When I put this to her, it’s obvious that she is not entirely sure of the answers herself. After all, she came into fashion through the back door, with no training and, initially at least, no great interest in the business. She had always had an abiding curiosity about appearance and what it says about us, though.

“I realise how powerful and important clothes are, especially for women,” she says. “They have to be useful for your life, of course, but they must also express your individual sentiment.” That Prada accepts that all fashion is role-play is what makes her such a force. She understands that the job of the truly ground-breaking designer is to decide on the roles, dress them and then present them in such a way that people all over the world want to join the cast.

Prada herself makes an unlikely éminence grise. She is neither dowdy, nor overwhelmingly chic. In fact, she looks entirely normal. Her figure is that of a woman in her fifties. Her hair is cut like most other women’s hair. She rarely wears make-up. Contrast her with another Italian icon, Donatella Versace, and you realise that she is a fashion outsider. Certainly, she avoids socialising with most other designers and runs a mile from social events. As she said to me, years ago: “I am a wife and a mother” — she has two teenage boys — “and I have many more interests than fashion. Fashion is just my job.”

But today she has amended her tune: “I’ve become impatient when people claim they don’t care about clothes. They still dress every morning, and if they are going to reject fashion, they still need clothes to show it. Style rebellion is still a form of self-expression.”

She is right. We all play roles. The question is how she managed to persuade women that sexiness could be predicated on librarian chic, that frumpy could be cool. What did she know that the rest didn’t?

A feminist and ex-communist, she was trained from youth to revolt against the status quo. She never believed that sexual dressing automatically involved becoming a sex object. This began during her school days. “I was mad about clothes when I was young,” she recalls. “But my mother was strict, and anything not serious was forbidden. I was always dreaming, though. And when I was 15, I broke out. I started looking for things that were inappropriate.”

Prada realised that choosing the inappropriate is what moves creativity forward. In the 19th century, impressionism was seen as inappropriate. So was surrealism, some 40 years later. Likewise, Andy Warhol and Damien Hirst. The list is long and important. These are the people who have changed our thinking for ever.

Many seasons ago, Prada took us back to school, with a new version of womanhood, the girl-woman. Slim, virginal, as uncurvaceous as a young boy and clad in a pleated skirt, neatly pressed blouse and a pair of clumpy shoes, the look said: this is where the new woman starts. People were as shocked and outraged as they always are when confronted with the new. This isn’t Italian fashion, they claimed. In fact, it isn’t fashion at all. But Prada had realised that the overly sexy power woman personified by the supermodels was over.

Of course, it’s useful to have money to help make your point, and her company’s focus on shoes and bags meant that she could afford advertising. This ensured her message was publicised worldwide. But it wasn’t easy. “I was never happy doing interviews, because I never wanted people to know my thinking,” she says.

She wanted the clothes to speak for themselves. Like learning a new language, we slowly began to understand the grammar and the syntax of the new fashion-speak. But what could the Prada girl do as she grew up? Turn into a big-busted, typically Italian sex symbol? Hardly. Like her creator, she had to have an intellectual element if she was to possess the conviction necessary to keep the story developing. She kept the librarian chic, but she gave the look its own unique sexiness. This was a librarian who jazzed up her clothes with killer heels and bags, and revelled in the luxury of astrakhan fur and exquisite embroidery.

“If you have something to tell, you must do it through an idea. For me, that is always through clothes,” she says. “This leads us into considerations of imagination, desire, the longing to be beautiful and the obsessions of narcissism. These are big subjects.”

The fact that she even thinks about them at all confirms that, in the fashion world, Prada is not just a big subject herself, but that she is, quite frankly, the biggest.

AN EVENING WITH MIUCCIA PRADA

Join Style and Colin McDowell for an evening of conversation with Miuccia Prada. She will be discussing her life, her look and her design philosophy. The talk will take place at the British Museum, London WC1, at 7pm on November 23. Tickets are £20; to book, call 0870 842 2242

Key looks for the Prada girl

Fur tippet Introduced in 2000, it is now a vital aspect of Prada’s ladylike aesthetic. Gym knickers Perhaps it is an offshoot of her fascination with uniforms, but Prada has consistently shown big gym-style knickers on the catwalk. Pleated skirt The just-on-the-knee pleated skirt, beloved of librarians, piano teachers, maiden aunts and other sensible types, is such a Prada staple that the woman herself usually takes her catwalk bow in one. The belted cardigan Prada invented a new way to wear it — she added a skinny belt

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