Oscar Wilde's famous quip 'All art is quite useless' might not be as outrageous or demonstrably false as is often supposed. No-one denies that much art begins life with practical aims in mind: religious, moral, political, propagandistic, or the aggrandising of its subjects. But those works that survive the test of time will move into contexts where for new audiences any initial instrumental values recede and the works come to be valued for their own...