I now, of course, believe this disdain for the national was immature and premature as well as loftily dismissive of majority opinion. How did I come to change my mind about that and about large-scale immigration?

No doubt becoming a more grounded person and mixing with a wider spread of people knocked some of the undergraduate ideological gaucheness out of me as I entered my thirties. But what I like to think really changed my mind was good ideas, or openness to better ideas than I had been carrying around. And it was one of those good, simple ideas that inspired the most important political moment in my life since failing to play in the Eton v Harrow match. 

In February 2004 I published a 6,000-word essay entitled "Too Diverse?" in Prospect magazine (which I then edited) about what I called the "progressive dilemma" — the conflict between diversity and social solidarity, two of the great principles of the Left. The essay was reprinted in the Guardian at the prompting of Will Hutton and raised a storm of often angry argument. I was accused of being a "liberal racist" and paid my penance on the race and immigration conference circuit for part of the next few years, where I tried to articulate — often with difficulty — why it is possible to worry about the effects of "difference" without being a racist.

It was David Willetts, the leading Tory, who had first drawn my attention to the "progressive dilemma". Speaking at a Prospect debate on the welfare state in 1998, he noted that if values and lifestyles become too diverse it becomes more difficult to sustain common norms and hence the legitimacy of a risk-pooling welfare state. "This is America versus Sweden. You can have a Swedish welfare state provided you are a homogeneous society with intensely shared values. In the US you have a very diverse, individualistic society where people feel fewer obligations to fellow citizens. Progressives want diversity but they thereby undermine part of the moral consensus on which a large welfare state rests."

That is to say, people are readier to share and co-operate with people whom they trust or with whom they believe they have significant attributes, and interests, in common. That "in-group" can be, and is, extended to include people of very different racial, ethnic or class background. But it does not happen automatically or immediately: consider the long history of class conflict and co-operation in Britain that led to the 1945 settlement. 

Willetts's dilemma seemed to me a true and powerful idea. I remember thinking when I first heard it: why is this issue not discussed more, particularly on the Left? My own attempt to give it greater salience was that rather abstract, tentative essay "Too Diverse?". I had no idea it would provoke such a response. When it did so, I felt briefly like a religious heretic.