His method was not to copy but to absorb. His study in the early 1800s of a series of Claude seaport scenes in the Angerstein collection left a lifelong imprint on his art. Whenever he subsequently thought of Claude he imagined "amber-coloured ether" and "every hue and tone of summer's evident heat, rich, harmonious, true and clear". The Frenchman's principles of composition became his own: the high viewpoint with framing trees melding into a middle ground with water before reaching a hazy, mountainous horizon is the starting point for innumerable Turner landscapes. And no matter that it was Wales, Yorkshire or the Thames valley he was painting, the light of the Roman campagna suffuses all.

Similarly with the Dutch, he learned that boats ploughing through foaming sea could have more than anecdotal interest. There was real artistry (and for Turner aesthetics were always more important than mere representation) in catching the shifts of sunlight, wind and water. He painted some 60 "Dutch" works but an early chance to show what he had learned came when the Duke of Bridgewater commissioned him to paint a companion piece to his A Rising Gale (1672) by the "Vandyke of Ship painters", Willem van de Velde. The response was Dutch Boats in a Gale, also known as the Bridgewater Sea-piece (1801), a lurching swell of a picture in which Turner brought an added painterliness to the Dutchman's observational accuracy. The picture, revealingly a foot or so bigger than van de Velde's, was a great critical success and confirmed Turner's estimation of his own talent; as well it might, hung as it was alongside the Duke's other works by Titian, Raphael, Claude and Poussin. This was exalted company for the 25-year-old son of a cockney barber. 

The question Turner asked of himself was how to cope with the past. His answer was to shape an entirely personal art out of it. The steam clouds of colour and shadowy forms of his later landscapes are the direct descendants of his earlier, more faithful Claudean and Dutch works. He transformed them by turning up the dial on their expressive and poetic qualities and making paint itself the message rather than the medium. 

Turner's ambition was to become an Old Master while still alive. This exhibition, in which his models and the work he spun from them will be hung together, will help show how successful he was. He never doubted himself: in his will he gifted to the nation two major landscapes of his own on the condition that they be hung in the National Gallery next to two views by Claude. His intentions could not have been any clearer. Although it is hard to imagine immodest Turner proclaiming Newton's celebrated words, "If I have seen further, it is by standing on the shoulders of giants," he, more than any artist, would have acknowledged the truth of it.