When the painting The Madonna of the Veil was found in 1930, it was hailed as a great work by the 15th-century master Botticelli and sold for $25,000. But, as revealed in the National Gallery's new exhibition "Close Examination" (running until 12th September), it was soon exposed as a clever forgery. The forger, Umberto Giunti (1886-1970) went to the trouble of drilling "worm holes" in the panel. Forensic analysis reveals the paint in Mary's robe is Prussian blue, a pigment that wasn't available before the early 18th century. Yet does the deceit matter, given the result was beautiful—albeit criminal—and had convinced connoisseurs? Philosophers have had plenty to say about this question. Arthur Koestler argued that, from an aesthetic point of view, it is snobbery to prefer an original above a good forgery. If a picture has aesthetic qualities, it has them whoever painted it. Attribution is simply about rarity: it may increase the value of postage stamps, but art is not philately. The 20th-century American philosopher Nelson Goodman strongly disagreed. It's no good describing a painting as indistinguishable from other Botticellis, he argued: it makes a difference who is doing the looking and whether or not they know they are looking at a forgery. For Goodman, seeing is a theory-laden activity, not the mere neutral reception of data. What you know affects what you see and what you look for. It is surprising how modern the Madonna's eyes begin to look once you've read a caption telling you that the painting isn't genuine. The philosopher of art Arthur Danto took a different line. You might think that in art all that matters is appearance. But, he argued, it isn't. The history of an object is part of what it is, even if that history is hidden. The Madonna of the Veil may look exactly like a Botticelli, but there's more to art than surface appearance. That's why a bicycle wheel or a bottle rack could become art when selected and used by Marcel Duchamp. In art, as in so many areas of life, what you see isn't necessarily what you get. Even the best forged Botticelli can never be as original as an original.