My heart sinks at the prospect of the film of Prince Caspian, released today. In 1959 C. S. Lewis declared his absolute opposition (“adamant isn’t in it!” he wrote) to a live action adaptation of the Narnia books. “Anthropomorphic animals, when taken out of narrative into actual visibility, always turn into buffoonery or nightmare,” he said, and “a human, pantomime Aslan would to me be blasphemy”.

Douglas Gresham, the film’s producer, has argued that technology can now do the book justice. At least the lion won’t be human. And no doubt this film, like its predecessor in the Chronicles of Narnia series, will be technically impressive and faithful to the events of the book. It will even lead to a surge in sales of Lewis’s books, bolstering the consensus that adaptations of children’s books stimulate an interest in literature.

But, however good it is, it will be another story spoilt. In the sense that it can’t unfold on the screens of children’s imaginations any more. Words do stuff that pictures never can. They allow readers to make their own movies. And the imagination already does special effects.

I know a child who regards himself as working class who, having read the Harry Potter books, declared, with dismay, when he saw the first film: “They made Harry a middle-class boy!”

Harry is written so that readers can identify with him. The film steals him, and makes him something else. Before the film there were as many Harry Potters as there were readers; now there is only Daniel Radcliffe – except, maybe, in the heads of readers who got to the books before the films. In my mind I cling to my own Harry, who is much smaller and skinnier than Ron, and my Hermione, who is plain and frizzy-haired with big teeth, as in the books, and also a lot more like me than like Emma Watson. But these personalised characters are in danger of being squeezed out by someone else’s image.

The producer’s case for Caspian does not address a point that Lewis seemed to be making when he used the word “blasphemy”: Aslan is not just a lion, he’s a deity. In the narrative, and in the reader’s head, he can be both. “Actual visibility” makes him a lion. Movies turn emotional truths into mere technical wizardry.

The same was true of the daemons in The Golden Compass. In print they were both real creatures and a visible manifestation of human individuality, or soul, a clever translation of abstract into concrete. On screen they were pets.

Stories are powerful things that can seem to have a life beyond the words they are made of, but they can’t really. As Jonathan Miller once said when discussing film adaptations, Cézanne’s apples are not apples but “paint apples”, and equally characters in a book are “word people”. If you make them out of actors instead, and then fill out the scenes with what Miller called “all the boring art director’s boring detail”, you have made something quite distinct from – and sometimes a travesty of – the original.

I’m not against films. Sometimes adaptations give you more than the book could: Judy Garland singing Over the Rainbow, for instance.

Films can surpass books as works of art. But don’t kid yourself that viewing is any kind of substitute for reading. See adaptations by all means. But read the books first before they are ruined.

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