At a conference in Geneva 13 years ago, Professor Stephen Hawking was stricken with pneumonia. Despite emergency operations, his condition deteriorated. Soon he was close to death, comatose and on a life-support machine. Jane, his wife at the time, who had nursed him since he discovered in his early twenties that he had degenerative motor neurone disease, was asked if she wished the machine to be switched off, but she elected to give him the chance of life.

With the aid of a tracheostomy that removed his power of speech, he survived to write A Brief History of Time. This month he celebrates the tenth anniversary of the publication of the book, which has been translated into 40 languages, has sold one copy for every 750 men, women and children on earth, and has remained in the Sunday Times’s bestseller list for 237 weeks.

Critics have compared the book to the biblical Revelations, a text providing a path to a fuller understanding of the mystery of creation; others have mocked its arid impenetrability, suggesting that it is the century’s most unread book.

The truth, as always, lies somewhere in between. Hawking himself jokes that, if you understood everything in the book, you would be ready to start a PhD in theoretical physics. What the book does successfully convey is the wonder of the Universe, of how revealed creation is stranger and more mysterious than anything dreamt up by even the wackiest New-Age occultist.

With the popularity of A Brief History, Hawking, now 56, became one of the icons of the age - a seer grappling with the fundamental questions of existence, a prophet unravelling the complexities of the cosmos, a friend of Hollywood, a television star, a role model for the disabled, a medical curiosity (no one has lived longer with his disease), a father and now a grandfather.

Yet, no matter how much you read about Stephen Hawking or how often you see him on television, nothing prepares you for the shock of actually meeting him. He sits in a high-tech, motorised wheelchair in his darkened office in the Department of Applied Mathematics and Theoretical Physics at Cambridge. He is frail and motionless before a computer screen. His limbs are withered - he cannot weigh more than five stone -and his head lolls wearily to one side. His skin has the smoothness and pallor of a baby’s.

What little movement he has is restricted to two fingers of his left hand and to some facial muscles: he can raise his eyebrows and smile - radiantly at times. He is otherwise paralysed and requires constant nursing.

On one wall of his office there is a white screen covered in equations; against another there is a ceiling-high bookshelf. Around the room are photographs of Elaine, his second wife, his three children and his baby grandson. There is also a witty photomontage of him sitting at a table with Einstein, Newton, Marilyn Monroe and a less familiar figure, who turns out to be Data from Star Trek.

Communication is an endless difficulty. In the three hours I spent with him, Hawking answered only 11 questions. Each word of every sentence - sometimes even individual letters - is selected from an index on his computer monitor. He has a computerised speech synthesizer, through which he also communicates but, unless he is selecting pre-programmed phrases to be delivered in that familiar metallic American accent - yes, no, can I have some tea? - his responses are agonisingly slow.

Much has been written about the paradox of his condition, of a remarkable mind freely roaming the cosmos but imprisoned in a ruined body. Hawking does not recognise this description, nor does he believe that he is two separate entities, mind and body. “I have sometimes imagined myself as different people,” he says, patiently scrolling through his index of words. “I think we all do that. But I have never felt myself as a perfect soul living in an imperfect body.

“I don’t think there is a distinction between the body and soul. Which means that although I may take pride in my intelligence, I have to accept that the disability is also part of me and not something I can blame on a poor body I happened to pick up at an auction.”

This last comment is characteristic of Hawking’s mischievous sense of humour; his books are notable for their sardonic one-liners, and many of his remarks end in jokes.

He was born on January 8, 1942, coincidentally the 300th anniversary of the death of the Italian astronomer Galileo Galilei, one of his great intellectual heroes. His father, Frank, was a research scientist; his mother, Isobel, was bohemian and politically radical. Hawking, a gauche, introspective child, went to St Albans School, where he was a gifted if not outstanding pupil. Then to Oxford, where he was more assured, raffish and intellectually imperious. He took a First in physics and moved to Cambridge as a postgraduate student in cosmology.

Not long after arriving there, he noticed that his speech was becoming slurred and his movements shaky. A specialist diagnosed motor neurone disease and gave him two years to live. He was 21. For a few months he was lost, helplessly seeking refuge in music but not, as has been suggested, in alcohol. “The realisation I had an incurable disease that was likely to kill me in a few years was a shock,” he has said. “How could something like

this happen to me? Why should I be cut off like this?”

It is difficult to say what prevented his total collapse. His daughter, Lucy, speaks of his “indomitable will” and astonishing capacity for hard work. Others mention the selfless devotion of his first wife, whom he had known socially as an adolescent in St Albans and married after discovering he had the disease. “I wanted to find some purpose to my existence, and I suppose I found it in the idea of looking after Stephen,” Jane said.

Reflecting on the early years of his illness, Hawking says: “When I was first diagnosed I couldn’t see more than a year ahead. I didn’t expect to have children, let alone grandchildren.”

The couple separated in 1990. Hawking had fallen in love with Elaine Mason, one of his nurses and wife of David Mason, a computer engineer who designed a portable version of the voice synthesizer through which he communicates. The end of the marriage was messy and difficult, exacerbated by the usual gossip and constant intrusion. Jane Hawking, since happily remarried, refused to speak of her distress, although she once told reporters outside her house that she was weary of devoting her life to the “greater glory of Stephen Hawking”.

Of that period Hawking will only say: “There are aspects [of my celebrity] I don’t like, but it would be hypocritical to complain. I can generally ignore it by going off into 11 dimensions, but my wife finds it much more difficult. I didn’t take much notice [of the media gossip]. I thought it would die down and it has. I’m hardly the only person that has got divorced and remarried.”

Why should we expect Stephen Hawking to be different from the rest of us, to live in a state of elevated self sacrifice? Why shouldn’t he be afflicted by the same driven passions and confusions of the human muddle? “My image of myself has not really been affected by my condition,” he says. “Physical ability was never very important to me, so its loss was not a disaster. I don’t know how blindness affects people, but I expect it doesn’t remove their self-esteem. I feel like I have a handicap such as blindness or deafness, which is a nuisance but doesn’t affect my validity as a person.”

Perhaps the secret of Hawking’s remarkable longevity is his refusal to reflect on what might have been, reflect on the forlorn possibility of an alternative life lived free of illness. Rather, he insists on projecting forward, thinking only of discoveries still to come. His work, he says, is his relaxation - and, one suspects, his fickle mistress, too.

“I want to understand how it all works,” he says. “At the moment we have tantalising glimpses of the fundamental laws of the Universe. It is like pieces of a jigsaw. But we don’t know the total picture or even if there is one. I have already lived much longer than expected, and I would be disappointed if I didn’t live long enough to be sure that there was indeed a picture into which everything fitted.”

He allows himself a slight smile. One hopes time will not defeat Stephen Hawking before he finds the last piece of his immense metaphysical jigsaw—a jigsaw that, in truth, may be forever beyond the possibility of completion, by him or anyone else.