By Martin Kemp, a history of art professor at Oxford University, is the author of Leonardo da Vinci: The Marvellous Works of Nature and Man, and curator of the V&A's forthcoming Leonardo exhibition (THE GUARDIAN, 22/05/06):
The Leonardo silly season never closes. I can bear witness to its enduring nature, having personally appeared in both the Sun and the Sunday Sport - in quotes at least - in connection with absurd stories about Mona Lisa's sexual life. Until the film of The Da Vinci Code, the previous highpoint for this silliness was the Sport's story that Lisa was smiling with her mouth closed because her teeth had been blackened by mercury treatment for syphilis.
Anyone involved with Leonardo over the course of the past 30 years, as I have been, becomes accustomed to the legends and nonsense - and can even relish them for their bizarre charm. However, Dan Brown's book is very much another matter.
After initially welcoming The Da Vinci Code as another piece of cult nonsense, and seen my book sales rise to unusually healthy levels, I have come to regard its impact as wholly pernicious. As I have been besieged by the media, including private TV production companies that value sensation above any vestige of historical accuracy, it has become clear that Brown's book has perverted not only any proper sense of who Leonardo was and what he did, but also proper conceptions of history and fiction.
There is no essential problem about historical fiction as fiction. I do not watch Macbeth to garner documentation about Scottish kings. However, the parading of fiction as fact - openly encouraged by Brown's irresponsible claims at the start of the book - transforms fiction into an outright lie rather than a vehicle for particular kinds of imaginative truth.
Opportunistic exploitations of the book's success - stories of Leonardo as the forger of the Turin shroud, as a Gnostic or Cathar heretic and so on - also serve to highlight the way that the writing of history is not generally regarded as a serious professional business, dependent on responsible ways of handling evidence. If I need an appendix operation, I do not go to a surgeon who has no credentials and professional standing. Why is it that anybody who nosily claims to have access to sensational research is given space as a credible historian?
If the uncovered "secrets" involve grand conspiracies, so much the better. There is now a wide taste for conspiracy theories. As political parties, governments and multinationals spin the truth into increasingly gross configurations, we feel increasingly that what we are presented with as "evidence" is the result of systematic campaigns to mislead. We are reconfiguring the past in the image of our times. Dan Brown's book does this with high efficiency.
But having seen the film, I have ceased to worry. It is so wonderfully bad that it is difficult to imagine how anyone can take it seriously. With a fragmented plot that is less plausible than a Harry Potter movie, it combines the historical seriousness of Monty Python's Life of Brian with the deep emotional characterisation of the Wacky Races.
The insipid Tom Hanks (as Robert Langdon) and prettily blank Audrey Tatou (as Sophie Neveu) blunder through the episodes with a growing air of bewilderment, as if they expected to be on the set of a different movie. Ian McKellen on the other hand, recognises exactly where he is. He camps up the part of Sir Leigh Teabing for all its worth. His virtuoso display of English eccentricity degenerating into gibbering insanity magnificently torpedoes the credibility of the film, and with it (I hope) that of the book. The whole farrago is so sublimely bad that it might become a cult classic.