DAVID Cameron accuses Leave campaigners of stoking intolerance and division with extreme warnings on immigration.

The Prime Minister thinks these campaigners, for all their motley ideological hues, have “become very narrowly focused” on a single, somewhat peripheral question – at any rate not the most important, given the range and weight of the other political and economic issues involved in the referendum.

For once, I find myself agreeing with Dave. The only realistic point to be made about migration is that a referendum in a host nation is not going to make much difference to it, whether the result is Yes or No, Remain or Leave.

Migration is basically an economic phenomenon. It is the removal of units of one factor of production, in this case labour, from a place where they are doing no good at all to a place where they are capable of doing some good. This follows as a normal result of people’s calculation of their own advantage. If they are right, there will be a net benefit to society.

An analogy might be with lumps of coal: they are useless lying inert underground where they have been since the days of the dinosaurs, but useful in all sorts of ways once we bring them to the surface and make them available to companies or households for burning. We cannot change the qualities or applications of the coal by saying Yes or No to it.

What economics thinks of the matter is fairly obvious, so it does not surprise or dismay me when Cameron fails to hit his target of limiting net immigration into the UK to 100,000 a year. He failed to hit the target in 2010, when he first set it, and he has failed in every single year since. At this moment, the limited progress he made at the beginning is going into reverse, with the result that in 2015, the UK took in 333,000 immigrants. He will continue to fail in the future, which for him is not going to be long anyway before he sets off into his sunset of consultancy and international speaking tours.

But what did you expect? Cameron is a feeble Prime Minister, but it would not matter if he were an amalgam of Winston Churchill and Clement Attlee, of William Gladstone and Benjamin Disraeli rolled into one. Cameron would still fail to control migration. This is because the forces that cause migration are not his to control.

The UK economy under Cameron has not done well, and may soon be doing a good deal worse. Even so, it has outperformed the economies of all our main partners in the EU except Germany.

A certain amount of economic growth has been achieved, then, and for an economy to grow it needs three things. It needs some resource or advance in technology not possessed by other countries, which it can therefore exploit to its own advantage. It needs the capital to finance this development. Finally it needs labour, in other words a workforce to man the new factories and learn the new skills that will allow the actual new production to take place.

The reason the UK has been able to grow even amid a dismal global environment is that it fulfils these conditions. For all its faults, it is still quite an inventive economy, in anything from the ATM to the world-wide web. It is inventive, too, in its creation of financial instruments in the City of London, so that it can find ample capital to finance its own expansion. Finally, even though its indigenous population is in decline, it has been able to attract enough immigrants to provide the workforce required for the expansion.

It is unlikely any of these three forces for economic expansion can work without the other two. If we have no new products to make, then the capital will go elsewhere and so will the workforce. If we have no capital, then we cannot spend it on investment to employ our people. And unless we can supply our needs for labour, our wages will go sky-high and our markets will vanish because our goods will be too dear for foreign customers.

I believe in free markets, and in the power of individual human agents within them, so I do not think that in the end national governments can do much to stem the basic economic forces as I have just set them out. That is true in particular of any power they might think they can wield over migration, especially as migrants have minds and wills of their own.

If it comes to the negotiation of a new deal with Europe, the UK will anyway seek to maintain its membership of the single market, with the free movement of capital and labour, of goods and services. So the Polish plumbers and the Bulgarian barmen will still be with us.

For the other main body of migrants, all those hapless refugees the world’s trouble spots, the UK might announce tougher border controls or even try to exclude them altogether. But such people, however wretched, have shown us that they have courage.

As a last resort, they will come crammed in the backs of lorries or on flimsy boats even at the risk, often realised, of death. They do not seem likely to let themselves be deterred by some new treaty among the statesmen of Europe.

These migrants come because, at the other end of the chains of exploitation along which they pass, the converse of the basic economic forces operates on them. They have been living in countries where there are few resources to exploit, and only primitive technology to exploit them with.

Neither they nor their families or communities, or governments, have any money. Yet they reproduce at a high rate.

It’s an old economic adage that a country that cannot exports its goods will export its people.

In the past the UK has taken in large groups of foreigners, from the Huguenots to the Jews, and at the end of the day found this acceptance only to have been of benefit to all. We have saved from persecution and death people who have turned out to be benefactors to the whole human race.

I find it incredible that in its post-imperial decline the UK may have a majority that wants to close, so far as possible, its frontiers to migrants.

It is a case of self-harm by a senile polity. And strangely, it will signal that this polity is scarcely interested any longer in economic growth, because migration and economic growth go together.

In other words, the UK will continue on its course of relative decline. When we get the chance, let’s take Scotland out of it.