Some years ago, I stayed in Manhattan with a drama school colleague. This girl, a teen soap sensation, was also a very spoilt lesbian heiress with her own apartment on Park Avenue. After two nights, I admitted to her that the endless whine of police sirens was terrifying to a suburban Brit. She shrugged slim shoulders and said, "You'll get used to it." Milly had "never in my life" used public transport or ventured "downtown".

I stayed in New York for three sweltering weeks in abject luxury. It didn't take as long as I'd feared (hoped?) to become immune to the box-dwellers' plight or the tragedy of the vastly obese black lady on the corner selling all her belongings "for less than five bucks". We spent our days and nights whizzing via cab between cocktail bar and flashy club. On Milly's doorstep, we were greeted obsequiously by the uniformed concierge - soon the sirens became little less than the soundtrack to my exciting Big Apple adventure.

One afternoon, I did something that has loitered on my conscience for over a decade. I walked not just past, but over, someone in serious need of help. On the corner of our block, an old man in the hobo uniform of grey tatty suit and mismatched shoes lay in the centre of the pavement, immobile. It was a busy day and it was almost impossible to avoid stepping on him.

"Oh my god," I said as we approached that poor man. "We should do something."

"Nah," said Milly. "He's just a drunk." We were steps away now. There was a puddle of urine leaking from his pants. No bottle of liquor was in sight.

As I stepped over his outstretched, puffy right arm, I saw the blood. "Oh my god. Maybe he's been shot. Help! Help!"

My friend put her hand over my mouth and we were pushed down the avenue by those behind us desperate to get away from the human detritus.

"He probably just fell over," Milly hissed tersely, annoyed that I would risk making a scene so near her apartment building. I couldn't understand why the way he became injured should make any difference as to whether or not we had a duty to help him. But it did, it does. It's the same old Victoriana dressed up as the hard-headed realism that new Labour feeds us: there are the deserving poor (those in low-paid jobs who pay their taxes) and the wastrels who bring disaster on their own heads and so deserve nothing but our contempt.

This weekend, two male friends and I were waiting for a cab at 1am on the crack-ridden streets of Soho. Suddenly, a tiny, skinny man/woman skittered over to us. My oh-so-liberal pal whirled in her direction and started screaming and shouting at what I now saw was a weasel-faced woman of indeterminate age.

"Oi, you, go on, piss off!" he started yelling, as she jigged and jerked involuntarily nearby. "You wanna get yourself sorted out, love. What the hell do you think you're doing, running around all night like this? What's your problem?" He took a menacing step towards her. "Go on!" he shouted "Go home. That's where you belong, not here. Go HOME!" The street rat scurried away, buzzing and hissing as it went.

I was in a state of shock.

"What the hell do you think you're doing?" I asked.

"Helping her, that's what," he said coolly.

"Helping her by yelling at her and abusing her in the street?"

"Helping her by telling her the truth. She's on crack. I saw her doing it earlier, she hangs out with that guy over there by the cash machine. They beg, hustle and take drugs."

Apparently, those of us "legally" stoned and on the street in the middle of the night have the right to abuse those who have to beg in order to get high.