‘Surely,’ mused the Dutch artist M.C. Escher, ‘it is a bit absurd to draw a few lines and then claim: “This is a house.”’ He made a good point. That is what almost all artists since the days of Lascaux have done: put down some splodges of paint or a line or two and proclaimed, ‘This is a bison’, ‘This is a man’, ‘This is Mona Lisa’.