The bonds that bind the European Union are fraying. And the looser they become, the easier it will be for the UK to slip those bonds entirely and make its escape.
Or, to put it another way: the EU's migration crisis could well hasten Britain's exit. If you're in the mood for real drama, how's this for a new mathematical formula? Greek debt crisis + EU migration crisis + Brexit = end of EU.
Fanciful? Perhaps. But after the latest emergency summit in Brussels on Wednesday, even the president of the European Council, the former Polish prime minister Donald Tusk, said: "What is at stake is … the future of Schengen, the sense of order in Europe and the common European spirit." In other words, the future of the EU itself.
The European Union is built on a labyrinthine system of rules and regulations designed to ensure that all members have equal rights and responsibilities. Once member states start tearing up the rule book with impunity, the structure soon starts to crumble.
The rule book is already in a sorry state. Rules about propping up weak economies were jettisoned when Greece teetered on the brink of debt default. The same thing happened to the rules on processing applications for asylum (the so-called Dublin convention) when first Italy and Greece, and then Hungary, buckled under the sheer weight of numbers.
Then Angela Merkel said "Willkommen" to just about everyone, regardless of the Dublin convention, only to kick the door shut again and tear up the Schengen free-travel agreement. In 1989, Hungary hastened the end of the Cold War by opening its borders with Austria, thus enabling citizens of the Soviet bloc countries free access to the West; in 2015, it started re-building the fences, and in doing so may have hastened the end of the EU, or at least of Schengen.
It's not a pretty picture. Mrs Merkel said in Brussels on Wednesday, as leaders gathered to discuss the migration crisis: “It cannot be that Europe says ‘We can’t handle this.’” It sounded like a whistle in the dark -- because the truth is that the EU can't handle it, doesn't know how to handle it, and can't agree on how to handle it.
For David Cameron, it's all a very mixed blessing. On the one hand, he is no longer the only EU leader unhappy at the way the institution is being run. Hungary, the Czech Republic, Slovakia and Romania are all deeply unhappy at the idea that other EU countries can simply gang up on them and order them to take in people whom they don't want to take in. (Prediction: they won't do it.)
He also has the satisfaction of noting that his fellow EU leaders have, belatedly, come round to his way of thinking that the most effective way to try to slow the numbers of people leaving the refugee camps in Turkey is to provide sufficient funding so that conditions in the camps, especially with winter approaching, remain tolerable.
But on the other hand, as long as the EU is embroiled in ugly recriminations over migration, no one will have much appetite for the nitty-gritty of the UK's demands for a re-negotiated relationship with Brussels. Besides which, if we take Mr Cameron at his word -- that he wants the UK to remain a member of the EU -- a rising tide of anti-EU sentiment is not what he was hoping for.
Nigel Farage and UKIP have made significant electoral headway over the past couple of years by equating EU membership in voters' minds with immigration policy. It is, of course, true that as long as the UK remains in the EU, it can't impose restrictions on the entry of citizens of other EU member states. But this is wholly unrelated to the treatment of people seeking asylum from Syria, Iraq, Afghanistan, Eritrea or elsewhere -- yet in many people's minds, the two issues have become fused into one over-riding fear: too many immigrants.
Now add to this unstable mix the election of Jeremy Corbyn as leader of the Labour party. He says he can't imagine a situation in which Labour would campaign for Britain to leave the EU -- yet his support for British membership remains less than wholehearted and he has in the past been extremely critical of what he has regarded as the EU's pro-free market ideology.
If you believe the opinion polls (and yes, I know that's a big "if"), the public mood is shifting towards a referendum vote in favour of leaving the EU. The day after the general election last May, I wrote: "It is not entirely fanciful to imagine that by the time of the next election, Scotland will have split away from the UK, and the UK, or what remains of it, will have left the European Union."
I'm not so sure about Scotland, but the prospect of the UK leaving the EU is looking even less fanciful now. Mr Cameron has a tough task ahead of him.