In a recent interview, a journalist told British artist Damien Hirst that he didn't think there was anything especially difficult about putting a dead tiger shark into a tank of formaldehyde. Anyone could do it. "Yes, anybody could," replied Hirst. "But I did."
Was Hirst making a post-modernist statement? Is the dead shark piece, entitled "The Physical Impossibility of Death in the Mind of Someone Living", a post-modern work?
To nod thoughtfully, perhaps a shade mysteriously, then to comment meaningfully "Mmm... very post-modern" has become one of the great cliches of our age. This comment could have been made about a film, a book, a building, a pop song - about life itself. But if you were to challenge it - "Ah yes, but what exactly do you mean by post-modern?" - you might get a bewildering variety of definitions.
Although post-modernism as a term was initially more applicable to architecture, it is now a world-wide movement in all the arts and creative disciplines. But pinning down exactly what it means and what it embraces proves extraordinarily elusive, rather like nailing jelly to the wall.
Post-modernism began, like so many modern movements, as a reaction to what had ruled immediately before. In this case, it was modernism. The creative energy and forward-looking exhilaration so characteristic of the Sixties gradually drained away and with this came a massive rejection of the major art movements of this time - pop art, op art and minimalism.
Modernism appeared to be absurdly obsessed with structure. Instead of opening up possibilities, it seemed to close them off. Worse still, it seemed to have developed a dangerous whiff of elitism, despite its original intentions. New York critic John Perreau expressed this new-found disgust in the early 1970s. "We need more than silent cubes, blank canvasses, and gleaming white walls. We are sick to death of cold plazas and monotonous 'curtain wall' skyscrapers... as well as interiors that are more like empty meat lockers than rooms to live in."
Post-modernism initially could be seen as the total rejection by artists and art movements of being labelled and categorised. Doubtless that is why it is now so hard to define. The freedom of individuals to express themselves in whatever way and in whatever discipline began to replace a need for the formal movements. These, it was argued, were more often than not invented by critics, in particular the American modernist critic Clement Greenberg, rather than artists.
Post-modernism became "the reassertion of the presence of worldliness, fecundity, variety, embodied spirituality, humour and pluralism", as Charles Jencks, one of the founders of the post-modern movement, describes it in his definitive book What is Post-modernism?
But coexisting with this wide-as-the-sea definition is a more specific interpretation of the movement. The modernists rejected the sensuous and natural.They loved machines and order, and they were none too keen on history or art of the past. In contrast, post-modernists have drawn on a wide range of styles, themes and ideas from the past. The viewer needs to recognise these references to understand the "post-modern" interpretation the artist is suggesting.
For "Self" (1991), the British artist Mark Quinn extracted eight pints of blood from his body (the average amount found in an adult) over a five-month period. The blood was then poured into a cast of his head and frozen.
The piece was inspired by William Blake's death mask, but Quinn's face is alive, suspended in frozen animation. It is, of course, the material which gives the work its macabre fascination and edge. Blood means life, but in the age of AIDS it can also mean death. This blood sculpture makes references to many such sculptures from the past, from the busts of Roman emperors onwards.
Quinn has also made contorted portraits of historical figures. His bust of French queen Marie-Antoinette was made out of bread dough, baked and then cast in bronze. Quinn makes us think of the past but puts his work into a unique context - Marie-Antoinette's head has to be kept in a refrigeration unit made specifically and at vast expense by the US space agency, NASA. It's also important that the blood in "Self" is Quinn's- it's a self-portrait.
Contemporary arts' ongoing preoccupation with the artist's own body is a part of the post-modern cult of the self. Witness Mona Hartoum's videos of the inside of her body, or Tracey Emin's tent with the names of all her past lovers embroidered inside.
Chris Ofili, winner of last year's Turner Prize, is also concerned with bodily matters, but in his case he looks to elephants for inspiration. Ofili has used elephant dung to give texture to his paintings and to act as a link to his African origins. He first used it on his canvases on a visit back to Zimbabwe, but now collects it from zoos. He also uses paints, beads and glitter dots.
Ofili's vibrant images might be seen as a post-modern use of personal imagery to draw attention to cultural issues. He looks to the past, to his roots, to say something about British culture.
In the Royal Academy's hugely successful "Sensations" show last autumn, one of the pieces which caused the most controversy was the Chapman Brothers' "Great Deeds Against the Dead", a fibreglass sculpture showing dismembered parts of male bodies grotesquely violated. This horrific and graphic image of violence is in fact a direct transcription from Francisco Goya's "Disasters of War", inspired by the atrocities the Spanish experienced during the Napoleonic invasion.
Famous - if not notorious - for their fascination with mutilations of the body, Dinos and Jake Chapman have put an old master into a modern context which is shocking and repellent to the viewer, yet undeniably thought-provoking.
Post-modernists have also often rejected the idea of art having to be shown exclusively in galleries and museums. Some critics considered The Museum of Modern Art in New York an oxymoron. As writer and critic Gertrude Stein once famously said: "A museum can be either a museum or it can be modern, but it can't be both".
Rachel Whiteread's "House", a concrete cast of the interior of an east London terraced house, won the 1993 Turner Prize, rapidly became the most controversial piece of modern sculpture since Carl Andre's bricks, and was finally demolished - to many people's disappointment. But in its brief life it became a monument to the frailty and resilience of people's lives, exploring not cosy domesticity, but the darker side of dwelling.
Whiteread uses her art, like many post-modern artists, to express her feelings about society. She says: "I'm a socialist and I think it shows in my work. I've lived in London most of my life and growing up in the Thatcher years, seeing the deprivation and more and more homeless people everywhere, I feel very sad about what is happening here. You see things crumble around you and you're helpless to do anything."
Despite the incredible changes in the Sixties, fashion and the arts were far more narrowly defined. Were you a Mod or a Rocker? Did you like the Stones or the Beatles? Did you wear minis or maxis? What has happened in the world of fashion, pop and the arts is the appearance of a far wider variety of ideas and styles, which make constant references to, or even revive, ideas from the past.
But is the concept of post-modernism now past its sell-by date? Are we entering a period of post-post-modernism? As the millennium draws near, one thing is certain - more change, rebellion and rejection of the past will follow.
Rachel Barnes is an art historian, author and lecturer at the Tate and National Gallery in London.