Look closer, however, and the discussion about God’s existence, which takes place between a small boy and a creature from outer space, is qualified with “I don’t knows”, “I’m not so sures” and a sprinkling of question marks.

“This is not a Christian confession, although some people reading it have thought so,” explains Gaarder over a Coke and a cigarette at a London hotel. He does believe there may be a “force” in the universe, “which dragged us up from the oceans and gave us eyes for seeing and brains for thinking”, but he would not call it God. This belief in a higher purpose surprised the biologist whom he consulted about the book, but Gaarder’s hope is that Hello? Is Anybody There? may act as a kind of bridge between science and religion, between the rational and the emotional.

“This is a personal book; it is, in a way, my credo, the story of my life in nature. I would like to help children ask questions and have an open mind. I want to share with them my sense of awe, my experience that life is a mystery and that we are part of it for a very short time only. This book, like all my books, comes out of that inspiration.”

Jostein Gaarder became an international best-selling author three years ago with the publication of Sophie’s World. Intended to introduce teenagers to the history of philosophy, the book was also snapped up by an army of philosophy-hungry adult readers, becoming a surprise hit in 40 countries and selling more than 10 million copies. Two novels followed: The Solitaire Mystery, which Gaarder wrote before Sophie’s World but was published in Britain a year later; and The Christmas Mystery, which is enjoyed by nine-year-olds upwards.

Hello? Is Anybody There? is Gaarder’s first attempt to address an even younger readership. It was commissioned in response to reforms in his native Norway under which children will start school at six rather than seven. But although a bright six-year-old might enjoy having the book read aloud, Gaarder now sees it more as a book for eight or nine-year-olds, to be read and talked about at home as well as in the classroom.

Sophie’s World, he admits, was unashamedly didactic, presenting chunks of philosophy in novelised form. In Hello? Is Anybody There? he is still keen to impart information (the legacy, perhaps, of his years as a teacher in a Norwegian high school), but in this case it is about evolution and the emergence of human life.

The book is more open, more exploratory. It begins when eight-year-old Joe, hours before the birth of his baby brother, is visited by Mika, who falls from a spaceship and lands upside down in Joe’s apple tree. The two strike up a friendship, and out of their mutual curiosity arises a long conversation about the nature of the universe.

Sally Gardner’s charming drawings bring out Mika’s baby-like aspects - she even has him dressed in a Baby-gro - and the way in which, for Joe, he is a dress rehearsal for the new baby brother waiting to discover the world.

Joe and Mika’s conversation touches on some big topics - gravity, evolution, human reproduction, God, time - but never pauses long enough to become heavy-handed or boring. There is a rather comical, but pertinent, passage early on where Mika bows in response to one of Joe’s questions. “Where we come from, we always bow when someone asks an interesting question,” he says. “And the deeper the question, the deeper the bow.”

At first, Joe is so flummoxed by this that he starts bowing whenever Mika says something that sounds clever. But Mika soon puts him straight: “When you bow, you give way, and you must never give way to an answer. An answer is always the stretch of road behind you. Only a question can point the way forward.”

Young children, Gaarder agrees, are naturally questioning and much more open to experiencing and finding out about the world than teenagers. Bedtime is often the time when a small child will suddenly come out with the big questions: Where do we come from? Who made the world? Do you believe in God? It is a time of safety and comfort in the semi-darkness, away from the business of the day, where the child has, for as long as he can hold on to it, the exclusive attention of a parent. Hello? Is Anybody There? is the perfect book for such times, guiding both parent and child through some of the larger mysteries, and giving both of them ample opportunity to join in with ideas of their own.

“The most important thing parents can do for their children is to read with them,” says Gaarder. “So many children today are just sitting in front of the television, and we are becoming very poor at sharing experiences between the generations. But if you have a child on your knee with a book, you are sharing it, you have the close physical contact, and the child can start to talk to you. We talk about interactivity on computers, but really there’s more interaction in reading a book.”

Having spent much of the past three years publicising his books, and his message, around the world, speaking at conferences and visiting classrooms,Gaarder admits that not only is he weary of travelling, he is also a little sick of hearing the word “philosophical”, and the verb “to wonder”. But judging from his sales figures, the message is getting across. As for schools, Gaarder’s ambition is not that philosophy should be enshrined as a subject on the timetable, but more that, as an attitude of mind, it should find a place in subjects from English to biology.

“Often you learn more by asking questions, than from the answers you get,” he says. “It’s about developing a critical attitude.”

Hello? Is Anybody There? by Jostein Gaarder, illustrated by Sally Gardner,is published on November 10 by Orion Children’s Books, #163;10.