SHE could not be straight with us even in her resignation letter. There, Amber Rudd gave as her reason for stepping down that she had “inadvertently misled” Parliament over targets for removing illegal immigrants from the UK. Yet all the evidence dribbling out over the past couple of weeks goes to show how she not only knew about targets but had even boasted to the Prime Minister of being able to tighten them.

Over the weekend, the press was finally in a position to publish the full document only referred to obliquely in earlier reports, no doubt after a leak by disgruntled officials inside the Home Office. It showed Rudd setting out before Theresa May an “ambitious but deliverable” aim to deport 10 per cent more illegal immigrants over the next few years. Nothing inadvertent about that.

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But the implausible theme of “I didn’t know that I didn’t know” continued to the end – and of course into the resignation letter, where Rudd took “full responsibility” for the fact she was not aware of “information provided to (her) office which makes mention of targets”.

I suppose members of the Cabinet getting towards the end of the gangplank are bound to wax desperate in their excuses, in a political system where economy with the truth has anyway long become routine. But, in a series of appearances in the House of Commons and before its committees, Rudd’s demeanour had grown ever more wretched, her explanations ever more convoluted. At the moment she resigned on Sunday night, she should have been preparing a definitive statement to MPs the next day. The intellectual effort would clearly have taxed this graduate of the University of Edinburgh beyond her limits, and off the end of the gangplank she duly jumped.

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Her shadow and nemesis, Labour’s Diane Abbott, conceded Rudd had “done the right thing” but saw still more mileage for the opposition at Westminster. She demanded that “the architect of the crisis”, Theresa May herself, should come before the Commons to explain “whether she knew that Amber Rudd was misleading Parliament and the public last week”.

That hit the nail on the head. The sole exoneration I would be inclined to consider from Rudd is that it takes a good while for all ministers to impose their own priorities on any department they take over. She had been at the Home Office for less than two years, whereas before her May had presided there for six years and found the leisure to remould it in line with her own unpleasant and oppressive prejudices.

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We know on past form that Rudd has a more liberal outlook – as a Remainer in the EU, for example. But even if she had dared to go against the boss who hand-picked her as a successor, it would have been in practical terms impossible. Just as the course of the Titanic could not be altered in time to miss the iceberg, so under its own institutional inertia the Home Office would have carried on creating an atmosphere of hostility to immigrants till some crisis, such as Windrush, caused a crash. And so things have turned out.

It would be nice if the resulting wreckage did prompt the start of a rethink on migration policy, though we should not hold our breaths: this is not a government that readily learns from its mistakes, but on the contrary presses on towards collision at all the greater speed. Reality does not matter so much as short-sighted and short-term political imperatives – in this case, for example, the perceived need to placate the prejudices of potential Ukip voters.

Yet the reality is that the UK has always absorbed successive waves of immigrants, from the Huguenots fleeing religious persecution 300 years ago to that shipload on the Windrush which came to help with post-war reconstruction. From the Pakistani corner shopkeeper to the Polish plumber, they have become an integral part of our open, liberal society too. I would find it impossible to argue that this has been bad for us.

Apart from anything else, the phenomenon has set a pattern for mobile Brits, not least mobile Scots, who are to be found in every corner of the world bettering themselves and the societies they have settled in. It is one of the blessings of globalisation, which in many other respects is what allows the citizens of these islands to survive and prosper.

For all these reasons it is quixotic to suppose migration can ever be controlled by any UK Government, not down to the level of fine detail that this government appears to expect. Its own target set in 2010 of cutting net migration to less than 100,000 a year has been consistently missed, and missed not by any small margin but missed by a mile. I would say a policy that has failed over eight years is never going to work and should simply be abandoned.

Why has the policy failed? The basic reason lies in the fact that the UK is one of the world’s big, rich economies, and it is easy for fresh arrivals to find a job, easier than in the continental countries with their high rates of unemployment.

This labour market is besides little regulated so that fit young foreigners especially, with no roots or dependants, can exploit one of the few advantages they have, their mobility: this is why so many of them risked so much to get over here from the Jungle at Calais.

As a lure for them from less than 100 miles away lay the great global city of London, with its endless demand for labour of all kinds, a place where the mobile migrants can keep one step ahead of any authorities pursuing them.

If these authorities ever did impose restrictions of the kind May thinks ideal, with rigorous checks at entry and detention or deportation at the drop of a hat, the migrants would just invent new forms of illegality. We would see boat people coming across the English Channel and the North Sea, with some of them drowning on the way, or vanishing after a safe landing into underground networks of their compatriots who would hire them out to anybody willing to pay. In an economy by now short of labour, there would be plenty.

These would be mainly metropolitan problems, but let’s recall that the UK has far from a uniform labour market and that the requirements in Scotland are rather different from those in London. Our economic (and political) future depends on raising our rate of growth above its present miserable level, so that born Scots have the courage to forge their own future and others want to be part of it. With an aging population, one essential is a rejuvenated workforce which, given our low birth-rate too, can only come from immigration.

I am confident that in the long run these natural economic forces will overcome the cruel and irrational constraints that the present Tory government seeks to place on them. Indeed they will be the downfall of the present Tory government, perhaps sooner than we expect. Amber Rudd may come to think she is well out of it. I would prefer a grimmer fate for Theresa May.