While other countries have their violent social revolutions, we in Britain tend to confine our class conflict to less bloody battles. Which door you're expected to use has long been a bone of contention. The term "tradesman's entrance" may have fallen victim to the forces of class war, but the concept is resurfacing in luxury housing developments, especially in London. To get planning permission for these prestigious projects, in some of the most desirable postcodes in the capital, the developers are required to include some affordable or social housing. The less well-off tenants are then expected to use separate entrances - so called "poor doors". This form of social apartheid has been dubbed "Dickensian" in a report out this week. It's argued that so publicly dividing society into the "haves" and "have not's" is a symptom of a much greater harm than being deprived of a 24-hour concierge to salute you when you walk through the door. How should people living in a diverse society relate to and interact with one another? The authors of this week's report believe that promoting social integration is a moral good - in the same way as - we've accepted - diversity is a good thing in other aspects of society. Should it be the business of the state to engineer integration? Developers have to maximize their profits to pay for the subsidised social housing - putting in premium services for those paying the market rate for their flats is part of that. Why should those people who are not paying the market price for their home get the same service as those who are paying the full whack? We're quite happy to accept the principle that you get what you pay for in other areas of life - flying for example - so why not in housing? And if we accept the idea of engineering social integration in housing, why not do it in other areas - for example by bussing school children from deprived areas to schools in more affluent districts? Does trying to promote social integration go against the grain of human nature? Studies of mixed housing in the US show that - far from encouraging integration by some kind of social osmosis - it actually entrenches divisions. And what about in our own lives? Do we as individuals have a moral duty to mix socially with people who are different from us?