THE former SNP minister Alex Neil, who has announced his retirement from elected office, is that rare thing in the increasingly conformist Scottish National Party – a genuinely independent thinker. You don’t have to agree with him to respect a politician who has consistently put career aside in the interest of what he believes.

Now he is leaving along with a raft of SNP luminaries including Jeane Freeman, Mike Russell, Roseanna Cunningham, Bruce Crawford, Linda Fabiani and others. You have to ask whether they would be heading quite so eagerly for the exit if they really believed independence was just around the corner – just an election away. They are all in their sixties – but as American politics shows, 70 is the new 50.

Any way you look at it, this is a turning point for the SNP. It is not just a generational change that is needed but an intellectual one. In his valedictory message, Mr Neil told the party in no uncertain terms that the economic case for independence needs to be rewritten. And not for the first time, he is right.

The prospectus of 2014 is now radically out of date. Following Brexit, we are a different world. Failure to recognise this could lead to the SNP’s towering electoral support disappearing like snow on a dyke – back in the days when we had much of the white stuff.

If and when Scotland leaves the UK, and it is looking more and more like a matter of time only, it will have to face challenges that were never part of the 2014 vision. A hard border will emerge in the UK and Scotland will be out of the European Union.

SNP ministers tend to laugh this off as if it is just another scare story like 2014’s Project Fear, but it isn’t. The first act of Scottish independence will be to manage this border and, as has been the case with Ireland, it will be profoundly compromising.

The 2013 Independence White Paper, Scotland’s Future, was a remarkable document in many ways. The 670 pages, largely drafted by Nicola Sturgeon, presented a brilliant step-by-step guide to pain-free secession. It offered independence without borders, without separation, without disruption, without new currencies –- indeed without actually leaving the United Kingdom under the Crown.

It was an offer the Scots couldn’t refuse. Inconveniently, they did.

2014 was the culmination of the policy of Independence in Europe, the prospectus devised by Alex Salmond from an original idea by Jim Sillars. This idea was that Scotland could evolve seamlessly into the ranks of successful small European nations without breaking with our major trading partner, England.

In 2014, no one thought that the UK would be daft enough to leave the EU. Inconveniently, it did.

When Labour’s Ed Balls talked of “border posts at Gretna” in 2014 he was talking, well, his own name. It could be laughed off since both the rUK and Scotland would have been coexisting peacefully in the European single market. There would be no need for borders any more than between France and Germany.

Similarly, the awkward issues of currency and debt could largely be ignored. Both the rUK and Scotland were happily opted out from the euro. There was no reason for that status quo not to prevail. Now, after Brexit there is.

If and when Scotland leaves the UK, the first task of an independent Scotland will not be to rejoin Europe. It will be to negotiate a new union with England to ensure free trade, a common travel area and regulatory alignment.

There is no getting away from this. Scotland will have to adopt the trading regime that prevails in our major trading partner, at least in the medium term. It will also have to accept the dictates of the Bank of England on interest rates and debt, since the SNP’s plan is to keep the pound for a decade.

This will lock Scotland into precisely the path it wishes to avoid, since the majority view here is that Scotland should rejoin the European Union at the earliest opportunity. Brussels will no longer seek to obstruct Scottish membership, but this doesn’t mean membership is a foregone conclusion, or that it could happen overnight. At the very least accession will take two to four years.

Scotland’s temporary adoption of sterling may not be an irresolvable issue, but it will be one. All new member states must accept the Maastricht Treaty and the euro, at least in principle. More importantly, countries seeking membership are required to have an independent central bank. Scotland will not possess one, unless it bites the bullet and adopts its own currency.

And of course, when Scotland does finally rejoin the EU, that very act will create a hard border with the rest of the UK. The integrity of the single market requires that EU borders are securely policed and free from regulatory seepage. Brussels will not allow another fudge like Ireland.

If the UK had adopted a soft Brexit, or even accepted regulatory alignment with the EU, this would be less of a problem. But this is not happening. We are headed for a hard Brexit in January, and that is going to necessitate what will inevitably be called a “hard Scexit”.

As Mr Neil has indicated, there needs to be a serious debate in the SNP about whether and how Scotland wishes to rejoin the European Union. He argues that it would more sensible to become a member of the European Free Trade Association rather than join the European Union directly.

I’m not sure that entirely answers the borders question, but it certainly needs to be discussed. It might be easier to manage relations with the UK were Scotland semi-detached from the EU. Some will argue that Scotland should on principle stand aloof from the free market EU and its neoliberal doctrines, as Norway did in the 1990s.

None of these issues is insurmountable, of course. This is not just another Project Fear. Independence voters are quite capable of understanding and accepting that independence may need some short-term sacrifices, just as Brexit supporters did over leaving the EU. But what voters won’t forgive is a party that pretends nothing has changed when everything has.

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