With his pimento-shaped face, reminiscent of an overstuffed hamster, Francis Bacon appears in photos taken by his contemporaries and in a famous portrait by his friend Lucian Freud - stolen in 1988 never to be seen again - as one of the most recognisable artists of the 20th century. Doyen of Soho drinking clubs, he led a reprobate life that has been well documented, from an Anglo-Irish childhood, with a repressive father who threw him out for showing an overdeveloped penchant for stable grooms and for his mother's underwear, to his sadomasochistic love affairs with numerous men of the demi-monde.
The new Bacon retrospective at Tate Britain, the first since 1985, allows for a reassessment of his work in an age when shock and violence are common fare, in the art world and in daily life. An avowed nihilist and atheist, he was fraught with contradictions. "You can," he claimed, "be optimistic and totally without hope . . . I think of life as meaningless; [but] we create attitudes that give it meaning while we exist." Painting, alcohol and sex were the ways he sought that meaning.
Bacon, widely regarded as Britain's greatest painter of the figure, aimed to inherit a place in the pantheon beside Michelangelo, Velázquez and Rembrandt. He insisted that his pictures "were to deserve either the National Gallery or the dustbin, with nothing in between" - and undoubtedly won that gamble. Yet despite his extraordinary innovation and recasting of the human form, he cannot be seen as a true modernist. He was, for most of his career, sidelined by the American critics, who saw him as too figurative, too narrative, and too concerned with European art history and Christian iconography. Neither did he share their boundless optimism nor care much for the abstract expressionism promoted by the American critic Clement Greenberg. As he said: "I do not believe in abstract art because you must have a starting point in reality."
Today, as one looks back, more than a decade after his death in 1992, Bacon's sensibility seems supremely European. His postwar angst springs from the same ground as that of Giaco metti and Jean Dubuffet, Jean-Paul Sartre, Albert Camus and Jean Cocteau, whose bleak dictum "If you see your whole life in a mirror, you will see death at work" Bacon admired. His 1955 painting based on the life mask of William Blake, that great outsider of British literature, nails his colours to the mast of iconoclasm and individuality. He lived by his own rules, both in his art and in his relish for the bohemian lowlife (homosexuality was still illegal) of Soho and the Colony Room. T S Eliot was a huge influence. The poet juggled with religious imagery for a secular age, whilst Bacon was a committed atheist, but both caught something of the existential isolation and abjection that defined postwar Europe.
Yet Bacon strongly denied a narrative message. He wanted his paintings to address the viewer's "nervous system directly" and to "unlock the valves of feeling" with his distorted forms, derived through chance, accident and appropriation. His paintings, he claimed, were a form of "exhilarated despair", and mankind "nothing but meat". He rejected the idea that his screaming popes, based on Velázquez's Portrait of Innocent X, shut in their claustrophobic glass cases, had anything to do with the image of the Nazi war criminal Adolf Eichmann, standing in a glass witness box during his trial in Jerusalem, or that his mauled, contorted bodies were born out of the horrors of the Second World War. Yet the famed detritus of his studio, posthumously saved by his heir John Edwards, reveals not only Bacon's passion for photography and film, but that his paintings were informed by images as diverse as illustrations from medical textbooks on diseases of the mouth, or the nanny's blood-spattered face from Battleship Potemkin. They were not, in other words, totally intuitive. It has long been acknowledged that Eadweard Muybridge's early photographs of movement were fundamental to Bacon's work.
So where should we place him now? To stand in front of his Three Studies for Figures at the Base of a Crucifixion, painted in the 1940s, is still a deeply visceral and gut-wrenching experience. Bacon had come to know Aeschylus's Oresteia through Eliot's 1939 play The Family Reunion. The artist's three writhing Eumenides are barely recognisable as human figures. They have no eyes, only silently screaming mouths, bespeaking the fascination of that first generation of post-Freudians with the id and "the hidden presence of animal trends in the unconscious". Bacon's screaming baboons, sniffing dogs and bulls all blur the line between culture and abject nature.
There is also something prescient about both the popes and Bacon's men in suits. Study for Figure II (1953-55) shows a solitary man with blank eyes and gaping mouth, in a jaundice- yellow suit, isolated on some sort of platform against an empty, black space. This figure, which bears an uncanny resemblance to George W Bush, is one of Eliot's hollow men, heads stuffed with straw, whose "dried voices, when/We whisper together/Are quiet and meaningless". There is nothing, Bacon seems to be saying, so isolating and dehumanising as power. His impact has not diminished one jot.
"Francis Bacon" is at Tate Britain, London SW1, until 4 January 2009. www.tate.org.uk