John Bayley has written a memoir of his wife, Iris Murdoch, who now has Alzheimer's. He tells Harriet Swain why in it he has chosen to reveal some of the most intimate details of her life

Fig leaves growing up the outside wall fill the drawing-room windows of Iris Murdoch and John Bayley's Oxford cottage, creating a dim, green-tinged light completely at odds with the sunny day outside.

Through the gloom and the dust, the tumbling piles of books and items of clothing, flashes the odd unexpected treasure - a bright pre-Raphaelite print, a plate of elderly chocolates.

"We don't exactly keep a dust museum, like Dickens's Miss Haversham," writes Bayley in Iris, his memoir of his wife, published this month. "If the dust is undisturbed it seems to fade easily into the general background. Like the clothes, books, old newspapers, letters and cardboard boxes. Some of them may be useful sometime." He says they have never, from their earliest married days, attempted to do much about the house.

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In their faded, well-worn clothes, the academic couple blend comfortably into this environment. When you visit them you enter a different world, a feeling exaggerated by the way they cope with Murdoch's illness. For the last couple of years Murdoch, 79, has been suffering from Alzheimer's disease. She can no longer form coherent sentences, concentrate properly or remember the titles of the novels and handful of philosophy books she has written. The Teletubbies are one of her favourite entertainments. But in the cosy dusk of the drawing-room, this is difficult to discern. She sits with an intelligent look of listening, occasionally chuckling at points where appropriate, nodding, saying "Oh yes," or "I don't know". And her husband, a former Oxford professor of English literature, continually involves her, asks if she remembers. Soon, never expecting an answer comes to seem perfectly natural.

Perhaps it works because the impetus for this strange rhythm of conversation seems to be not only Murdoch's illness but Bayley's extreme self-effacement. His speech is punctuated by expressions of modesty. He insists that he only scraped into Eton because his elder brother was already there; that he was "a total failure" in the army and got a scholarship to Oxford only because of the war. Finally, he stresses that he was hopeless at history once he did get to Oxford and managed a first in English just because he loved reading books. At his first dinner party with his future wife he was aware that his hosts looked disappointed when she said she wanted to leave and more philosophical when he announced the same thing. "It was Iris they had wanted, and almost greedily, to stay." Frightened in the early days of the impassioned intellectual circles in which she moved, his role, he says, was to provide "mild relief in an atmosphere that had become too intense".

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Now he finds himself not only acting as the main conduit with the outside world for a woman he describes as a genius but also writing about the most intimate details of her life. Why did he write it? "To cheer myself up," he says, simply. "Because it was a very sad situation" and because "it (Alzheimer's) is a condition that isn't known much about. I thought also that Iris would approve because I hoped to show something of what it is to be good." While neither has ever been that taken with God, they have both been interested in goodness. He claims he was first attracted to her because he sensed she was good and has come to feel it more and more.

This aside, the book contains plenty of juicy gossip about fellow academics and the Oxford of the couple's youth. He speaks of the then all-female college St Anne's, where Murdoch was a philosophy don, being "a hotbed of emotion". While "the dons in general were not, so to speak, professional lesbians," he writes, "a lot of feelings ran beneath the surface," although he had Murdoch's word that no advances were made to students. He describes well the claustrophobic academic world - failing to impress when asked about his research at college drinks, the excruciating arrogance of fellow guests at a literary party.

But really - and here it is not cheerfully mischievous but more complicated - the book is about Murdoch and their life together. Did he ever discuss writing the book with her? "We never have talked much about our books to each other," he says, somewhat disingenuously, since neither of them has written anything quite so personal before. Bayley's book discusses the first time they made love, Murdoch's previous lovers and the difficulties these days of getting her undressed ready for a swim, whereas he says he never appeared in any guise in her novels. "I failed to be one of the sort of people she was interested in," he says. Murdoch was always more concerned with the inner dimensions of people than with their physical presence, and in that way Bayley was too close. But also, "I don't think, without being falsely modest, that I or Iris would consider myself a remarkably interesting person. Interesting people aren't tremendously easy to get on with."

Bayley says he first fell in love with Murdoch when he spotted her from his college window, riding her bicycle. He was 28, she was 34. At his next meeting, he was gratified by the thought that "since she had no obvious female charms she was not likely to appeal to other men". But he soon found how wrong he was, and had quite a job persuading her to marry him. Over the years, Bayley became a critic and professor of English, eventually writing a few novels himself, while Murdoch became steadily more famous as a novelist and philosopher, winning the Booker Prize for The Sea, The Sea and being put forward for a Nobel prize for literature. She would construct elaborate metaphysical worlds, often completing an entire novel in her head before writing it down in long hand.

The first sign that something was wrong came four years ago, when Murdoch appeared to have trouble responding in a discussion about her work in Israel. Over the next months, her memory and speech became progressively worse, until the diagnosis of Alzheimer's. Since then, Bayley has painstakingly guided her through an increasingly confusing world, which, paradoxically, he says, is becoming more real for her, a woman who has lived much of her life in the imagination. It has also given a momentum to their marriage. "Many people - perhaps most - get married with a definite end in view," he says. "I don't think we did. We just wanted to get married full stop. We didn't want to have children, we didn't want to do anything as a team. We never thought we would develop."

Now, though, circumstances have forced them to progress. "Purposefully, persistently, involuntarily, our marriage is now getting somewhere," he writes. As the conversation winds up, the two share a little wordless joke, a mutual grin, hands reaching out. For Bayley, "married intimacy is not only secretive but mainly a matter of humour" - a private world almost impossible to describe.

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PASSIONS OF MURDOCH'S ACADEMIC LIFE

Those mentioned by John Bayley include:

Miss Elaine Griffiths, a senior member of the English faculty when he was a young don and a fellow of St Anne's who "liked a good strong drop of gin" and, at the party where he first met his wife, was "looking at Iris with a wistful expression which even at that awkward moment surprised me."

Brigid Brophy, a novelist, who "tried very hard indeed" to persuade Iris into bed "before and after we were married".

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'The great Professor Fraenkel' (classicist Eduard Fraenkel) who as he discussed texts with Iris "stroked her arms and held her hand". "That there was anything dangerous or degrading in his behaviour, which would nowadays constitute a shocking example of sexual harassment, never occurred to her."

Isobel Henderson, Iris' tutor at Somerville who allegedly sent her to the professor saying, "I expect he'll paw you about a bit".

'A Jewish Italian professor', another wartime refugee, from London University," with a wife and daughter, who spent every Saturday with Iris "talking all evening about the ancient world while he kissed her sometimes and held her hand". Apparently his wife "accepted the relationship".

The Dichter (Elias Canetti, who won a Nobel prize for his novel Auto da Fe), referred to by Bayley as "the Hampstead Monster". He "had several mistresses whom Iris knew" and a wife "who was sometimes in the flat when the Dichter made love to Iris, possessing her as if he were a god".

Isaiah Berlin "truly and unselfconsciously benevolent" but "he had the common, touch, and some people spoke patronisingly of him for that reason, implying that his fame and reputation were almost entirely due to his extraordinary powers of getting on socially, rather than to any real originality or achievement of his own".

A 'strong-minded female philosopher' who practised "telegamy" and "a mathematical logician of international standing who was a bachelor" and was "Viennese but possibly with Baltic origins", who asked to borrow Iris's room for a day to make herring soup and left it in such a state that Iris was forced to move. They strained the soup through a blue chiffon scarf her mother had given her for her birthday.

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Iris: A Memoir, Duckworth, Pounds 16.95