What is the difference between great art and tat? In the theatre, Dr Johnson’s rule of thumb seems adequate: if people are still prepared to revive a play a century after its premiere, it probably matters, whether or not they call it ‘dramatic art’. But it’s trickier with paintings, which have a relationship to the word ‘art’ that baffled me for years. When I was a child, there was ‘art’ and then there were pictures, and the latter were to be preferred. Art turned up in Look and Learn, World of Wonder and a board game called Masterpiece, and it was small, blurred and usually incomprehensible, even when Kenneth Clark was standing in front of it sounding enthusiastic on television. Why was a naked man wrapped in a curtain jumping over a wagon from behind a tree, twisting his head oddly as he did so to look towards a girl who seemed to be pushing an imaginary door in an unnaturally blue sky? And what were leopards doing there? Why was the Virgin Mary, among some equally blue angels in the right-hand panel of an unattractively hinged double set, trying to throw the baby Jesus across the intervening frame at the kneeling king on the left? And why was I supposed to care about the mustard-coloured pot on the mustard-coloured table with the mustard-coloured dead sunflowers in it?