In 1942, Patricia Highsmith wrote in her diary: "The abnormal point of view is always the best for depicting 20th-century life, not only because so many of us are abnormal, realising it or not, but because 20th-century life is established and maintained through abnormality." When she wrote this, Highsmith had produced none of the novels that make her one of the great 20th-century writers. A gifted painter, she only opted for a life of writing a year later, when she left Barnard College. She did not publish her first novel, Strangers on a Train, until 1950. The character for which she is famous appeared first in 1955, with the publication of The Talented Mr Ripley. Yet in the same early diary entry, Highsmith set out the central theme she was to pursue through nearly all her books over the coming five decades: "I should love to do a novel . . . about one abnormal character seeing present-day life, very ordinary life, yet arresting through it, abnormality, until at the end the reader sees, and with little reluctance, that he is not abnormal at all, and that the main character might as well be himself."
Highsmith's theme is the normal amorality of ordinary life. We all like to believe we are moral, but it is a pretence that is easily dropped. Her 1974 novel, Ripley's Game - a film version of which has just been released in which John Malkovich gives what may be the definitive performance of Highsmith's part-time art-forger and occasional murderer - shows how quickly ordinary people can become killers. Meeting at a party an English picture framer who is dying from leukaemia, Ripley entraps him in a scheme to assassinate a Mafia figure. When the well-meaning, ineffectual Englishman is first approached as a potential hit man, he is horrified; but the shock does not last long. Like other conventionally decent people, he takes for granted that a sense of acting morally is the very basis of his life. In the event, it proves to be insignificant, a minor obstacle in the way of providing for his family after his death. A large sum of money soon dispels his qualms.
For Highsmith, the notion that it is normal to be moral obscures the way humans really live. "I find the public passion for justice quite boring and artificial," she wrote "for neither life nor nature cares if justice is ever done or not." It is as if Tom Ripley was speaking, and in fact Highsmith identified closely with her creation. She wrote that she often had the feeling that Ripley was writing the novels and she merely typing them, and when she gave a copy of the second Ripley novel to a friend she signed it "from Tom/Pat". Most people conceal their amorality from themselves. When circumstances force them to face it, they forget their scruples, or - like some of Highsmith's more cruelly realised characters - collapse into psychological disintegration. In contrast, Ripley thrives on his amorality. Untroubled by guilt or remorse, he is free to live as he pleases.
In real life, Highsmith was far from the debonair immoralist she created and dreamt of being. Born in 1921 in Fort Worth, Texas, after an attempted abortion, she had an unhappy childhood. Her parents divorced just before her birth; a few years later her mother married a man she hated and often fantasised about killing. She spent her teens reading Edgar Allan Poe and studying the case histories collected in Karl Menninger's once-famous textbook of psychopathology, The Human Mind. Unlike Ripley, who is virtually asexual, Highsmith was highly sexed. Mainly gay, she had many male and female lovers with some of whom she lived, but none became a long-term partner. In her late twenties, she fell deliriously in love with a married woman she glimpsed in the toy department at Bloomingdale's department store. The two never met, and the romance Highsmith sought transpired only on paper - she turned the woman into the central character of her lesbian novel Carol (originally published under a pseudonym as The Price of Salt). Her writing was her life, and her most constant companion the bottle (she was always a heavy drinker). If she found any enduring happiness it was with her beloved cats. Her fondness for animals inspired The Animal-Lover's Book of Beastly Murders, a delightful collection of short stories in which cats, dogs, pigs, hamsters and insects take violent revenge on their human overlords.
She viewed the human species with a cool, corrosive eye. It is one of the achievements of Andrew Wilson's powerful and at times beautiful biography that it shows how her coldness was a shell she grew, snail-like, to protect her intensely troubled personality. Her eccentricities are faithfully chronicled - her passion for snails, which led her to smuggle them across borders by hiding them under her breasts, the stinginess that came out in later years when she told a lover to bring a hot-water bottle so the radiators need not be kept on - not as neurotic tics, but as ways in which she coped with the intractable difficulty of her life. Beautiful Shadow is a long book, but not a page is wasted: the accumulation of details, many fascinating or intriguing in themselves, yields an unforgettable portrait of a great 20th-century writer.
It is not only her private life that Wilson illuminates. By using her diaries and notebooks, he shows that her writing was stirred by some of the most radical modern thinkers (Kierkegaard was a notable influence). That does not mean she was a writer of ideas. She saw herself as an entertainer: she did not construct her novels to illustrate any philosophy, but let her view of life emerge along with the story. In the terms of prevailing opinion, it is a highly subversive view. Highsmith belongs to a tradition that rejects or ignores the liberal humanist faith that humans can create a better world for themselves. Beckett and Proust, Pessoa and Musil belong in this European tradition, and so do Dostoevsky and Kafka, two of her favourite writers. Like Dostoevsky's novels and Kafka's stories, her novels can be read as satires on humanist hope.
It's a curious fact that, while philosophers prattle on about personal autonomy and every politician has a vision of progress to sell, few great modern writers have subscribed to the secular faith in humanity that has replaced Christianity as the unthinking creed of conventional people. Wilson quotes Will Self as saying that the experience of reading his first Highsmith novel was a physical experience of being confronted with evil, and it is true that all her novels involve an encounter with evil. The core of the humanist creed is the denial of evil: once they enjoy good education, democratic government and a reasonable level of prosperity, human beings will normally behave well. Cruelty and oppression may never disappear entirely, but over time they will become rare and exotic, like vanquished diseases. Pervasive as it has become in politics, popular culture and the academy, it is not a belief to which Highsmith ever subscribed, or found remotely plausible. Her novels effect a savage deconstruction of this secular faith. That is the source of their unsettling power.
We like to think of the 20th century as a time of advancing enlightenment. Actually, it was a century dominated by one mad project after another. Highsmith's fascination with the unnoticed amorality of everyday life may not have left her a happy person, but it gave her a rare insight into the driving forces of the 20th century. She was not a political writer - she said of herself that she was "a Social Democrat - or something" - and it is a mistake to turn to her for remedies for current ills. Thankfully, she has none to offer. Instead, she shows humans in their normal madness, entertaining her readers as she insinuates the forbidden truth.
John Gray's most recent book is Al-Qaeda and What It Means to Be Modern (published by Faber and Faber)