All human beings - and even animals - need borders, limits and constraints. They may be physical, ‘an Englishman’s home is his castle.’ They may be moral or habitual - we prefer not to spit or swear on a public bus. There’s no law against it. It’s just that it goes against our natural decent inclinations. They are nearly always imaginary - our animals, even our free-range pigs, very rarely stray over the border which is our front gate. They know their limitations and stick by them.

Attempts to build huge physical borders to keep people in - or out - very rarely work. Hadrian discovered that to his cost. The Mongol hordes swarmed over the Great Wall of China. Few people would hail the Berlin Wall as being a huge success; we all detest the needless wall in Israel and even more strongly that surrounding the ghetto in Warsaw. Mr Trump’s $5 billion wall with Mexico is an absurd piece of political narcissism. And the fence around Hungary will not stop the vast flow of migrants from the Middle East and North Africa. Walls of this kind rarely work.

If you want to keep people in a place - or out of it - you have to create the conditions which will make them not want to cross it. The reunification of Germany, the withdrawal of the Romans, and today, a longed-for increased peace and prosperity throughout the Middle East is the only thing which will stop the vast tide of humanity seeking a better life in Europe.

So it is with the Northern Irish border. No-one with half a brain, on either side of the Brexit argument, believes that a ‘hard’ border with the Republic is either desirable or possible. The Brits deployed 26,000 soldiers on the border during the Troubles. Yet it remained entirely porous. I remember watching smuggler ‘Slab Murphy’ moving large quantities of petrol across the border within a few hundred yards of the Army ‘sangar’ which housed our surveillance equipment. There will not be a ‘hard’ border. But there must be - and already is - a border. There are different tax regimes and VAT north and south of it.

96% of the world’s trade is never inspected by customs men, who are largely interested in drugs, tobacco, and alcohol, as well as people and bush meat, in which there is a large and wholly illegal trade. Those inspections are very largely intelligence-led, and do not involve ‘hard’ borders at all.

So if the Southern Irish are concerned about better or cheaper goods being imported into their country, then they must be ready to face the competition, and realise that it comes about largely because of uncompetitive EU rules and regulations. There are a variety of ways of electronically and procedurally monitoring goods that cross the border. Goods (or people) which cross illegally (in either direction) will remain smuggled goods, unsaleable except on the black market. Illegal immigrants remain just that - illegal - and therefore unable to claim benefits, work legally, go to hospital or educate their children.

The whole question of the Irish border is a chimera - a problem thought up by the EU as a means of stopping Brexit. So I wish the PM well as she sets off to Brussels to persuade them that we need borders, just not ’hard’ ones.