Picasso was not being entirely ironical when he claimed that the most important quality of a great painting was that it should be very expensive. Art dealers, gallery owners, auctioneers – not to mention many painters – would all concur. But who defines, who polices, greatness in art? Enter the experts, the connoisseurs, the authoritative givers of attributions of authenticity. And behind them follow the dealers, the investors and the collectors, followed in turn by the restorers and improvers; while lurking in the wings are the outright forgers and the fakers of paintings – and of authentications. This is a murky world. An old joke in the art trade is that of the 165 pictures known to have been painted by Titian, 365 of them are still in existence in America.Bernard Ber