In 1605, an unknown Spanish author published a novel that rewrote the rules of literature. Michael North asks why generation after generation has fallen for Don Quixote

There is an apocryphal tale about a group of Japanese tourists who went to Campo de Criptana in Spain to see the imagined inspiration for the windmills that Don Quixote attacks on his skinny horse. One of the group asked the tour guide which mill the hero had tilted at. On being told that it was only a story invented for a novel, the tourist and his fellow countrymen promptly got back on their bus with their illusions shattered.

Such is the impact on the human psyche of Miguel de Cervantes' novel, published 400 years ago. Like Shakespeare's Hamlet , it has been absorbed into each generation's culture and has created a new vocabulary - countless languages have a form of the adjective "quixotic" or the phrase "tilting at windmills". And even if you have not read the book, you will know of its existence, or at least of its curious hero.

"In Spain, it is not much read but it is in the atmosphere. It's more real than any real person," says the Spanish novelist Juan Pedro Aparicio.

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Aparicio, who is the new director of the Cervantes Institute in London, a body charged with disseminating Spanish culture, says that when Don Quixote first appeared in 1605 on the streets of Madrid, it was a shock bestseller, penned by a nobody.

The Spanish public instantly fell in love with this tale of a lowly nobleman driven potty by reading too many chivalry novels, who transforms himself into an errant knight, complete with homemade helmet and lance. He, his malnourished horse, Rocinante, his pragmatic squire, Sancho Panza, and a donkey go on a picaresque journey across the central Spanish plain - fighting windmills and saving princesses masquerading as prostitutes.

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The book captured the imagination of the rest of Europe too, particularly in Britain where the novel was almost immediately recognised as a great work of art.

In this 400th anniversary year, Venezuelan president Hugo Chavez has had a million copies of Don Quixote distributed free to his people. He hopes to "feed" the country once again "with that spirit who went out to undo injustices in the world".

Around the planet, governments, academic institutions and Cervantes institutes have organised marathon readings of the book, as well as lectures and festivals, to mark the anniversary. "The Quixote", as it is reverently called today, has sold more copies than any other volume after the Bible, and in 2002 was voted "the most outstanding book of all time" by 100 prominent writers, chosen above masterpieces such as Homer's Odyssey and Tolstoy's War and Peace . Among the writers selecting it was Milan Kundera, who said: "The artist need answer to no one except Cervantes."

So why has the book had such an impact? Why has it so influenced writers that the 20th-century US critic Lionel Trilling judged that "all fiction is a recreation of the theme of Don Quixote "?

"The key to this lies in Cervantes' claim that his intention was to destroy the romances of chivalry," says Edwin Williamson, professor of Spanish studies at Oxford University. "There had been numerous burlesques and parodies of romance before Cervantes, but what made the crucial difference was Cervantes' brilliant idea of making a country gentleman go mad from too much reading. At a stroke, the intricate poetic world of chivalric romance that had entranced the European imagination for some 500 years was reduced to an illusion inside a madman's head." Williamson observes that the basic principle of Cervantes' narrative was the collision of illusion and experience, a principle that underpins the development of the realist novel that evolved in subsequent centuries.

Works that owe a heavy debt to Don Quixote span the centuries since 1605: Fielding's Tom Jones (1749), Goethe's Wilhelm Meister (1795), Stendhal's Le Rouge et le Noir (1830), Dickens's The Pickwick Papers (1836), Flaubert's Madame Bovary (1856), Twain's Huckleberry Finn (1885), Melville's Moby Dick (1851) and Rushdie's The Satanic Verses (1988).

Flaubert wrote: "I have found all my origins in the book I knew by heart even before I was able to read: Don Quixote ." While Aparicio comments: "As a writer, Don Quixote is a continuous source of information and inspiration."

The novel was a quantum leap in literature, not least because of the creation of its two central characters. Rodrigo Cacho, assistant professor of Hispanic studies at the University of British Columbia, says: "[Jorge Luis] Borges was right when he said that the Quixote is immortal, especially because of its two protagonists who have entered our common imagination."

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Williamson contends that Cervantes invented the "modern conception of literary character in which there is a sense of complexity, of a specific individuality, of the growth of an inner life, which is very far removed from the stock types that predominated in medieval literature. Cervantes created two such characters... and locked them in a relationship in which the destiny of each critically depends on the actions of the other, in which there is dialogue and conflict, affection and envious rivalry, a relationship that Cervantes will come to use as a prism through which to observe - and criticise - his own society."

Howard Young, professor of romance languages at Pomona College in the US, says the relationship between Don Quixote and Sancho constitutes "the longest and most enduring conversation in all literature".

Young sees the gamut of human relations explored through the record of their friendship, which occupies 90 per cent of the novel. "They deceive each other, mock each other, forgive each other, argue about the state of the world," he observes.

Contemporary parallels of Cervantes' double act can be found in Laurel and Hardy or the film Withnail and I .

A further innovative feature that made Cervantes' work the first "modern" novel was the interplay between illusion and reality - the chivalric imaginings of Don Quixote that make every tumbledown inn a castle and every syphilitic prostitute a fair damsel worth fighting for, in stark contrast to the brutal, paltry reality of 17th-century Spain's common men, who give the knight and his man a beating at every turn.

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Anthony Close, a Cervantes specialist and emeritus professor at Cambridge University, says this conflict "opened up a rich psychological field and a sector of reality - the everyday - that had remained virtually untouched by literature before Cervantes. Having crystallised in the 18th century and developed in the 19th, the novel... operates something like the twofold perspective that Cervantes invented, foregrounding a daydreaming or idealistic subject, often inspired by literature, whose illusions more or less resemble Don Quixote's chivalresque-literary mania." Close adds: "That Don Quixote became first a classic and later a legend is thanks largely to the historical coincidence that the modern mentality, from the 18th century onwards, has found in the book a vehicle well adapted to its expression."

Cervantes' use of multiple layers of narration has also had many imitators - the book's narrator tells how he found the adventures of Don Quixote in a notebook lying in the street and had the story translated from Arabic by a Moor. Williamson says Cervantes' invention of the "unreliable narrator", used to great ironic effect, became a "ubiquitous modern device".

Such devices are also a dream for postmodern critical theorists. James A.

Parr, professor of Spanish at the University of California at Riverside, in the US, says the novel "not only responds to any critical approach one might wish to apply" but anticipates "several important facets of modern and postmodern theory". Parr elaborates on Cervantes' prescience of narratology, deconstruction, metalepsis (infraction of narrative level), analeptic prolepsis (flashback to immediately flash forward), and a host of other devices. He adds: "It is a book that speaks to readers of 2005 as eloquently as it did to those of 1605."

The book has fuelled an academic industry. Paul Julian Smith, professor of Spanish at Cambridge, talks about the "recent emphasis on the 'homosociality' of the novel: the way in which the woman [Dulcinea, Don Quixote's imagined lady-in-waiting] is just a foil for an exploration of a relationship between two men [Don Quixote and Sancho]".

However, such theories hardly explain the enduring nature of Cervantes' huge book - a two-parter of more than 1,000 pages. From the first line - "In a village in La Mancha, the name of which I cannot quite recall" - the book is threaded through with a throwaway and compelling irony as well as with hilarious episodes. "What giants?" asks Sancho when Don Quixote points at the windmills and urges him to engage in mortal combat.

Cacho says: "What I love about the novel is its massive dose of entertainment. Cervantes has made his readers laugh for centuries using multiple devices." Joaquin Garrido, a former Cervantes Institute president in the US now teaching at the Universidad Complutense in Madrid, adds: "It is such a funny book. When we read the whole two parts in both English and Spanish in Albuquerque last April, people kept laughing."

But there are also the more universal themes that give a great work of art, in Close's words, "the capacity for self-renewal".

Roberto Gonz¨¢lez Echevarr!a, professor of Hispanic and comparative literature at Yale University, notes: "It focuses on literature's foremost appeal: to become another, to leave a typically embattled self for another closer to one's desires and aspirations." For him, Don Quixote is about the "soul's pith, the flickering light of being". In the travails of that preposterous, deluded romantic, the reader finds hope.

A LOSER'S SUCCESS

Despised by the literary elite of Spain and buffeted in life by the "slings and arrows of outrageous fortune", Miguel de Cervantes was, according to novelist Juan Pedro Aparicio, a loser. But he kept bouncing back, making him a symbol of hope for the underdog.

Cervantes was born in 1547 in Alcal¨¢ de Henares, near Madrid, to a lowly nobleman, a surgeon. His literary debut is marked by four poems published by the humanist L"pez de Hoyos.

After a spell in Rome, Cervantes became a professional soldier, fighting against the Turks at the sea battle of Lepanto in Greece in 1571, where his left hand was maimed by a harquebus shot. He later took part in Juan of Austria's campaigns in Corfu and Tunis.

In 1575, Cervantes set sail for home with his brother Rodrigo on the galley El Sol . The ship was captured by the Turks and the brothers were taken to Algiers as slaves. Cervantes attempted to escape four times but it was five years before his family raised the ransom to free him.

He returned to Spain expecting a war hero's welcome but instead found penury. Following his marriage in 1585 to Catalina de Salazar, who was 22 years his junior, Cervantes published the pastoral novel La Galatea but it was poorly received and he never wrote the promised sequel.

He was forced onto the roads of Andalusia for ten years, first raising capital for the doomed Spanish Armada and then as a tax collector. As a result of money troubles with the Government, he was jailed in Argamasilla in La Mancha, near Seville, where, legend has it, he started to write Don Quixote. The novel, published in 1605, was an instant success.

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In the final nine years of his life, Cervantes wrote prolifically, publishing in 1613 the Novelas ejemplares, another acclaimed masterpiece, and the second part of Don Quixote in 1615 as a masterly riposte to a fake sequel. Cervantes completed his last novel, Los trabajos de Persiles y Sigismunda , three days before his death on April 22, 1616.