Thomas Sutcliffe: Special effects? I don't believe it!

I don't suppose that students of epistemology are going to form a big chunk of the audience for Peter Jackson's King Kong, but there is one moment they should enjoy if they're dragged along to a screening for the Philosophy Department Christmas outing.

It comes shortly after the film-makers have been shaken off a log into a canyon infested with carnivorous cockroaches and giant spiders. This is not, you might have thought, a moment for introspection, but Jackson takes a beat or two to point up the fact that grief and loss have struck several members of the party. Jimmy the cabin-boy has lost his substitute father, Jack the playwright thinks his last chance to rescue Ann has disappeared and Carl Denham, the unscrupulous film-maker who has got them all into this mess, has finally lost his precious footage.

The money shot shows Jack Black lifting his head from the mud to contemplate the wreckage of his Bell and Howell camera, film stock spilling like entrails into the destructive light. And for once, Black doesn't roll his Belushi eyeballs but lets them moisten with emotion.

What delivers an extra frisson to this scene is its calculated innocence. Black wants the film not just as money-making raw material, but as proof. It harks back to a time when celluloid counted as a kind of evidential gold standard - and yet it sits at the heart of a movie that proves with virtually every frame that you can't believe anything you see on a screen. That's where special effects have been going for the last 20 or 30 years - busily eroding the distinction between the simulated and the real.

And, though most special effects want to be noticed, there have already been several occasions in which CGI has insinuated itself absolutely seamlessly into cinema's promise that what it shows you once had some real existence in front of the lens. When Oliver Reed died half-way through the filming of Gladiator, he was digitally resurrected for a few essential scenes - and I doubt that anyone but a trivia buff could tell you which they were. Similarly, the audience for Robert Benton's film of Philip Roth's The Human Stain were mostly unaware that the New England porch on which Anthony Hopkins danced with Nicole Kidman only ever existed on a hard disc.

Our need for some kind of visual affidavit of the truth hasn't disappeared entirely, of course. It's a basic human appetite - and it finds itself satisfied these days by footage that is pointedly insufficient in cinematic terms: the smeary monochrome of CCTV footage that chops a critical moment into crude slices of time, or the jolting incompetence of home-video cameras, registering the physical excitement of its operators much better than the wildly swaying image they are attempting to capture.

If you really wanted to persuade a sceptical audience that you'd found a tropical island inhabited by dinosaurs and a gorilla the size of a house, your footage would have to carry just these hallmarks of disbelief - the crash zoom and the flailing frame. You'd have to film it in Run-Like-Hellovision, or people would just assume you'd downloaded the allosaurs from an animation website.

Which gives us a paradox. The more convincing special effects become, in visual terms, the less convincing they become in empirical ones. Which is perhaps why Jackson takes the pleasure in special effects to a new level in King Kong. Quite a few of the scenes in his film are spectacularly fake - by which I mean that their artifice is consciously part of the visual pleasure. It's not quite the same thing as you see in The Lion, the Witch and the Wardrobe - which features several effects of cheesy retro clunkiness, including a blue-screen background that looks as if it was cut out with a pair of nursery school scissors. In King Kong, by contrast, what's at work is a knowing cinematic nostalgia - an affection for a time when we could be fooled by simpler wonders.

It's particularly noticeable in the widescreen shots of the giant wall the villagers have built to hold back Kong. It's a model shot - fantastically intricate and enhanced by digital gouts of flame, but at the same time conspicuously a triumph of craftsmanship and miniaturisation. And as the camera ranges over it, your gaze is curiously poised between the credulous and the appreciative. Half of you gasps at the gothic barbarity of the scene, while the other half is thinking: "Every single one of those trees was handmade."

Like every fantasy film ever made, King Kong hopes there will be moments when you forget there are such things as special effects - but sometimes it doesn't want you to believe the evidence of your eyes, it just wants you to admire how beautifully the evidence has been constructed.