The filming of David Mitchell's acclaimed novel Cloud Atlas, like any act that defies commonsense and carries a high risk of failure, deserves the epithet "brave".

There should be a special medal for adaptations like this, which push the boundaries of reason and in so doing disembowel the original book as if it were the enemy, and not the source of inspiration. Released in the United States last week, Tom Tykwer and the Wachowski brothers' film has been hailed by an American critic as "one of the most ambitious films ever made". One English reviewer was more forthright, describing it as "a roaming behemoth of a movie" that "carries all the hallmarks of a giant folly".

Given the inventive structure of Mitchell's novel, which tells six interlinked stories, each set in different historical and futuristic eras, it was always likely that a film version would draw as many barbs as bouquets. What fascinates me, however, is not the challenge of turning so intricate a novel into film, but why one would want to in the first place. As novelist Lev Grossman writes in Time magazine, "There's a weird aura of manifest destiny around successful novels, a pervasive belief that they must progress through the stages of life and become movies, as the caterpillar becomes the butterfly: the movie industry treats narrative like a precious nonrenewable resource that must be carefully recycled and never just wasted on mere paper."

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As any novelist will tell you, long before their book reaches the bookshelves their agent will have been in earnest discussions with film directors, pushing its merits for screen. Some of the most hyperventilating press releases that fill my inbox come from publishers ecstatic to announce that one of their authors has sold their work to a movie mogul. To judge by the tone of such bulletins, this and not the writing of a good book, is the true triumph, the real measure of a writer's credentials. Clearly it's not sufficient for a novel to be a creation of printed words alone; no, like a triathlete it must multi-task, proving itself fit for more than one medium, able to vault, race and twist itself any which way, as if to be a novel and nothing but a novel is a dereliction of duty.

On one level, Tykwer and the Wachowskis' ambition is impressive. Surely we'd think them dull if they had merely taken a strongly plotted linear novel, which tells a decent story, and turned that into a good film? At least they have exercised their imaginations to turn something profoundly bookish into almost three hours' entertainment for cinemagoers.

Yet the bigger question remains. What is it that draws filmmakers to literary fiction, as if they were moths to its flame? Why must they appropriate and reshape a work that was intended for the page, and can only be fully appreciated in that form? Ignoring this, they force it instead on to a procrustean bed and mangle it beyond recognition. It's as if ambitious movie-makers feel compelled to put their stamp on literature's elite novels. In so doing, they are reaching for the top of the tree, bypassing the more modest and filmable works on the lower branches. They might as well come right out and say that in their opinion, no book has earned its laurels until it has been put on screen.

"Have you read Cloud Atlas?" people will ask in a few months' time. "No, but I've seen it," will be the reply, as if that amounts to the same thing.

Thus book by book the literary canon is turned into fodder for film, and the need to read diminishes year by year. What perplexes me is that in a century of filmmaking, directors still don't grasp that books and films are not interchangeable. They are neither the same currency, nor do they simply need interpreters.

What works in the best novels, what lifts them beyond mere storytelling, cannot be reproduced on film. And vice versa. That should be a cause for celebration, but it seems to me that it only breeds envy.