Vive Yves!

''Ma plus belle histoire d'amour, c'est vous..." So sang Catherine Deneuve, serenading her old friend Yves Saint Laurent when he came out on to the catwalk on Tuesday night for the last time. At the Pompidou Centre, the great designer's swan-song show was a brilliantly produced journey through his own archives – the lovely high points of the grandest of fashion careers, spanning more than 40 years. The African collection, the Cossacks collection, the Gypsy collection... The first Le Smoking tuxedo, the Safari look, fashion inspired by the work of artists from Mondrian to Picasso, from Cocteau to Matisse... The perfect black evening gown, narrow in liquid satin, overblown in rustling taffeta, strapless, backless, plunging to the navel... Many of these garments have already attained iconic status.

For one brilliant hour, Saint Laurent's supremacy as fashion's great colourist, its most effortlessly cultured couturier and the best fashion friend of contemporary women was captured and sealed for eternity. As he stepped out to receive his ovation, it was hard to tell who was more moved, the man himself or his audience.

In the end, the legacy of Yves Saint Laurent is about far more than just clothes. He is the only designer in the world to have his own museum, a place in the Paris suburbs where devotees are not allowed within an arm's length of his fiercely protected archive unless wearing fresh white gloves. His image graced some of the last French francs ever minted. In 1999, French lovers received St Valentine cards boasting stamps of Saint Laurent's design. All of this testifies to the vast shadow he casts over culture, and French culture in particular.

So, what is it about Yves Henri Donat Mathieu Saint Laurent that makes him so special? And why is his stepping down so moving? Suffice it to say that when Paco Rabanne, say, took his final bow, no one shed a tear.

To begin with, there is, of course, the socio-political impact of Saint Laurent's clothes. His long-time friend and client, Paloma Picasso, puts it like this: "Yves gives women both extreme elegance and dash. He put trousers into a woman's wardrobe and made our lives easier."

In fact, Yves Saint Laurent did rather more than that. Of course, his Le smoking tuxedo – Saint Laurent sent the first one out in 1966 – was the first real eveningwear alternative for women who'd rather not wear overtly feminine gowns, and therefore had considerable impact. But as much as a decade before that, Saint Laurent – who took over from Christian Dior when the latter died suddenly in 1957 – also relieved women of the dowdy and overly formal way of dressing, and the legacy of the wasp-waisted, full-skirted New Look in a more general way.

Here was a designer who, despite the fact he worked in the heart of the bourgeois fashion establishment, was well aware that young people needed a more relaxed aesthetic, an aesthetic in line with more modern and permissive times. Where a look was too overtly feminine, he invested it with masculine dress solutions and, in feminising masculine dress some time later, vice versa.

In 1968, Saint Laurent sent out the first ever transparent blouse – for a woman to be seen bare-breasted now is run-of-the-mill, way back then, it was a brave move indeed and more about the liberation of women than their exploitation. The designer paid tribute to the looks of the street – the student riots of May 1968, the Beat movement – and interpreted them for the couture catwalk, so educating a hitherto extremely narrow-minded clientele in street culture.

More democratic than any of this, he also, that same year, opened the first designer ready-to-wear boutique – Yves Saint Laurent Rive Gauche. Clara Saint was his press officer at the time. She has since said: "It was the first boutique opened by a great couturier outside the walls of the fashion house. It was young, subversive and, considering the part of town [on rue de Tournon, in the heart of Left Bank Paris], it had a certain 'intellectual' connotation."

Up until then, haute couture allowed only the very privileged few who could afford it the magic of the Parisian designers' touch – the rest of the world had to make do with cheap and badly made copies. Saint Laurent put an end to such unashamed elitism, paving the way for every other designer worth mentioning to go on and do the same.

"He constructs clothes around the body, to hide or to reveal just enough," continues Paloma Picasso. "With him, you can be fat or thin and you will always turn out just right. He creates protective clothes, by day to look your best going to work, by night to be more extroverted, more seductive."

In this, too, Saint Laurent is a rarity. For a designer of his creative impact and cultural diversity to put women's needs first, over and above any pyrotechnics and/or a ferocious ego, makes him respected more than any more obviously attention-seeking designer working today.

Yves Saint Laurent is more well-read, better versed in the arts and educated in theatre than almost any of his contemporaries and, certainly, the younger generation of designers, but he is remarkably self-disciplined where airing his own, or others', artistic talent is concerned.

Jean-Paul Goude, the Eighties über-photographer and film director, and the man behind the creation of Grace Jones, puts it neatly: "Today, as opposed to some designers who don't seem to see the difference between the art world and the fashion industry, and who forget that a garment should enhance the individual who wears it, one can only admire Saint Laurent's humility. He has never stopped putting his tremendous talent at the service of the women he dresses."

Small wonder, then, that women the world over love Yves Saint Laurent for it.

The photographer Helmut Newton has enjoyed a closer relationship to a famously reclusive designer than most.

"Yves Saint Laurent has inspired my best fashion photos – Le Smoking in rue Aubriot in Paris; the man on his knees rolling down the stocking of a woman in a tailored suit at the Trocadero; a colour photo for French Vogue of all the mannequins in the couture salon; a series of couples in tailored suits kissing each other. I like the ambiguous side of his fashion for the grande bourgeoisie."

Newton, over and above Saint Laurent's other collaborators, was tuned in to the designer's darker and more overtly sexual side, the side that was so brilliantly expressed when he dressed the archetypal repressed bourgeoise, played by Catherine Deneuve, in Belle de Jour. The photographer also identified the one thing that, above all others, sets Saint Laurent aside: "There are so few people with whom I feel completely in accord. And he, he is so touching, so fragile."

Yves Saint Laurent has traced his own neuroses back to his youth and the fact that he was taunted by his classmates for his homosexuality from an early age. That same fragility, he has said, also furnished him with the will to succeed. "I told myself repeatedly, 'one day, you will be famous'. My name will be written in fiery letters on the Champs-Elysées."

The designer had his first full-scale nervous breakdown after being called up for his military service in 1960. Since that time, he has been forced to battle with, at times, extreme instability. His dependence on drugs – prescribed and otherwise – and alcohol; his sojourns in various clinics; his increasingly reclusive nature have all been well-documented. But however unbalanced Saint Laurent may have been, he has never failed to take his seasonal catwalk bows. At times, this has been precarious: his physical and mental fragility have been so obvious that rumours of his forthcoming retirement have surrounded him for well over 10 years.

When, two weeks ago, this finally happened, the notoriously shy Saint Laurent braved the media spotlight once more, as opposed to asking his long-time business partner and former lover Pierre Bergé to do so. Alongside the prerequisite grandeur and, in Saint Laurent's case, perfectly justified hyperbole ("I tell myself that I created the wardrobe of the contemporary woman, that I participated in the transformation of my times"), the designer also felt compelled to open up his less comfortable side. Given that he has refused all but the briefest face-to-face interviews for many years now, and is fiercely protected from the press while his contemporaries continue to court it, this was not only remarkable but brave.

"I have been through anguish," he continued, reading from a piece of paper and never letting his gaze meet the room full of reporters. "I have been through hell. I have known fear and terrible solitude, the false friends that are tranquillisers and drugs, the prison of depression and of health clinics.

"But I got through, dazzled but sobered-up. It is people who suffer from nervous problems who have started religions and created masterpieces. I am part of that family. I've rubbed shoulders with those who play with fire, but the most important thing is to learn how to face up to yourself."

Whether Yves Saint Laurent has finally been able to do just this will no doubt remain one of life's great mysteries. This is only fitting. Such an enigmatic conclusion is only to be expected of the career of the greatest living fashion legend of them all.