Our conversation turned to tribes.
We'd been talking about careers; about working your way up a company and realising too late that those who run it are just the sort of people you don't want to become. They're not your type; not your tribe.
So who is your tribe? I asked the man who was sitting beside me at lunch. He began to answer by saying his ancestry was Jewish.
But for him tribe wasn't about ethnicity or religion. It was about the people who raised him, those who helped form and shape his early years.
He felt he would always have a deeper resonance and understanding with them than with others.
I mulled this over and wondered whether I could say the same. On reflection, I didn't think I could.
I found the conversation interesting because the question of tribes has been fluttering around my mind since the inception of the referendum campaign.
For the first time since I moved to Scotland more than 30 years ago, I'm being asked to take sides.
Most people have made up their minds about where they stand. I'm in with the leftovers, that 20-some per cent who are still swithering and sucking their gums.
So which way shall I vote? Which side is mine? Where do I belong? Am I a pro-Independence or pro-Union?
It feels to me like I'm being asked to choose a tribe. It's a difficult question, one that I find hard to answer.
In my case the choice is complicated by the fact that I am Irish and Northern Irish at that.
Where I grew up, there were very definite sides but I didn't get to choose between orange and green.
The green wasn't stamped on my forehead or sewn in a yellow star on my sleeve but it was writ large in the freckles on my face and the mop of ruddy hair on my head.
And yet I didn't feel like a member of a tribe. Which of us does before we are labelled from outside? I tell people I have a rebel heart, and I do. I resist being ruled. I value self-determination. To me it is a prize worth the fight.
But I loathe the narrow nationalism I saw in my youth. I detest the "us and them" approach to life; and I hope not only because I started as one of "them".
I attend to the referendum debate and sometimes I shudder. Depending on whom you listen to, an independent Scotland will be glorious or doomed.
The Scottish nation is either morally superior or a dependency culture peopled by a bunch of whingeing scroungers.
We will either be rolling in clover or owners of Europe's biggest begging bowl. Assertion follows counter assertion until I want to shout: "Stop it."
To my mind voting for Independence is like getting married. If you don't want it enough to risk your entire future, don't do it. For when you walk up the aisle, or in this case into the ballot box to place an X against Yes, you throw your hat in the air.
So will I? Will I toss caution to the wind come September 18?
Alex Salmond has made it clear that residency determines the right to vote. It means I can vote but my two Scottish children cannot.
I told them I will vote for them if they concur but I need to know my own mind if they don't. I need to choose my tribe. Last week I spent a few days in England and looked at it anew, the possibility of separation at the forefront of my mind.
And what I came to realise during the visit was that the referendum campaign had had an effect on my view of the English without me realising it.
For a start, I was aware of the people I was meeting being English, of the possibility that soon they might belong to another country. Just a few years ago I wouldn't have made such a distinction.
I also realised that, since my visits south are usually to London, I've come to think of the English as a cosmopolitan urban people who are less embedded in the countryside than the Scots. Last week I spent some time in Derbyshire where I met a very successful young woman from Sheffield.
She is rooted in her community and very happy to be so. But every so often she works in London for a few days.
"It's like going to Paris," she said.
"It's your capital city. Don't you have a sense of ownership?" I asked.
"No. Not at all."
She didn't feel that London was hers. Westminster was even more remote. I was amazed.
She sounded very much like people do in Scotland, people who plan to vote Yes.
I probed further. She is left-wing, a great supporter of the welfare state, especially the NHS.
She loves the idea of her children being educated in a school with a rich range of social and ethnic diversity.
She was expressing many of the views I associate with the arguments of the Yes camp.
Instead of regarding her as English and therefore different, I found her reassuringly familiar.
What's more, she thought her views were pretty normal outside the home counties.
Which tribe was she in? Was it different to mine or the same?
I knew that Newcastle and the north-east of England in general identified with Scotland's sense of alienation from Westminster.
I'd no idea the feeling stretched so far south.
It helped me to crystallize my thinking; to realise that my own sense of tribe is not ethnic, religious or geographic.
Now that I am home and have reflected on other conversations I had last week in different parts of England, I see my tribe as a gathering of like souls. I identify with people of shared values and attitudes from wherever they originate.
I know who they are because when we meet, whether for the first time or when years of separation have gone by, we connect immediately.
Some will share my political leanings, others will not. Even that doesn't matter so long as they, like me, can agree to differ.
My tribe has no name. It has no labels of class or creed. It has no gender or age bracket.
It can't be identified by physical characteristics and it is contained within no geographic boundary.
If it has a hallmark it might be tolerance.
I still value self-determination. I still feel the pull of the prospect of waking one morning in a country whose future depended in part on my efforts.
I can understand that feeling of excitement, the thrill of history being written.
But if I was sitting down with a blank sheet of paper to draw a blue-print of the society in which I would like to live, it wouldn't be much different to the United Kingdom before the Iraq war.
The question for me now is whether that United Kingdom has been lost forever.
If I conclude it cannot be regained, will I have to choose, after all, a side based on geography?