Cindy Sherman has been doing the same thing, over and over, for years: she dresses up, then photographs herself. So why is she so important? You have to look behind the masks, writes FINTAN O'TOOLE

IT IS QUITE EASY, looking at the huge Cindy Sherman retrospective at the Museum of Modern Art in New York, to see that she is one of the most important visual artists of the past 30 years. It is much less easy to say precisely why. The thing we normally say about great art is that there is so much going on behind it, that it springs from a deep life of its own.

Sherman’s power lies in the unsettling and insidious notion that there is nothing at all behind it – or behind any of the ways in which we present ourselves. She is all about absence and disappearance.

If you do try to pin Sherman down, the first thing you come up with is that she is an artist who has made a career from one idea, repeated, over and over, albeit with striking variations. And it’s not even an original idea. Sherman dresses up, using wigs and make-up and costumes and lighting to create a disguise, and photographs herself. That’s pretty much it.

She didn’t even invent this idea. In the early 1970s, other feminist photographers, such as Suzy Lake, Hannah Wilke and Eleanor Antin, produced grid-like strips of self-portraits in various poses, sending up and analysing the stereotypes into which women were slotted in advertising, movies and pin-ups.

Even earlier, the surrealists and Dadaists had experimented with the ways the camera lies, in which photography could present not an image of a “real” person but a constructed persona that yet felt uncannily real.

Once it learns how to read a photograph, the brain is sucked into a trap: as neither of them is actually real, how can it tell a “real” pose from a visual fiction? Identity could be literally seen as scarily malleable: Man Ray photographed Marcel Duchamp in the invented persona of a society lady, Rrose Sélavy. The surrealist Claude Cahun (Lucy Schwob) did pretty much what Sherman does, creating gender-bending self-portraits that challenged ideas of sexuality, class and the self.

Sherman obviously emerges out of these influences. There’s a strip of 23 photo-booth-style shots from 1975 (when she was 21) in the MoMA show in which her face gradually dissolves from ordinary geek with big glasses to mask-like blankness and is then reconstructed into the image of a vampish floozy. It’s a brilliant comment on both femininity and beauty, but it’s not a new idea.

Yet one of the points about Sherman is that she’s emphatically not trying to be original: on the contrary, the idea that there is an original, an actual person who is represented by the image, is exactly what is challenged in her work. Indeed, she turns the whole idea of originality on its head.

Her big breakthrough moment is the wonderful Untitled Film Stills sequences, made between 1977 and 1980. The black-and-white images, almost all of Sherman alone in a specific but displaced interior or exterior location, are in fact highly original. Each is a fragment or moment of a scenario that eludes the viewer, a page torn from a lost narrative. Each is a complex invention in itself. Yet the twist that Sherman gives to the images is to make look them look entirely unoriginal. As viewers, we are made to feel that we have seen them before, that they are indeed stills from movies we remember.

The Untitled Film Stills, in fact, sent critics scurrying to find the movies to which the pictures refer. They seem as if they simply have to be re-creations of a movie by Hitchcock or Antonioni, one in which a woman alone is surely being watched by an unseen and probably sinister man. But there are no original movies.

And this is more than a very clever and superbly executed trick. It reproduces for the viewer the experience of false memory that is so typical of an image-saturated world in which our brains are full of half-remembered pictures. We live in a world where it’s not just images that trigger associations with other images. New experiences themselves seem to be memories: the first time you see New York, you think you’ve been there before, because you have been – in thousands of movies, advertisements, photographs and TV cop shows.

Sherman explores this condition more potently than any other artist. She gives us the sensation that we are actually remembering something we have not in fact previously witnessed. In other art, “we’ve seen it before” is a protest at boredom and unoriginality. In Sherman’s it’s the artist’s conscious creation. It induces not boredom but the feeling of entering a twilight zone occupied by the ghosts of those who have never actually lived, the memories of things that never happened, the traces and vestiges of nonevents.

This same quality attaches to her next big project, the 12 large, horizontal colour photographs of the Centerfolds series, unveiled in 1981. The saturated colour palette and widescreen format are deliberately reminiscent of the glory days of CinemaScope. But, within the frames, the world is not that of the big, epic male world of Westerns or thrillers – though each of these is actually evoked in one image each – but the tiny, isolated female one of pop songs about heartbreak and loneliness. Most of the images are of young girls lying down, clutching lonely-hearts ads or waiting for the telephone to ring.

There’s a deliberate disjunction between the form (epic) and the content (teenage sob story). One makes us expect action, the other shows complete stasis. The effect is disorienting: we hover somewhere between two trashy pop-culture narratives, unable to settle on the genre that will make sense of the images.

THE ONE PROBLEMwith the Untitled Film Stills and Centerfolds projects is that they fit too easily into the postmodernist theorising that was reaching its zenith at the time. Sherman is one of those artistic figures who, if she did not exist, would have had to be invented. She ticked all the critical boxes: subverting grand narratives, undermining patriarchal ideologies, questioning the nature of truth, breaking down the distinction between high art and popular culture, connecting with mass media (film, advertising, fashion and later pornography), showing the collapse of the "real" in a world where everything is culture.

None of this is untrue, but as a way of seeing Sherman it is limiting, perhaps even imprisoning.

The Centerfolds series, for example, is an obvious parody of Playboy poses. But once you’ve got that reference the work is not used up. It is only beginning. It transcends the postmodernist concept: after the idea there are images that convey something else – pain, fear, isolation, alienation.

So when Sherman was featured in the big postmodernism show at the Victoria and Albert Musum in London last year, her work leaped off the walls because it was so much better than almost everything around it. Its energy is fully intact, unlike much of the game-playing imagery around it. It does not feel smug or knowing. And, above all and in stark contrast to so much that goes under the postmodern label, it has a genuine mystery. Postmodern art is not mysterious: it is all high concept, and once you get the concept you get the work. Sherman’s concept, on the other hand, is almost childishly simple: lady dresses up and takes picture. But the more you see of it, the more mysterious – one might even dare to say spiritual – it gets.

I wrote “almost childishly”, but, in a sense, there’s no “almost” about it. One of the reasons Sherman retains this sense of mystery is precisely because her basic impulse is that of a little girl but her methodology is that of a very complex woman. There is a photograph in the exhibition materials of her and a friend, taken when Sherman was about 12, dressed up as old women. It is the kind of picture you can take on any Halloween night – except that the illusion, unsettlingly, is far too good. It crosses the line between dressing up and disguise.

Dressing up is fun, playful; disguise is sinister. One evokes the innocence of childhood, the other the guilt of the criminal, the assassin, the terrorist. One is Shirley Temple, the other is Professor Moriarty. One is Halloween, the other the faceless clown from whom every kid instinctively recoils at the circus. In one, the multiplicity of selves is a lark. In the other, it is potentially evil. One is an obvious performance with which you can play along, the other evokes the scary shape-shifter, the changeling, the vampire, the figure who cannot be pinned down or controlled and who therefore can do anything to us.

Sherman’s genius and mystery lie, I think, in the way she stands right on this borderline. Her images ought to feel funny and playful: what is she doing, after all, except dressing up? They ought to be satiric, mocking as they do existing images of women from movies and ads, from porn and fashion magazines, from old-master paintings and (in her most recent series) society portraits. And satire is fun. But Sherman’s images are no fun at all. Even at their most playful they have that nightmarish quality of disguise.

What they are, most unexpectedly, is emotional. The great dividing line between Sherman and most postmodern work is that, especially when you see her work as a whole, the sense of absence in it becomes highly charged and evocative. What it evokes is the very sense of humanity that is, so hauntingly, not there. Theologians talk of the via negativa – the attempt to approach God by contemplating His absence. What’s absent in Sherman’s via negativa is not God but humanity, the self that must exist somewhere behind all her constructs but that remains continually beyond our field of vision. We can’t help imagining some blur at the edge of the image that is the departing shadow of a human being.

This actually creates two very old-fashioned, decidedly unpostmodern feelings: the terror and pity that Aristotle believed were the emotions created by tragedy. Sherman’s work sometimes lurches into pure horror, especially in the 1980s work inspired by extreme pornography or the more recent psychedelic images of clowns. But it doesn’t have to: it is much more effective when the terror is insidious.

But there is pity, too. From the sad girls of the Centerfolds series to the ageing, sculpted society hostesses of her more recent work, their mask-like faces shored up against time by plastic surgery and Botox, there is no sense that Sherman is laughing at her creatures. There’s too much yearning for that, too much wistfulness, too strong a desire to escape the poses in which they are trapped. There is still, somewhere, the child’s memory of posing a doll just so and then wishing that some hand could touch it and it would come alive.

Cindy Sherman is at the Museum of Modern Art in New York until June 11th