Iris Murdoch's legacy is, in the end, her philosophy of love, best expressed in the love story she never wrote: her husband's memoir, writes Eve Patten

On a dark night during a thunderstorm, a middle-aged man suffers a seizure and drives his car, containing his wife, into a canal. The husband recovers but is tormented by one question: was his action deliberate or accidental? Another man drugs his friend's sandwich as a joke, but the friend jumps - or perhaps falls - out of a window. Who is responsible for his death? A woman becomes so obsessed with rescuing a butterfly trapped in her carriage that she leaves her luggage on the train. Is she virtuous or foolish?

This is the recognisable landscape of Murdochland, a fictional territory built on such uncertainties of human interaction. "Human" is perhaps too broad, for Murdoch's books are almost exclusively concerned with a small cast of English middle-class civil servants, academics, failed writers and intense adolescents, whose liaisons - sexual, adulterous or cerebral - are the stuff of painstaking philosophical meditation.

The typical Murdoch novel - and they're all typical, say her critics - builds on a moral or psychological problem and a set of characters who become archetypal: the enchanter, the adulterer, the failed priest, the haunted child. Frequently, there is a drowning or near drowning; regularly, a car accident. Families are usually quarrelling, and any valuable collections of stamps or china are inevitably destroyed.

While the novels are self-consciously formulaic, they change considerably over the five decades of Murdoch's career, in tandem with her evolving philosophical interests. Her first, Under The Net , from 1954, is a surprisingly humorous amalgam of Wittgenstein and French existentialism - Murdoch was a devotee of Jean-Paul Sartre - disguised as a classically British angry-young-man farce.

The novels of the 1960s, such as The Italian Girl and The Time Of The Angels, are skeleton-thin Gothic meditations driven by Heidegger. Those of the 1970s - A Fairly Honourable Defeat or The Black Prince - are convoluted Shakespearean comedies of trickery, seduction, mistaken identity and contrived adultery.

In the 1980s, in The Good Apprentice and A Message To The Planet, she began to touch on issues such as ecology, nuclear warfare and, somewhat belatedly, hallucinogenic drugs, a move not all her readers welcomed. She was accused of producing intellectual soap opera and, worse, of being too prolific, a charge fuelled by Murdoch's quip that she took only half an hour off between novels. Her last, Jackson's Dilemma, from 1995, brought the total to 26.

At the heart of the Murdoch enigma is the paradox of the philosophical novelist. Philosophy may have come a good deal more easily to her than fiction. Certainly, she looked to the novel as the barometer of life; "the novelist has his eye fixed on what we do, not on what we ought to do or must be presumed to do," she wrote in Sartre: Romantic Rationalist, her 1953 study.

And her best writing, in virtuoso works such as An Accidental Man or The Book And The Brotherhood, achieves a happy coincidence of theme and form. In her worst - Nuns And Soldiers, even whose title is vile - the philosophical agenda becomes baggage carried awkwardly by feckless, rather dull characters. The critic Walter Allen judged her about right. Murdoch had it in her, he claimed, to become the greatest novelist of her generation, "but her management of her gifts has been baffling".

Murdoch herself was well aware of the pitfalls open to the philosopher-novelist. Against Dryness, a 1961 essay, is a succinct discussion of the balance required in the novel between abstract ideas, which she terms the crystalline, and the untidy contingency of reality, or the journalistic. She struggled to integrate the former - the predictable shapes of myth, allegory and fable - with the latter, that unpredictable world of chance and coincidence that emerges, most obviously, in the recurrence of accidents in her fiction.

Transposed into literary history, this dilemma was really one of split allegiances. Intellectually attached to a Continental school that encouraged the arid novel of ideas, her mentors were Raymond Queneau and the great unsung Elias Canetti. Emotionally, however, she was always committed to a tradition of social realism inherited from Tolstoy and Dickens, novelists who relied, she said, on "the great hub of society".

Fusing the two was never straightforward, though it led to some interesting experiments, such as The Philosopher's Pupil, a wonderful pastiche from 1983 of arch realist George Eliot, bizarrely interspersed with bursts of Alain Robbe-Grillet and the nouveau roman.

As a moral philosopher, however, Murdoch should be treasured without reservation. Not just for her solid academic record but because, on a popular level, she worked against the grain of post-war scepticism and 1960s liberalism to defend a philosophy of love. Essentially, and above all her other philosophical leads, Murdoch looked to Plato, to the love of the good in whatever form it takes.

Her belief, outlined in The Sovereignty Of Good, from 1970, was that individuals must seek beyond the self - thereby escaping the clutches of existentialism - to a true recognition of the Other.

Her biographer Peter Conradi describes this as the process of unselfing, but her husband, John Bayley, put it better in his 1960 study of the novel: characters should arouse in their authors a feeling of love, he suggested, "a delight in their independent existence as other people".

This delight is not a difficult concept - some might consider it trite - but it provides an important connection between the lesser-known world of Murdoch's books and the now much-publicised details of her later life. Iris, Bayley's account of his brilliant wife in the grip of Alzheimer's disease, is so poignant because he continues to delight in her individuality, her "otherness", even in the distressing process of her mental degeneration.

Ironically, of course, his eloquent memoir, which is the basis for Richard Eyre's new film, is the kind of love story Murdoch would never have written.

• Dr Eve Patten is a critic and lecturer in English at Trinity College, Dublin
• Iris opens on February 1st; Iris: A Memoir Of Iris Murdoch, by John Bayley, is published by Abacus, £7.99 in UK