Much is known about his work, but what about the man behind the art?

GENIUS is a currency that can be devalued or inflated according to the narrow prejudices of the existing times. A cursory glance at the most tacky souvenir shop, or, alternatively, at the most magnificent building confirms that Charles Rennie Mackintosh's most enduring legacy is to have left his personal stamp. His genius is another matter. He was a man who never felt the need to explain and so, almost inevitably, he has created the conditions for a mass of explanations from art historians, writers, and fellow architects.

''It was a fascination with the personality of Mackintosh that attracted me,'' says Professor John McKean of his book, Charles Rennie Mackintosh: Architect, Artist, Icon, which is published this week.

McKean, Professor of Architecture at Brighton University, was born in Glasgow and studied at Mackintosh's masterwork, the Glasgow School of Art. He has already written two books on Mackintosh and contributed to the pamphlet celebrating the Mackintosh exhibition in the nineties.

However, it was no narrow provincialism that attracted him back to the great man, rather he was driven by a desire to explain as much as possible about an artist whose private travails bear as much an examination as his works which dominate the landscape and the culture of much of the city.

''This book tries not just to be a story of Mackintosh's works,'' he says. ''The whole idea was to try to get together a life. He seemed to be a number of different people and I have tried to weave his personality together as the book progresses.''

With the help of lavish illustrations from Colin Baxter, Professor McKean has achieved this task with an enviable fluency which does not avoid the discord of controversy. The most contentious assertion is Professor McKean's finding that Mackintosh suffered from Asperger's Syndrome.

This had its genesis when Professor McKean was discussing Mackintosh's personality with a friend who was also a child psychologist. She suddenly said: ''You sound just like an Asperger's Syndrome parent.''

Asperger's Syndrome is a condition which, at its most severe, can veer towards one end of the autistic spectrum.

The more Professor McKean investigated the symptoms, the more he became convinced that Mackintosh was a sufferer. ''The sufferers have a predominantly visual style of thinking and in Mackintosh this is obvious,'' he says. ''They keep their own company and are loners. They are also solitary players, not collaborators.''

An image from the book suddenly surfaces of Mackintosh sitting at the bottom of a hill at a pub in Killearn while work continues on a house above. If Mackintosh was not in charge - and in this instance the control had been taken away from him - he would often find solace in his own company and a pot of beer.

Other symptoms include poor motor co-ordination, a high incidence of depression, a predisposition to find intimate relationships later in life than their peers. All these chime with the life of Charles Rennie Mackintosh.

A list of further symptoms add weight to the theory that has been summarily dismissed by a series of art historians, although it has been endorsed by an expert at Guy's Hospital.

Professor McKean is acutely aware, however, that Asperger's is merely a way to group a series of physical, behavioural, and emotional responses. ''Genius is itself an abnormality,'' he says. His interest is how that baggage informed a life and created buildings, chairs, and paintings. ''There is no doubt Mackintosh suffered heavily from the melancholia of the artist. He always believed his depression was worse than anyone's he met.''

There is evidence that Mackintosh was embroiled in a circle of Asperger's/depression/ alcohol which may have been self-destructive, but, paradoxically, was vibrantly creative.

There are regular allusions to alcohol in the book, but Professor McKean says: ''I very much doubt Mackintosh was an alcoholic.'' He adds: ''I wanted to bring a little more understanding to his personality because that is what intrigued me. He's an enigma. I wanted to investigate the whole package and paint a

picture of his personality.''

That portrait has been completed with a sure hand. While there is much discussion of Mackintosh's work in the book, the undertow of a compelling personality always draws one back to Mackintosh the man: the physically gauche son of a policeman whose light hand could create almost unconsciously.

''One interesting aspect of Mackintosh,'' says Professor McKean, ''is that he was once accused of copying a clock that had been designed in Austria years earlier.'' Professor McKean believes this was not a conscious act of intellectual theft (after all, Mackintosh was a febrile innovator), but rather that an image had been impressed on Mackintosh's mind and simply surfaced years later.

He expands: ''Mackintosh as an artist was completely different from, say, Van Gogh. In Mackintosh's landscapes there is an eerie emptiness. There is no interaction with people, there is not a soul, they are amazingly empty. Van Gogh's empty chair, however, is resonant with the absence of a person, you can almost see the shadows of the people who would sit in the chair.

''This two-dimensional effect created by Mackintosh is in contrast to what other artists strive to evoke. Mackintosh's paintings are in many ways like a photograph, they don't seem to inhabit a social world.''

He is beguiled, too, by the enduring legacy of Mackintosh. ''The phenomenon is fascinating,'' he says. ''I am interested in what attracts people to Mackintosh and his copiers. Now that his copyright has lapsed, there is much that it is cheap and tacky but some of it is very nice. I suppose copying is the sincerest form of flattery.''

Indeed, the Japanese have taken this form of reproduction to the grandest scale by reproducing their very own Mackintosh House.

But the real thing is only a moment away. A friend said of Mackintosh: ''The thing is, he was just in a way an ordinary Glaswegian, and didn't hook up with anybody.'' Professor McKean has gone a long way to forging a link with Mackintosh's personality, but his genius can at least be gauged, however imperfectly, by our eyes.

A simple walk from Charing Cross, along Sauchiehall Street, and down towards Buchanan Street ensures that, however elusive his personality, a genius can survive in three-dimensional form.

n Charles Rennie Mackintosh: architect, artist, icon is published by Lomond Books at #10.