Modernism is a response to modern social and political conditions, says art historian Tim Clark. So how come both David's Death of Marat, painted in 1793, and Jackson Pollock's works qualify as modernist? Kam Patel finds out

Art historians, Tim Clark points out, have a thing about dates. One will say that modernism, the succession of avant-garde styles that have dominated 20th-century art, began with an essay by the French poet Charles Baudelaire in 1873, another that the modernist movement started with the Spanish painter Pablo Picasso in 1903. Others, fingering different dates, point to Cezanne or Matisse as instigators.

In his latest book, Farewell To An Idea, Clark, professor of modern art at the University of California, has been unable to resist the temptation to pick his own date: October 16, 1793, the day a hastily completed painting of Jean Paul Marat, the radical French revolutionary murdered in July of that year, was first exhibited in public. It was also the day of the beheading of Marie Antoinette, Queen of France.

In choosing the day Jacques-Louis David's Death of Marat went on show, Clark is conscious that he is pushing the birth of modernism back much further than most have yet dared. "At least the choice has the merit of being obviously far-fetched," he says, chuckling. "I am doing it because I want to think about modernism as a set of responses to modern social and political conditions." And it is not too controversial, he adds, to take the French revolution as the starting point of modern societies.

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According to Clark, modern societies arose from a decision by Europe and America to reorient themselves to the future, set aside past traditions and accept democracy. The ultimate good became progress, increasing productivity, glorifying the individual and building up financial, trade and, most recently, information networks.

To understand David's painting, which captures the essence of an artist caught up in the "hot, shapeless and dangerous public sphere" of the revolution, you need to understand the modern society on which it draws, Clark says. And all the different artistic styles classed as belonging to the modernist movement - cubism, surrealism, abstract expressionism etc - according to Clark's theory, have one thing in common. A modernist work is a painting produced at times of "maximum (social and political) stress", which replies to those extreme circumstances with an extreme response.

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It is one thing to argue that an obviously representational painting - depicting a murdered revolutionary leader - is a response to the upheaval of a changing social order. But how does one apply the theory to the more abstract art of just a few years ago? Can Jackson Pollock's action and drip paintings, his laceworks of coloured lines, for instance, be interpreted in terms of a response to the turbulent modern world?

Of course, says Clark, fresh from a visit to the Tate's Pollack exhibition:

"Pollock is as close as we get in the past 50 years to what the modern condition amounts to."

The hell-raising artist certainly lived through disorientating circumstances: the aftermath of two world wars, the cold war and the atom bomb. "It could be argued that his whole notion of space, density and 'explosiveness' has something to do with the emergence of the nuclear age, but it is also clear that what he is doing is abstract, it is not some disguised operation. Its aim is to evoke a new sense of scale and dealing with the world ... right from the beginning critics have said his work must be about the urban experience, and that seems right."

Clark has friends who knew Pollock and reckons he would have found the American artist, who died in a road accident in 1956, pretty obnoxious. "I do not think I would have lasted five minutes in his company ... he was a strangely mixed bag, a drunken lout who was profoundly thoughtful."

One of Clark's favourite works by Pollock is Number 1 (1948), featuring his famous hand prints. "For me it is about disorientation, loss and imbalance but then if you look at the handprints it is as if a figure is disappearing into the voidI" The linking of the interpretation of paintings to precise historical study of the political, social and cultural climate prevailing at the time they were produced is familiar terrain for Clark. He won considerable praise - and provoked various arguments - making such connections in the 1970s and 1980s, notably through examination of the works of the French painters Courbet and Manet.

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Until then, art history in Britain had focused on scholarly formalist analysis, histories of style and monographic studies of artists. Clark pioneered the new approach, first at Camberwell School of Art, then in Leeds before getting a professorship in the United States in 1979. Farewell is his most ambitious work. Ranging from the French revolution to the collapse of the Berlin Wall, Clark draws on the work of artists such as Pissaro, Cezanne and the Soviet constructivists to justify the distinct shifts that occur in painting in response to social, political and cultural upheavals.

So, during the French revolution, he contends, "painting finds itself called on to do a certain number of jobs, to become involved in a hot, shapeless and dangerous public sphere. In the France of 1793 you cannot invoke Christianity, and you certainly cannot celebrate the monarchy or the aristocracy. Artists can call on elements of ancient mythology but these have to be reworked ... everything is up for grabs, and painters are faced with the challenge of reinventing a language from the ground up."

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And one of the results of this was David's Marat. "Marat was a true maverick, extremely difficult to handle politically and personally," Clark says. "He was far more radical than the ruling government. He was hated and despised by many and that is why he was stabbed ... but the idea of an artist celebrating and sanctifying Marat was extraordinary. It's like somebody being called on to celebrate Michael Foot in his heyday, when everybody seemed to hate him, except here there was real blood on people's hands."

But Clark is fascinated to know why, despite all they have delivered, "modern societies have spawned an art that seems so disaffected with the norm ... Why is there an art that is continually goading the bourgeoisie, intent on shocking the public and why is modern art so obscure? Why can't it reorient itself towards scrutinising and celebrating the good things, current values?" The answer, he believes, is double edged. "There is a side of modernism that relishes modernity (modern social and political conditions), loves it, but there is also a side continually on the attack. Modern art is in a continual unresolved condition, celebrating modernity but hating it at the same time."

One of the most provocative aspects of Farewell is Clark's binding together of socialism and modernism. For Clark, modernism in the arts is tied in with the long "Age of Revolutions" that began in 1789. Did this age finish 200 years later with the collapse of the Iron Curtain? Did modernism and socialism live and die together?

The fall of the Berlin Wall in 1989, Clark agrees, was a moment when many people thought modernism in art, like the Soviet Empire and perhaps the socialist project as a whole, had come to an end. "In a sense I do think they went together, both addressing questions about what modernity was, how it could be completed and tensions within it could be resolved. Modernism and socialism also shared the same idea - it seems very dated now - that modernity might be a transitory state leading to a final completed state."

Nonetheless, Clark warns that attempts to draw a line under modernism have occurred before. "In the 1890s there was a reaction against symbolism. After the first world war, in the aftermath of cubism, there was a huge uproar, people saying they had had enough of this dreadful early 20th-century experiment, that it had just led to chaos and anarchy, we should reconstitute the past, the 'good things'."

But it did not happen then - and it is still too early to say whether the era of modernism in art is now finally over.

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Farewell to an Idea, Yale University Press, Pounds 30.