What is `modern'? Auctioneers Christie's have decided that Picasso is but the Impressionists aren't. So where does that leave Van Gogh and the `Young British Artists'? By Andrew Lambirth
The news that Christie's has decided to relegate the Impressionists back to the Salon raises once again the question of what constitutes the modern. Modern, for the Christie's auctioneer, now starts in 1900 rather than 1870. Thus Picasso, Braque, Kandinsky and gang are now the pioneers of modernism; Renoir, Monet, Van Gogh, Cezanne now live squarely in the 19th century.

Formerly, Impressionism was seen as the prelude to the 20th century developments of Fauvism, Cubism, Surrealism et al, and consequently marketed in the same sales. But from next spring, the Impressionists will be lumped with the very Salon artists against whom they rebelled - Bouguereau and co. Sales categories are being redefined, principally because the market analyses commissioned by Christie's report that people who buy Impressionists do not also buy Dubuffet or Jasper Johns. Therefore, in order to target potential buyers more efficiently, the pigeon-holes must be re-named.

The seizing upon arbitrary dates is misleading. Why 1900? "On, or about, December 1910 human character changed," wrote Virginia Woolf, rather specifically placing the watershed a decade later. Not until 1913 did Stravinsky's Le Sacre du Printemps cause a riot in the Paris Opera House. James Joyce's Ulysses was not published until 1922, the same year as TS Eliot's The Waste Land. And if 1900 is taken as the birth of modernism, then in what relation to it stands a figure like Oscar Wilde, who died that very year? Wilde's reputation as a precursor of 20th-century values, as a forerunner of the cult of personality, and as a writer and critic of continuing relevance, makes him an increasingly important participant in any modernist debate.

If modernism is largely categorised as being a period of intense innovation, it is at the expense of the fact that innovation has existed throughout art history. From Leonardo to El Greco, from Rembrandt to Vermeer, the best art works transcend the period in which they were made. If we look at the extraordinary late black paintings of Goya, for instance, we are struck by their freshness, their ability to speak directly to us, their - for want of a better word - modernity.

Historically, the origins of the Modern Movement can still be traced to the last years of the last century. But how do you identify a starting- point in what is really a process of continuous transition? The Impressionists made a break with accepted practice, by working en plein air, attempting to analyse tone and colour in broken brushstrokes, and to render the play of light on things. But much of what their art was about (a greater naturalism) had begun to be tentatively explored by earlier artists such as Courbet.

On the other hand, what precisely does the ward "modern" connote? Part of the problem lies in how the term is used. Logically something that is modern has happened recently, yet modern art itself dates back a century. What you have then is a contradiction, a split in the usage and semantics of the word "modern". It simultaneously refers to something which happened 100 years ago, yet also to the art of today. Over the last century, modernism has come to mean a body of art made all over the western world sharing a spirit of innovation. It encompasses great variety and experimentation, including the development of abstraction and the non-representational, the harnessing of the subconscious to generate visual imagery and the incorporation of everyday objects into the realm of art. And the "modern" to this day continues to do this.

Many art critics pronounce modernism dead. Not so Norman Rosenthal, internationally acclaimed as a curator of contemporary art and currently exhibitions secretary of the Royal Academy. Rosenthal believes that modernism is ongoing, and he has made it his job to recognise each successive shock of the new, to analyse and identify it. As a curator his position was made very clear in the recent blockbuster exhibition he co-curated with his long-time colleague Christos Joachimides. Entitled The Age of Modernism: Art in the 20th Century, it cogently put the argument that modernism is still very much with us. Starting with a 1907 study by Picasso for his seminal work Les Demoiselles d'Avignon, the exhibition traced the development of modernism up to the videos, photographs, installations and paintings of today. For Rosenthal and Joachimides, modernism is the driving force behind the most inventive of contemporary art.

Another way of looking at it is to think of the shockwaves generated by modernism as having diminished and died away. Or that the freedom granted by modernist innovation was gradually normalised. Right from the beginning of the century there had been extremist statements. In the 1920s the Russian painter Kazimir Malevich produced the most extraordinarily radical statement simply by painting a black square on a white ground. He compounded it by painting also a black cross and a black circle, on separate canvases. The circle is like a full-stop: it's as if Malevich is saying that this is as far as modernism can go. It was only the morning of an art movement, and yet one of its most inventive champions was already in genuine mourning for it.

Of course, not every artist responded to the shock of modernism at the same pace. The history of the movement from those early heady days of possibility is the story of the psychology of the artists' response. Once the shock had been normalised (which happened at different moments in different places) and artists realised that it was acceptable for them to make art with these new freedoms, it became a question of how far the freedoms could be taken. This is the period which might be called post-modern, and it was imbued with a serious name of intent which matched the early years of modernism, but with no restraints at all. The pluralism that has become a hallmark of our society flourished, and every different style co-existed.

Inevitably there was another layer of response, perhaps a backlash. Andy Warhol is a crucial linking figure here, If the post-modern was an attempt seriously to engage with the newly discovered freedoms modernism conferred, the post-post-modern is a denial of those freedoms, the crushing (or perhaps energising) realisation that in fact it is all going nowhere. Coincidentally, just before he died in 1966, Andre Breton, the self-styled Pope of Surrealism, lamented that it was no longer possible to shock people. The bright perfectionist dreams of modernism gave way to the utter disillusionment of post-post-modernism, and with it came the glorification of the trivial, the kitsch and the flip. "If you want to know all about Andy Warhol," he said, "just look at the surface of my paintings and my films, and there I am. There's nothing behind it."

This is where we stand today, with the exhibition of young British artists at the Royal Academy called "Sensation". Ironically, this is the show that replaced Rosenthal's "Age of Modernism" exhibition, which should have been at the Academy but was prevented by logistical complications from travelling from Berlin. Instead of a demonstration of what modernism means, we are presented with a display of largely flip and superficial art. Post-post-modernism is getting terminologically clumsy; when do we hear the last "post"? An alternative new category to suit the times might be millennianism, though not an exactly euphonious term. There is, after all, a nicely judged decadent and confused fin-de-siecle feel to today's art.

So, what charge, if any, does the word "modern" still carry? In accepted art-speak, "contemporary" has largely replaced it. Interesting then that the magazine Modern Painters, set up in emulation of John Ruskin and attempting to recapture the spirit of traditional British art, should now be actively seeking out the contemporary. Either it should change its name or cease sending David Bowie out to interview such art-stars of the minute as Tracey Emin, as it has for its current issue. The irony is that the very word "modern" is thought to be dated. To say a kitchen has "all mod cons" is to retrieve the unmistakable flavour of the 1950s. Yet there is no real consensus, and the vast proportion of the population is not suddenly about to stop using the word "modern" because in certain quarters it's regarded as outmoded. Whether modernism went to the taxidermist long ago, or whether it's with us still, in the end it's all a question of who's writing the essay