Once, when I was a still a student, I was crossing a large intersection in a scummy area of London-Elephant & Castle. As a student in a city you get the view, not of cathedrals, but of gutters-where morning-after-the-drunken-night puke sticks to your shoes, and people spit and the stench of urine in the underground is so powerful you learn to hold your breath.

You never think of clean areas because that's where a year of tuition fees can be blown in one shop. Also in the tiny Greek diner, or the greasy café you hear interesting conversations. Voices are never discreet so you hear the expletives and the bitching about men and the rage over women and the complaints about the NHS and shouting in strange languages and smells of stuff you know you should not ingest, but it gives you a thrill to be there anyhow.

That wintry day was like any other. Swishing cars with unforgiving and unseeing drivers sending black soot, cold wind, grit, to stragglers crossing the road. It happened so quickly I wondered if I imagined it.

As the traffic light flashed "walk" pedestrians on either side began crisscrossing, faces dodging one another as they did this the most natural and unconscious of acts in any city. A man in front of me, one I wouldn't have looked at twice-dressed ordinarily in a faded navy windbreaker and jeans, came face to face with an adolescent boy. I remember the boy clearly because he had ruddy cheeks. But more because of his air of vulnerability.

That swagger that hides all kinds of teenaged insecurities. Who sees all that in a passer-by? So I don't doubt my memory could be playing games with me given what happened next. The man in front of me randomly went up to this young boy and slapped him. Hard. Twice. Till his cheeks turned red. Perhaps he wasn't ruddy, just red from being slapped. Perhaps he was confident but as I remember him he was vulnerable because a stranger slapped him out of the blue. Broke every rule of anonymity and assaulted him for no reason and walked on as if nothing had happened. The lights were flashing, changing.

The boy stood there in the middle of the road, stunned, one hand frozen on his face. I saw this and carried on walking, carried along in the crush of the crowd. That's life in a city when you look at it from the bottom, more often than not, in a scummy area without the protection of being behind a wheel, or in a guarded edifice.

One scene in Martin Scorsese's 1976 film Taxi Driver:- a young man in a gloomy room (a space so poignantly utilitarian it suggests its scrappily lived state-stale, sour smells of alcohol, sweat, oily takeaways, soiled sheets) is slowly rocking his chair, pointing his gun at the actors in a soap opera on the television on which he has a foot. The tension of the deliberate rocking, the backward and forward drip, drip, drip of his emptiness, clawing onto the fake conversation between the actors on a grainy TV, is so unbearable that you almost want him to shoot at it, at them, to end it so you can breathe again.

You hold your breath while your heart thuds uncomfortably for the minute that seems to last forever. Relief comes as he kicks the television which crashes with an explosion. It's the ultimate portrait of an isolated, empty restlessness, of a dull loneliness that is so terrifying that most of us fill up our days trying to avoid it. Give us anything - an explosion, a slap would be better. Did I say a slap?

Then there is the backdrop of course, skilfully created, of New York from the bottom up. Not the flashy New York of Sex in the City, of designer labels, sushi salad cocktail bars, of opera and penthouses on the right side of the river, of canters around Central Park, but the underground city that is reality to hundreds of thousands of the people who drowned before they made it to the American dream. A New York of pimps, prostitutes, pushers, hustlers, guns, badly ventilated buildings, swirling with smoke, a New York of illegal immigrants, runaways, stowaways, drop-outs. A New York of greasy spoons, of the smell of oil, eggs, fries, bacon. Of dripping car exhausts. Of taxi drivers.

It was surreal, watching this Academy Award-winning film in the heart of Laventille, in the enterprisingly created CCA7, the art centre in the Fernandes Industrial Centre, where classics like this one are shown free every Thursday night. The dozen or so of us watching this film were sitting in a concrete space, watching a film, watching a man watch a television where actors are acting. So many barriers between us and reality. Yet art transcends that. And makes us remember the reality we live in but block out.

Robert de Niro and Jodie Foster chillingly recreate all cities. Robert de Niro is Everyman who becomes drained of hope and then drifts along in a world where fantasy and reality blur, where people shoot because they can, where absolutely contradictory pulls of unbelievable generosity and compassion merge with the unforgiving moment when a man is dead because someone near him has a gun and is angry enough at the world to pull a trigger.

Art brings us back with a jolt, as if to say, look what you're missing, you idiots. There's lessons in the anger. There's emptiness behind an anonymous slap. And depth behind the taxi driver who empties his life savings so a child prostitute could escape her pimp. So much lost humanity. It's right here. Right here. Just reach.