Metropolitan liberals have received a number of shocks in the past few months. One of the biggest was the discovery that many of England's poor, northern towns and cities are segmented along racial lines. Much in the same way as Edward VIII organised an expedition to the poverty-stricken south Wales coalfields in the 1930s, distinguished committees have visited, discovered and pronounced.

Still, the fact that things which have been obvious for years to anyone with experience of these areas are now wider public knowledge must be good news. The inhabitants of these towns have witnessed the developments with their own eyes, yet the prevailing ethos of multi-culturalism has restricted debate. At last, we might now be able to have a proper discussion.

It appears that far from celebrating diversity, the masses have taken every opportunity to separate themselves along racial lines. This is hardly surprising. A preference to mix with people similar to oneself is as old as humanity. British towns and cities have long been segregated along class and income lines. The Manchester bourgeoisie did not feature in Engels's description of life in the central and eastern districts of the city in 1844. And nor did they feature for the next 150 years, until a few expensive apartments were built for them on old industrial premises. The British exception is inner London, where the rich and the poor have traditionally lived cheek-by-jowl. But the rich spend a great deal of effort and money in insulating themselves from their immediate surroundings, relying on private education, private health care and private transport.

Many similar examples can be found. The housing division along religious lines in Ulster is now unusual in the west. More common is the segmentation within the working class between the respectable and the remainder, the former fleeing first to garden estates then to private ownership in a long process which began in the 1950s and continues to this day.

For many liberals, the events in northern towns reinforce their belief that Britain is a deeply racist society. Yet Britain is, in fact, a rather tolerant society. It has accommodated immigrants, by and large placidly, from very different cultural and economic backgrounds. Immigrants tend to live in poorer areas and to compete for jobs at the lower end of the labour market. They exert pressure on precisely those groups for whom life is already something of a struggle. Yet the number of serious racial incidents in Britain over the years has been very small.

Geographical segregation, whether on class, racial or religious grounds, is not a problem in itself, and is compatible with a liberal, open and tolerant society. It is also inevitable so long as people have some choice over where they live. Moreover, high degrees of segregation across a town or city can come about from very mild preferences at the individual level. In other words, individual citizens may have a perfectly understandable wish to live amongst people similar to themselves. And this preference need not be very strong for the end result to be distinct segregation, as the science of complexity theory can demonstrate.

The original insight into the question of geographical segregation on racial lines was provided in a paper by the American, Thomas Schelling, 30 years ago. Schelling imagined a large chessboard, not just of the usual eight by eight dimension, but much bigger. In his model, or game as we might think of it, there are equal numbers of two types of pieces which populate the board-"agents" in the jargon of complexity theory. The types in the game can represent different social classes, racial groups or whatever. Initially, these are placed at random across the board, with a small number of squares left vacant. These rules are all that is needed to set the game up. One more rule is needed to describe how the game progresses, namely the rule agents use to decide whether or not to move. It is very simple. An agent decides to move from its square to a vacant one, if less than a specified percentage of its neighbours are of the same type as itself.

The game progresses in a series of steps, with an agent being chosen each time to decide whether or not to move. If a neighbour is defined as one of the eight squares which surround any particular square then the rule for moving is as follows: if four or more out of these eight squares are occupied by agents of the same type, an agent will not move. If there are less than four, the agent moves to a vacant square elsewhere. So an agent is content with its neighbourhood if, adding itself to its eight neighbours, five out of nine pieces are the same type as itself. In other words, it is content if as many as four out of the nine are different.

In this version of the game, agents exhibit quite a high degree of tolerance for those of a different kind. They only move if they find themselves in a minority in their area. Yet across the chessboard as a whole, as the game progresses, the two types of pieces divide themselves into sharply segregated groups. From an initial configuration in which they are scattered amongst themselves haphazardly, a very distinct pattern emerges (see the illustration on the previous page).

Of course, this is a theoretical model, and the world is more complicated than this. In reality not everyone has the same ability to move, and people are not scattered at random across a city. But this does not undermine Schelling's central insight. Marked segregation can arise from only rather mild individual preferences.

The main problem faced by British society is neither one of racist attitudes nor of residential segregation. Rather, it is the ideology of multi-culturalism. Multi-culturalism, not in theory but as it actually exists, has come to mean the attempt to imprison individuals in the class and cultural background into which they were born. Thus the suggestion that everyone living in Britain ought to learn English is rejected by some multi-culturalists as linguistic colonialism. Similarly, it is sometimes suggested that ethnic minority or working-class children should be given a different style of education to middle-class children because they stand no chance of getting into good universities and having successful careers in the existing system.

There does appear to be a set of characteristics which is a necessary condition for the existence of a nation. A common experience of language, territory and economic life are defining features. America and Canada share a language but not territory, and the Canadians are proud of their distinct nationhood. Belgians share a geographic area and an economy, but the deepening linguistic divisions put increasing pressure on the survival of Belgium as a nation. But in addition to these factors there must be a community of culture. The history of Ireland is a classic example of the problems which arise when this does not exist.

David Blunkett, the home secretary, is absolutely right to insist upon such a community of culture in Britain, which has developed over the course of centuries. The minimalist liberal demand that people of all races and classes merely obey the law of the land is not enough if we want to live in a society with a sense of cohesion and neighbourliness. There is such a thing as "Britishness," amorphous and evolving though it may be. (Britishness is a good illustration of the familiar "defining an elephant" problem. It is hard to define but everyone recognises an elephant when they see one.)

Immigrant groups contribute to the evolution of such a culture, in more ways than Robin Cook's trivial example of the availability of chicken tikka masala. But at any point in time immigrants and their descendants are a small minority. Proper integration in these circumstances inevitably places on them the responsibility to adapt to the host culture, rather than the other way round. Suggestions from time to time in my home town of Rochdale that all schoolchildren learn Urdu have not been favourably received by the majority. As it happens, I believe it would stretch the minds of schoolchildren in the town in much the same way as, say, learning Ancient Greek would. But this is not how it is viewed.

There is, of course, an equal obligation on the host culture both to ensure that immigrants are equipped to adapt to their new surroundings and to ensure that those religious and cultural differences which are compatible with life in Britain are tolerated. More effort should be made to encourage poorly educated immigrants to develop language skills and to explain British law, institutions and traditions to them. Opinion surveys show that clear majorities among most ethnic minority groups themselves favour "adapting and blending in" to the way of life of the host community. Thoughtful voices from within the minorities are beginning to question the ideology of multi-culturalism and to recognise that it traps people in their ghettos, both cultural and economic.

Nations and cultures evolve. John Major's vision of Britishness-his famous old maids bicycling to communion-was rightly condemned on these grounds. It was an exercise in nostalgia. But if we are to be consistent in applying the idea of evolution to societies, we may have to recognise some uncomfortable truths. We have to recognise that at any given point in time not all cultures are equally "fit." The western tradition of scientific empiricism, discovered by the Greeks and reinvented by the Arabs, does appear to be extraordinarily powerful and enduring. It is largely the foundation of our present prosperity. It is superior to a culture which glorifies ignorance and grinding poverty. This is not to say that history is now at an end. Evolutionary theory tells us that at some unpredictable point in the future, a new dominant culture will emerge. But in the meantime it is sensible to insist on the primacy of the western empirical cast of mind.

The geographical divides in our towns tell us only that people have a mild preference for being surrounded by neighbours similar to themselves. Britain has been and remains a relatively tolerant and accommodating society. The social and economic problems of the north of England are not caused by people living amongst their own kind and they will not be solved by well-meaning attempts to stop them doing so.