Although immigration has boosted the economy and made Britain a more diverse and in some ways interesting place, it has also made us poorer, drained our resources and brought cultural practices we could happily do without. In January a gang of nine Muslim men — seven of Pakistani origin, two from North Africa — went on trial at the Old Bailey for the sex trafficking of children from the ages of 11 to 15. One of the victims sold into slavery was a girl of 11. She was branded with the initial of her "owner" abuser: "M" for Mohammed. The court heard that Mohammed "branded her to make her his property and to ensure others knew about it". This did not happen in some Saudi or Pakistani backwater, but in Oxfordshire from 2004 to 2012.

Of course gang-rape and child abuse are not the preserve of immigrants. But as recent child rape-gang cases in the north of England have also shown, there are specific cultural ideas and attitudes that some immigrants bring with them — about women, other races and sexual minorities — which are not even medieval yet. Attempts to impose parallel legal systems, "blasphemy laws", and other new "norms" of behaviour are subtler versions of the same. But so fearful of "racism" and so in retreat is the core culture that it can barely rouse itself even to point any of this out. What media reporting there is of cases such as that in Oxfordshire is not only scant and periodic, but fearful and hedged with caveats to the point of obliqueness.

Of course none of this ever comes up in any "acceptable" discussion on immigration. Only the good must be dwelt upon. The bad is ignored. But just as surely ignored is the other thing which was missing from our cosy, right-on Newsnight discussion. That is what we used to call the mainstream — the core — what used to be called "our culture". Again, nobody much likes to talk about this. Amid all the endless celebration of diversity, the greatest irony remains that the one thing no one can bring themselves to celebrate is the thing that allowed everyone here to celebrate in the first place.

But why is it ignored? Is it accident, design, policy or cock-up? Even those of us inclined to cock-up theories of history might recognise that the direction in which the argument has jumped in recent years has a deeply malevolent edge. It brings to mind what Samuel P. Huntington said about "multiculturalism" — that policy made up on the hoof after mass immigration began. In his final work, Who Are We?, Huntington wrote, "Multiculturalism is in its essence anti-European civilisation . . . It is basically an anti-Western ideology."

Among the staging-posts which allow the argument to get to that end are some of the most intellectually dishonest arguments of our time. All were on display in reaction to the census. They include: "It's nothing new." This popular argument goes: "Britain has always been a melting pot of people of different races and backgrounds."