She was his first love and he loved her on sight. He remembers the grubby tweed skirt and the brown cotton stockings she wore when they met. He remembers their first swim together, tearing their clothes off and slipping like water-rats into the old, brown, sluggish river and afterwards drying themselves on Iris's waist-slip. He still has the slip, retrieved from the back of a drawer, mottled with the powdery traces of 45-year-old mud.
River images course through their majestic love story, their life together, one in which they seem always to have floated serenely along a single current, like quaint, good-natured children, indifferent to fashion, housekeeping, procreation, ego, criticism, status.
He remembers their first kiss and the moment at which their life together "really began", when he crashed her beloved Hillman Minx and her reaction proved beyond doubt that she cared more for him than the car. He remembers the name of the dodgy champagne they drank on their wedding day and their insatiable appetite for spaghetti pomodoro on their Italian honeymoon and soon, the delicious, dawning awareness that they were beginning that "strange and beneficent process in marriage by which a couple can `move closer and closer apart' ", in the words of an Australian poet; whereby their most precious gift to each other - childless and equal - would be a shared solitude.
He remembers the first LP they bought together - Mussorgsky's Pictures from an Exhibition - and "how the Great Gate of Kiev seemed to resound in harmony" with their spaghetti and red wine as they listened. And later, when they grew fond of "song albums", how they used to chant an imaginary pop song "whose words had somehow come into existence between us". And how everything stopped at 20 minutes to two as they put their books down to listen to The Archers.
He remembers coming home one snowy evening to their cold, chaotic, rat-plagued country home "and uttering wild cries as we rushed about the garden together hand in hand, watching our feet make holes in the printless snow". And he remembers how she "always felt a tenderness for the feelings of torn open envelopes" or an abandoned wine bottle and how she could never bear to throw anything away. And how her "Christ-like" qualities of tolerance and amusement withstood even the couple of eminent academics who borrowed her rented room to make fish soup and left it and her reputation in ruins. And how this intellectual giant yearned to give credence not only to UFOs but to the Loch Ness monster, for whom she would sit waiting for hours in the heather above the loch.
The memories tumble out in a beautiful testament of love to a fairy-woman, now absent in all but the flesh, their joyful, eccentric innocence tempering the chronicle of her savage decline into the "insidious fog" of Alzheimer's disease, a fog "barely noticeable until everything around has disappeared".
The phenomenal brain which produced 27 remarkable novels including The Bell, The Severed Head and the Booker Prize-winning The Sea, The Sea as well as books on philosophy; the formidable intellect which garnered honorary doctorates from the major universities and made her a Dame of the British Empire, can hardly cope with doing up her shoelaces now.
But 44 years after that first thrilling swim in the sluggish, brown river, they are back again, only this time after a major struggle to get Iris's clothes off - a battle he wins, though conceding the socks - and her swimsuit on. It's always a battle to get her out of her trousers and sweater before getting into bed. She is unable to concentrate or to form coherent sentences, or to remember where she is or has been.
A woman he meets sometimes, whose husband has Alzheimer's, once remarked cheerily: "Like being chained to a corpse, isn't it?" He was repelled. "How could our cases be compared. Iris was Iris." He wasn't being unkind, just protecting the soul of his love. "It's not an uncommon reaction, as I've come to realise, among Alzheimer partners. One needs very much to feel that the unique individuality of one's spouse has not been lost in the common symptoms of a clinical condition." And Iris is Iris, not was, neither a corpse nor a trap, but the same fairy-woman radiating the same old gentleness - never the raging frustration typical of many with the condition - and, in spite of the anxious and perpetual queries, never complaining. The disease, he believes, has simply exaggerated a natural goodness in her.
By all accounts, Alzheimer's is hardest on those "who hug their identity most closely to themselves" but Iris was so devoid of a sense of identity that she hardly knew what it was - a factor which he believes "seemed to float her more gently" into the Alzheimer's "world of preoccupied emptiness".
In the sluggish, brown river, they smile happily at each other as they paddle to and fro, her muddy stockinged feet being investigated by shoals of tiny fish. Once they would have swum the hundred yards or so across and back. "Now it is too much trouble . . ."
As they approach the bank to scramble out - a difficult, inelegant operation at the best of times - and he turns, arms out to help Iris, her face contracts into that familiar look of child-like dread which communicates itself to him. "Suppose her arm muscles failed her and she slipped back into deep water, forgetting how to swim . . .? I knew on the spot that we must never come to bathe here again."
Later, her overwhelming fear of trees means that they can never visit the bluebells in Wytham Wood again. The sense of a world contracting is searing as he hurries her past the two gigantic sycamores and back to the car: "Soon be back in Teletubby land," he says soothingly.
Teletubbies. With their virtual reality landscape, sunlit grass, blue sky, artificial flowers and rabbits hopping about, they have become a part of the morning ritual, one of the few things they can watch together, in the same spirit. "There are the rabbits!" he says "quite excitedly" and Iris settles down, always returning the beaming smile of the baby in the sky. (His woman friend chained to the corpse simply tells her husband every evening, "There's another snooker programme on" and then plays the same old one on the video. It was always a new one for him, she reasoned).
In communicating with Iris, he has found, tone is what matters - playful, jokey, childish. "All is OK with a child or cat exclamation. `The bad cat - what are we going to do with her?' I imitate the fond way her father used to say `Have you no sense at all?' in his mock-exasperated Belfast accent. Iris's face always softens if I mention her father in this way. Instead of crying she starts to smile."
Although she never showed any interest in children before, she loves them now, on television or real life. "It seems almost too appropriate. I tell her she is nearly four years old now - isn't that wonderful?"
But her fear of people generally is so acute that he can't bring himself yet to arrange for carers to keep her company. Sometimes however, he will be ruthless about getting her to go places he thinks she will enjoy. At one such party, he noted happily, she was playing the game, her face animated, appearing to listen closely to a riveting account of how things are done in an insurance adjustment office. Only then he heard her ask : "What do you do?" From the face opposite, it was evident that the question had been repeated a few times already. (It could just as easily be "Where do you come from?" or "What are you doing now?") Undiscouraged, the insurance man began all over again. Back home, Bayleytries to keep her interested in the party, saying how much people had liked seeing her. But Iris begins to ask anxiously, "When do we go?" And he wonders just how many times she asked the insurance man what it was that he did.
She has become obsessive about rescuing things - not just abandoned wine bottles now but old leaves, sticks, sweet papers, cigarette ends, even dead worms. A Coca Cola tin, a rusty spanner, a single shoe. It's just another way in which she has become more like herself. But what is staggeringly new and different to her husband, is her terror of being alone, of being cut off for even a few seconds from the familiar object - a feature of Alzheimer's. "If Iris could climb inside my skin now, or enter me as if I had a pouch like a kangaroo, she would do so . . . I sit at the kitchen table, and make desperate efforts to keep it as my own preserve, as it has always been. Iris seems to understand this and when prompted goes obediently into the sittingroom where the TV is switched on. In less than a minute she is back again."
He is haunted by the notion that she possesses some "terrible lucidity", some inner knowledge of what is happening to her. How much does she know? Is she saying inside herself, like the blindman in Faulkner's novel, "When are they going to let me out?". Escape. She is self-aware enough to tell a friend, twice, that she feels she is now "sailing into the darkness". And disturbingly, nothing embarrasses her at this stage but her own tears, this woman who used to weep quite openly. She stops, as though ashamed, when she sees he has noticed. "It makes me feel she is secretly but fully conscious of what has happened to her, and wants to conceal it from me . . ." The ultimate nightmare.
And through it all, not a syllable of self-pity or self-congratulation creeps into the text.
On the contrary, his accounts of his own furious, irritated outbursts are mercilessly honest. The wild wish to shout in her ear when the television breaks down and the sedatives don't work: " `It's worse for me. It's much worse!' . . . When are they going to let me out?"
The summer's day when, goaded by the memory of the unpleasant smell of her "daft and elderly" mother "nosed now from Iris herself in the muggy heat", he finally cut loose over her compulsive watering of the houseplants, half-killing them and flooding the floor with stagnant water. "I told you not to! I told you not to!" Then, more cold, and deadly: "You're mad. You're dotty. You don't know anything, remember anything, care about anything". And on and on, until he caught himself in the mirror - "a horrid face, plum colour".
Or on the nightmare bus journey from Gatwick to Oxford, the high point of which occurred when she took a distraught woman's bag, and spilt its contents in the gangway. Having returned the bag with a whispered apology, "I get Iris into a seat and give her a violent surreptitious punch on the arm by which I am holding her".
Or, that time, trying to ease his isolation, when he made a savage comment about the grimness of their outlook and she looked merely relieved and intelligent. "But I love you", she said.
There is no hope now and little fear. His survival strategy is taken from the Rev Sydney Smith: "Take short views of human life - never further than dinner or tea". She sleeps on like a cat beside him in the big Victorian bed, while he works, the clack-clack of the typewriter a familiar, soothing sound. "Now I feel us fused together", he says about the marriage that flourished on being "closer and closer apart" for more than 40 years. "Every day we are physically closer; and Iris's little `mouse cry', as I think of it, signifying loneliness in the next room, the wish to be back beside me, seems less and less forlorn, more simple, more natural. She is not sailing into the dark: the voyage is over, and under the dark escort of Alzheimer's she has arrived somewhere. So have I".
She murmurs in her sleep and her hand comes out from under the quilt. "I put mine on it and stroke her fingernails for a moment, noticing how long they are, and how dirty. I must cut them and clean them again this morning."
Iris: A Memoir of Iris Murdoch by John Bayley, published by Duckworth, £16.95 in UK