The definition of art is not something that anyone would lightly undertake. Nor would it normally be left to a US customs official to decide. But that is exactly what happened in October 1926 when Marcel Duchamp arrived on the New York dockside accompanying 20 modernist masterpieces from Brancusi's studio that were destined for selling exhibitions in New York and Chicago. Duchamp at that time had given up art in favour of chess, and was trying to eke out a living by art-dealing with his friend Henri-Pierre Roché, mainly in Brancusi.

The point was that ordinary merchandise was subject to duty at 40 per cent, while art was not. And the customs official on duty at the time happened to be an amateur sculptor – just the sort of person to have bumptiously confident views about matters aesthetic. He took one look at the Brancusis, concluded that they weren't art, and levied $4,000 duty.