A parlour game for the jaded: you're assistant to a straight-from-central-casting film mogul, broad of suit and pouched of chin. He swivels in his chair and says, frowning: "What we really need is another one of those novels. You know: the classy sort. Long words and kinky sex and a coupla nice awards no one's ever heard of. One of those unfilmable ones. Run out and get me one, why doncha?"

You run down to Borders to scan the shelves. The Life and Opinions of Tristram Shandy is displayed prominently, but - gah! - in a film-tie in sleeve, with Steve Coogan on the cover. You remember Michael Winterbottom's film is coming out. You move on.

Naked Lunch? Done. Crash? Done. Moby Dick? Done. American Psycho? The English Patient? The Master and Margarita? Done, done, done. Ulysses? Done twice. The films in question may be travesties, but they're wound, reeled and printed, and hung on the wall like so many moose heads.

There must be a novel, somewhere, mustn't there, that is truly unfilmable? You consider what might it be that would put a film-maker off? Length? Pah! A mere 208 minutes sufficed for King Vidor to unroll the whole of War and Peace on to the screen.

Offensive or controversial content might seem a surer bet. Hollywood can only sigh at the thought of all the filth that writers have always got away with, with barely an obscenity trial to hold them up on the way to the bank. Yet the bounds of celluloid acceptability tend to expand with the years. And often do so on the back of literary adaptations. So you tick off Lolita, Portnoy's Complaint, Intimacy.

How about sheer logistics, then? Fantastical settings, invented creatures and cast numbers beyond the scope of on-set catering: these are the matter of a few pen strokes on the page, but they can redline the buzzingest treatment at birth. But not any more. Peter Jackson's Lord of the Rings trilogy is a fearsome statement of the power of cinema. There is nothing, now, in heaven, hell or Middle Earth, Jackson seems to be saying, that cannot be conjured up out of the digital soup of ones and zeroes.

So is the unfilmable novel extinct? Are there any left? Where should you look for that stubborn grain of inadaptability, the quintessence of the irréalisable? Look inside, my friend. First-person narration has always been one of the novel's preferred modes. It is something no other art form can master. Stuck on a train, or tucked up in bed, we are able, in reading a book, to experience another person's world in a way that film can't get its pretty head around at all.

The world that film shows us is objective, to a fault. Faced with a first-person narrator, there are a few tricks a director can turn to - invented friends for the narrator to confide in, point-of-view shots. And then there is the voiceover, the last refuge of the cinematic scoundrel.

But we can move beyond interiority. Further in. What can words do that pictures can't? Why, abstraction. I nominate as the ultimate unfilmable novel the final part of Beckett's Trilogy: The Unnamed. A hundred pages of mental cogitation without context. Characters, places, times drift in and out of view, without ever fixing themselves down. The narrator sets out his stall at one point thus: "These things I say, and shall say, if I can, are no longer, or are not yet, or never were, or never will be, or if they were, if they are, if they will be, were not here, are not here, will not be here, but elsewhere." Get your script doctors to work on that, Mr Mogul.