Mary Woolstonecraft - mother of Mary Shelley and a member of Dr Johnson's circle - wrote in her Vindication of the Rights of Women, that the ideals of Voltaire and Rousseau should apply to women. High-born women held literary salons, especially in Paris. "Ladies like the Marquise Du Deffand don't exist any more," says Prof Jean-Didier Vincent, a French neuro-physiologist who wrote the best-selling Biology of the Passions, as well as a book on the sex-life of Casanova. "They were women of extraordinary freedom of thought and morals. They made love with whomever they wanted, went from one partner to another, ignored all the taboos."
The sexual decadence - or freedom, depending on who you talk to - of the 18th-century aristocracy still fascinates. Publishers report a rush on the books of the Marquis de Sade - who paid for his writings by spending most of his life in prison. In the century when the Church was first openly questioned, Sade went too far. His novel Juliette, for example, recounts satanic Masses conducted by lascivious priests who whipped, sodomised and murdered young girls.
The 18th century is a popular topic in gay and lesbian studies programmes in the US, because for the first time homosexuality was practised relatively openly, in "Molly Clubs" throughout Europe.
Homosexual characters began to appear in literature. "Society was very homosexual," Prof Vincent says. "In the army most of the officers were homosexual, for example the Duc de Vendome, who won the Spanish war of secession, and Turenne, the Marechal de Villiers."
The Venetian-born male courtesan Giacomo Casanova (1725-1798) embodied the reckless, cosmopolitan existence of 18th-century Europe. "He was insolent. He loved luxury and the rich and powerful, but at the same time he loved being free," says Prof Vincent. French was the lingua franca of the day, and Casanova wrote his 12-volume memoirs of seduction in French. He is believed to have been the model for Mozart's Don Giovanni - and may have helped to write the libretto. "You can study almost any aspect of 18th-century life through the life of Casanova," Andrew Miller says, "food, travel, morality, medicine . . ." Miller's second novel, published last year, chronicles Casanova's brief exile in London.
Promiscuity had a price; venereal disease was rampant. "If you had a good constitution, you might get over it," says Miller, who did extensive research on 18th-century medicine before writing Ingenious Pain. "James Boswell, the biographer of Samuel Johnson, was constantly going down with what he called "signor gonorhoea". They treated it with mercury, which is highly toxic and makes your teeth fall out. A man like Boswell from a good family would not have been considered unrespectable for going with whores. Being rather mean, he went with the cheapest ones."
While 18th-century medicine was often primitive, the period produced some of the millennium's most brilliant music. Mozart has never gone out of fashion. Now the works of almost forgotten composers such as Jean-Philippe Rameau are being revived in France by groups such as William Christie's Arts Florissants and Marc Minkovsky's Musiciens du Louvre.
Academe too, is filled with 18th-century nostalgics. Witness the success of the 10th Enlightenment Congress, organised by Prof Andrew Carpenter at UCD last month; more than 3,000 academics from around the world attended, and they will convene again in Los Angeles in four years' time. "There is nothing equivalent for the 16th, 17th or 19th centuries," Prof Cronk says. "There are 18th-century societies in Britain, Ireland, France - even in Hungary and Estonia; it's absolutely flourishing."
Pierre Rosenberg, the director of the Louvre is helping to prepare three major 18th-century exhibitions this autumn. To mark the 300th anniversary of the birth of the painter Jean-Simeon Chardin, the French luxury conglomerate LVMH is sponsoring a major exhibition of the painter's works at the Grand Palais. Over the next year, the Chardin exhibition will travel to Dusseldorf, London and New York. Chardin refused to paint religious, mythological or historical themes, winning the affection of 18th-century philosophes - and helping to usher in modern painting. A hundred years later, Manet and Cezanne were influenced by him.
From next October until January, the Louvre has scheduled an exhibition about its founder, the artist, diplomat, writer and occasional spy Dominique Vivant Denon (1747-1825), who adeptly manoeuvred from the court of Louis XV through the French revolution and into the favour of Napoleon Bonaparte. "Denon was very much a man of the 18th century," Rosenberg says. "Under the (Napoleonic) Empire he transformed the Louvre into the world's first modern museum. He had in mind the ideas of the 18th century and of the French revolution - to make art accessible to the masses; to make art clear; to present it chronologically. He shared the obsession of the philosophes and encyclopedistes with classification."
For the past 10 years, Charles Wirz, a Rousseau specialist and the director of the Voltaire Institute and Museum in Geneva, has corresponded with eager teachers in Tokyo, where the theories of Jean-Jacques Rousseau (1712 - 1778) are very much in vogue. The idea of raising children as "noble savages" uncorrupted by society seems to please Japanese parents traumatised by their own regimented upbringing. "The idea is to create an environment conducive to the effect you want to achieve, without the child realising it," Wirz says. "You work on the environment - not the student."
Rousseau had a similar effect in 18th-century France. When he counselled women to breast-feed their babies rather than entrust them to wet nurses, fashionable pre-revolution ladies responded by nursing their babies in opera boxes. Rousseau's idealisation of work and country life influenced the royal palace and spread beyond French borders. The dauphin was taught to plough.
Marie-Antoinette built a mock dairy farm at Versailles so that she could pretend to be a milkmaid. In the BBC's Aristocrats, Sarah Lennox dressed up as a peasant, pitching hay in the hope of catching the eye of George III.
For the rich, the 18th century was a game of seduction in "pleasure gardens" and masquerade balls. But the vast majority of Europeans were illiterate peasants for whom life was cruel. Prof Vincent nonetheless confesses he would have liked to have lived in that period. "I love freedom, beauty, women," he says. "Yes, I'm nostalgic for the 18th century. But I admit there was deep injustice; I admit it had to end - like all great moments of pleasure."