So if artwork is unfinished it’s not really by the artist? “A visit to any gallery will throw up plenty of examples of unfinished art. Last year’s Velázquez exhibition at the National Gallery featured several pictures that the painter had not completed; they had great lacunae or only one level of paint. There is also a roaring interest in sketches, which are by definition not the finished work; the queues at the Victoria and Albert museum for the Da Vinci exhibition bore this out.”