It was obviously an English face. The little boy, aged 9 or 10, in 17th-century clothes, had been captured in oils; his portrait was displayed in the Pierpont Morgan library in New York: a wonderful institution - once a rich collecter's private study, now a gallery.

Outside it was snowing. Within was a glowing treasure chest - of porcelain, photography and portraiture of every kind, but above all books and manuscripts. The faces of the tourists brushing past us were cosmopolitan-metropolitan; the faces on the walls were from every epoch, continent and walk of life.

But something about this boy said “English”. His clothes, his hair, his complexion could have been from any Northern European nation. But his glance, I thought, could only have been English. I walked over to the painting. It was John Milton, as a child.

Do you ever, as I do, walk through a crowd and see a face, and think: “She (or he) must be English. I can't say why but I'd put a hundred pounds on it”? Or see the face of (say) the young man sitting opposite you on the London Underground, and think: “He's not English. Neither his clothes, his hair colour, his eye colour, his skin nor any of the particular components of that face mark him out as a stranger - and yet I know he's foreign”? I even wait, sometimes, hopeful of overhearing the person speak, so I can see if I am right; and I almost always am. This is not true of every face - perhaps not true of most. Many could be from anywhere; but some just couldn't be English. And some could only be.

How can this be? How do we discern it? Can a face change as a foreigner is assimilated? I do not know. I only know that from time to time I can sense the presence of a countryman. The glance is somehow open to me - as, four centuries and an ocean away from 21st-century England, that boy Milton's was.

I'm in America for an event organised by the debating forum Intelligence 2. On Tuesday night, with snow still falling, I negotiated icy streets to the Asia Society on Park Avenue in New York, one of a team of three opposing the motion that “America should be the world's policeman”.

I was nervous. It's 30 years since I've spent much time here, but I know that Americans are different, in subtle but important ways. I hope I know how to pitch a speech in Britain, but felt insecure faced by an audience of clever, successful (and predominantly rich) New Yorkers, and an opposing team consisting of two influential American foreign-policy thinkers and academics, and the young British Atlantacist neocon, Douglas Murray.

I decided to attack them. No British debate is complete without an opening salvo of an ad hominem nature, so I described my priestly foreign-policy-guru opponents as an example of the Right's outsourcing of its higher mental processes to experts, and Mr Murray as a talented limelight-seeker.

Laughter died. I even heard a hiss. The audience thought such attacks bad form. One lady told me afterwards she found such discourtesy disgraceful. Oddly enough I remember making a similar mistake at Yale, 30 years ago. Though hardly slouches in the aggression stakes, Americans retain a certain ceremoniousness in public debate. The facts may be slurs but the slurs should be factual: my audience felt uncomfortable about personal remarks.

The rest of the speech went fine, and we won the vote (just), but I doubt my knockabout helped.

And there was an illuminating moment at dinner afterwards. I was talking to a warm, glamorous, funny lady, a successful businesswoman and politically well connected. “One of your team said a shocking thing in the debate,” she said. “He said America can't police the world, even if we should.

Can't?” she exclaimed. “When I started with nothing, in Indiana. I didn't say ‘can't' to myself. Never use the word ‘can't' next to the word ‘America'. That put me right off your side of the argument.”

We flew out and back from Luton. Really - Luton. Direct. To Newark airport. It was brilliant. You can arrive as little as half an hour before take-off, get a super breakfast, and check-in takes about five minutes. Newark, too, is less hassle than JFK.

Some 30 years ago, on a Campari advertisement, the proto-Essex-girl Lorraine Chase was asked by a businessman: “Were you truly wafted here from paradise?” “Nah, Luton airport,” she replied. Campari may be a little out of fashion, but Lorraine was right about the airport. New York snow permitting, I too will be wafting in from Luton this morning.

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