Are we all, without realising it, taking part in a vast witness protection programme? Did we observe, at some time in the distant past, a deeply disturbing event in which we were closely implicated? Were we then assigned new identities, new personalities, fears and dreams so convincing that we have forgotten who we really are?

These questions crowded my head as I watched A History of Violence, a film as brilliant and provocative as anything David Cronenberg has directed. All Cronenberg's films make us edge back into our seats, gripped by the story unfolding on the screen but aware that something unpleasant is going on in the seats around us.

That unpleasantness, needless to say, is ourselves, a damp bundle of passions, needs and neuroses that conceal our secret nature. The disturbing event we witnessed in the past is the experience of being alive, a state of affairs that Cronenberg most definitely does not take at face value.

Existence, in Cronenberg's eyes, is the ultimate pathological state. He sees us as fragile creatures with only a sketchy idea of who we are, nervous of testing our physical and mental limits. The characters in Cronenberg's films behave as if they are inhabiting their minds and bodies for the first time at the moment we observe them, fumbling with the controls like drivers in a strange vehicle. Will it rise vertically into the air, invert itself, or suddenly self-destruct?

Cronenberg has modestly described himself as looking like a Beverly Hills gynaecologist. Having worked with him on the making of Crash, I know that in person he is good company, with the reassuring manner of a neurosurgeon explaining how he is going to remove the inoperable tumour buried deep in your brain. Remarkably for a film-maker working entirely within commercial cinema, he has remained faithful to his central project, and his films constitute a sustained autopsy into the nature of existence.

All Cronenberg's films, up to and including A History of Violence, are concerned with two questions: who are we, and what is the real nature of consciousness? Together, the films seem to parallel the growth of the mind from the womb onwards. Early films such as Scanners and The Dead Zone explore the blurred frontiers between mind and body, very much a new-born baby's perception of reality.

In Videodrome, this growing mind has made its first move into the outer world, appropriately by switching on a TV set, a parable of how tenuous reality has become in a media-dominated world. The Fly, Cronenberg's most successful film, has echoes of Kafka's Metamorphosis, where a despised son sees himself transformed into an insect. Here Jeff Goldblum, filled with almost adolescent doubt and self-loathing, finds himself in a doomed love affair with Geena Davis. She watches cheerfully as he walks across ceilings, and I assume that his transformation into a giant fly takes place entirely within his own mind.

Naked Lunch moves beyond sex into the night world of heroin overdoses, and Crash, a love story that treats the car crash as a religious sacrament, enlists technology in an attempt to escape even death itself. Lastly, in A History of Violence society as a whole is embraced and then quietly dismantled.

The title, A History of Violence, is the key to the film, and should be read not as a tale or story of violence, but as it might appear in a social worker's case notes: "This family has a history of violence." The family, of course, is the human family, a primate species with an unbelievable appetite for cruelty and violence. If its behaviour in the 20th century is any guide, the human race inhabits a huge sink estate ravaged by unending feuds and civil wars, a no-go area abandoned by the authorities, though no one can remember who they are, or even if they exist.

The film is set in a small town in rural Ohio, a peaceful backwater where the only thing that changes is the single traffic light. Tom Stall, played by Viggo Mortensen, runs a pleasant cafe, and "I'll have some of that nice cherry pie" sums up the Norman Rockwell ethos. Tom is relaxed and likable, and is happily married to Edie (Maria Bello). They have a six-year-old daughter, Sarah, party-dress sweet and adorable, who we know is going to get it before too long, and a teenage son, Jack, with a droll line in humour. Asked by his bored girlfriend what the town's future holds for them, he replies: "We grow up, get jobs, get married and become alcoholics."

It seems unlikely. This is one town where David Lynch will never come calling, though Tom and Edie have playfully naughty imaginations. When the children are staying with friends, she dresses up as a high-school cheerleader and they have passionate sex on the daughter's bed. But it all seems as innocent as the stuffed toys lying around them.

Sadly, a darker world intrudes. One evening Tom is about to close up when two hoodlums enter the cafe, on the run after killing a motel manager. Seeing that he and his staff are in serious danger, Tom springs into action. During a violent struggle he is stabbed but seizes one of the weapons and shoots both men dead. The town rallies round, acclaiming its new hero. Wife and children proudly drive Tom home from the hospital. He mumbles modestly into the national TV cameras. He is a hero to his son, and the cafe is packed with well-wishers.

But far away, in Philadelphia, others have been watching the TV news. A week or so later, three very threatening men enter the cafe. Their leader, Carl Fogaty, is played by Ed Harris in a star turn that rivals Dennis Hopper's psychotic gangster in Blue Velvet. In black suit and ice-white shirt, sunglasses covering a damaged eye, he is stylised violence in every gesture. He greets Tom like an old acquaintance, glad to have found him at last. He claims that 20 years earlier Tom was a member of the Philadelphia mob. His job is to take Tom back to see his brother, now a mob boss and keen to settle certain unfinished business.

Tom maintains that he has never seen Fogaty before, but he is vague about his family background, and both his wife and son are unsure whether to believe him. Every certainty in their tranquil world has been overturned. Edie stares around their comfortable family home, realising that it may be no more than a stage set. The leader of the gang remarks to her: "Ask Tom where he learned to fight so well . . ."

What is so interesting about the film is the speed with which the wife accepts that her husband, for all his courage, is part of the criminals' violent world, in spirit, if not in actual fact. A dark pit has opened in the floor of the living room, and she can see the appetite for cruelty and murder that underpins the foundations of her domestic life. Her husband's loving embraces hide brutal reflexes honed by aeons of archaic violence. This is a nightmare replay of The Desperate Hours, where escaping convicts seize a middle-class family in their sedate suburban home - but with the difference that the family must accept that their previous picture of their docile lives was a complete illusion. Now they know the truth and realise who they really are. Their family has a history of violence.

· A History of Violence is released on September 30.