TO study philosophy increases your identity, giving you more strength as an individual. If we learn a little about thinking, it gives us a stronger self, because philosophy doesn't ask questions like what have you got? but what are you?.

Jostein Gaarder talks like an evangelist. The author of Sophie's World, the g novel that doubles as an introduction to philosophy, he has an unquestionable commitment to the cause of questioning everything. And it's not an austere or overly rational activity either. For me philosophy is not something just cerebral, it's also a question of sensuality. When I ask the questions who am I? what is love? what is Nature? what is the Earth? I'm talking about physical things. And I don't think at all that my mind is more important than my body philosophy includes both."

In his new book, The Solitaire Mystery, he underlines his argument that the best introduction to philosophy is to hang on to something of your childish curiosity.

"We are born curious. Young children ask curious questions "does God exist? Why do the stars twinkle? How do birds fly?" he says.

While the idea that adults have a lot to learn from children is hardly new, what is striking is Gaarder's assertion that it's through such apparently childish questions that we can make a start on the more difficult work of Socrates, Descartes and Hume.

"You don't have to teach children philosophy, they are philosophers already. It's more important to teach adults. As we grow up we get so accustomed to the world, the world becomes a habit."

Gaarder, himself a boyish looking 44, was for many years a philosophy teacher in his native Norway, where all students going to university have to take an introductory course in philosophy.

"It's not a coincidence that the writer of Sophie's World is a Norwegian," he says. Nor is it a coincidence that the teaching in the book is set within the framework of a novel.

"I think the story is our mother tongue. Our human brain is made for stories, more than it's made for storing information. A good teacher is a good storyteller, and I've done both. And in Sophie's World I did both too."

The surprise publishing success of recent years, the novel concentrates on a 14 year old girl receiving philosophy classes from a mysterious teacher.

As the narrative develops, the reader gets a history of philosophy from the preSocratics to Sartre.

To begin with, Gaarder was surprised at the success. "It seems as if the English word I've used most in the last two years is baffled". When I sent the manuscript to the publishing house, they hesitated, and when they decided to publish it, I wrote them a postcard thanking them for doing this completely non commercial act."

NOW, he has a couple of theories as to the world wide success of the book. The palatable mixture of entertainment and philosophy helped, but he sees the main reason as a widespread desire to find out about the big philosophical issues.

"One thing I've learnt out of this is that human beings are human beings we have the same needs and ask the same questions," he says.

Asking questions is all very well, but Gaarder is adamant that philosophy has practical benefits for society.

"What is happiness? What is a good life? What is a just society? These are questions that don't have a specific answer. But it's crucial that all generations find their own answers to these questions.

"It's extremely important that we encourage young people to ask these questions, because if you don't ask the question, you have no possibility of achieving that happiness or justice."

He also sees philosophy as a unifying force in an increasingly fragmented society. "All societies need to have something in common, but we've lost the physical meeting place in the centre of town. We now have television, and as long as we only had one TV channel it served as the nation's meeting place.

"But in the 1980s there was this explosion of media, and we lost the common platform. There are lots of people living in the great cities of the world who aren't part of civilisation, in that they don't know anything about their spiritual background.

"For me this is a question of identity I feel I have a wider and deeper identity than my physical body I'm a Norwegian, I'm European, and I'm a member of the human race living at the end of the 20th century."

One political attempt to create this common platform" is the United Nations, an organisation for which Gaarder has a lot of respect. One of the characters in Sophie's World is a major in the Norwegian contingent of the UN peace keeping force in the Lebanon.

"The greatest practical achievement of philosophy is the Human Rights charter produced by the UN in 1948," he says. "Statements like "All humans are equal", were not taken out of the air, they were based on philosophical reflection you see traces of John Locke and Voltaire and others."

The Solitaire Mystery, Gaarder's new book, has a similar story within a story framework to Sophie's World, but there is more fantasy and no overt philosophy teaching.

"All my books are philosophical, but Sophie's World is the only philosophy book. With that book I had no artistic ambitions, no literary ambitions," he says. The Solitaire Mystery is more sensual than Sophie's World it's more dependent on story, and destiny and love."

The book follows 13 year old Hans Thomas as he travels with his father from Norway to Greece to search for his mother who has gone to Athens. On the way, Hans Thomas finds a tiny book which contains a story about a ship wrecked sailor who lands on an island inhabited by living playing cards.

The two stories in the book are closely interconnected, with the motif of playing cards used to illustrate the need for philosophical curiosity. It's the joker in the pack who asks the important questions about who we are and where we came from.

I THINK we are all born jokers in life's game of solitaire," says Gaarder.

"As we grow up and get used to the world we become the boring eight of clubs or nine of diamonds. But there is still a joker living in all of us, and many things in life can bring it out. For instance, we fall in love and we see the world in quite a new way."

This ability to marvel at the world around him shines through in his work.

"When people get old they start looking in a new way at this fantastic, enigmatic fairy tale we live in. It's sad if you have to get old or ill to see this, and here I think that literature and philosophy can bring us into contact with the joker in all of us."

Two more of Gaarder's books will soon be available in English The Christmas Mystery appears in November, and Through a Glass Darkly early next year.

For Gaarder himself, after some promotional work in England this week he's off to a new cottage by the sea in southern Norway. To contemplate the fairy tale of life.