Some quotations can be tricky to source. John Maynard Keynes is frequently cited as saying: “When the facts change, I change my mind, what do you do?”

Others credit alternative economists. Or Churchill, generally a reliable stand-by.

Either way, it is an apt comment. The SNP are facing the consequences of one altered fact which is that Scotland is no longer in the European Union. Cue irony.

What is the core justification for indyref2? Why, Brexit of course. Nationalists say Scotland’s departure from the EU, against the majority wish of her citizenry, provides the material change in circumstances which enables the referendum timetable to be truncated from “once in a generation”.

Next, what is the core new conundrum confronting independence campaigners? Why, Brexit of course.

Question. If an independent Scotland rejoined the EU, would that create a trade and customs border between Scotland and England?

Yes, as things stand, there would be such a border because Scotland would be governed by EU rules and England would not.

In a study of this issue, two leading experts, Katy Hayward and Nicola McEwen, say the Scottish economy would “clearly be exposed to any trade disruption with the rest of the UK”.

They add that could be offset by “new trading opportunities with the rest of the European Union”, while noting that this might take time.

Another question. Would a newly independent Scotland seek to rejoin the European Union? Definitely? After seeing the disquiet in Northern Ireland, where EU trade rules continue to apply despite the UK’s departure?

That question of whether Scotland should rejoin is presently being whispered within SNP circles. One report suggesting there might be a distinctive referendum on Europe, post independence, was fairly speedily dismissed.

This week, The Herald opened an extensive and much-needed discourse on Scotland’s Future. The series will eschew partisan propaganda, replacing it with serious scrutiny and debate.

Up first, the European Union. Alex Neil, the former SNP Cabinet Secretary, forecast that Nicola Sturgeon would lose indyref2 if she posited Scottish membership of the EU as an intrinsic part of the independence offer.

Now, Mr Neil might be expected to say this, given that he campaigned for Brexit during the 2016 referendum campaign. However, he might well attract a sympathetic hearing from sections of the SNP.

I trust Alex Neil will not be upset if I note that both he and I are old enough to remember when the SNP was decidedly sceptical about the EU, in its previous guises.

In the 1970s, when the UK joined the Common Market, much Nationalist opinion tended to be that they did not want to replace or supplement London hegemony with rule from Brussels.

There was even bitter talk that the EEC wanted to take control of “Scotland’s oil”. Remember when the black gold was seen as a desirable asset, and not an ecological embarrassment?

Those sceptical sentiments have been submerged within the SNP. But they still exist, for some.

It is perfectly feasible that some fervent Remainers outwith the SNP might find independence more appealing if it guaranteed a return to the EU. It is equally feasible that some independence supporters might be deterred.

Just as I remember when the SNP distrusted Brussels, I remember why the party switched, heavily influenced by voices such as Jim Sillars, Mr Neil’s former mentor.

The SNP had long been seeking ways to counter claims that independence only involved fragmentation, the break-up of the UK.

So they embraced Europe. They argued: “Don’t think of us leaving the UK, think of us joining the EEC on an equal footing with England, our friends and neighbours”. Not leaving, but joining.

That has been the underlying mantra of the SNP since the development of the modern party. Remember Winnie Ewing and “stop the world, Scotland wants to get on”. Succinct and sharp, like the best slogans.

Europe provided a platform for that slogan to be deployed in practice. Might it now dump a large, unwieldy boulder upon the path to independence?

I think it might. Yes, Scots voted to retain EU membership. But I do not believe that was a decision attended by universal zeal or fervour. I think it was a choice on balance, substantially influenced by the largest party, the SNP.

Which means that Scots will look at the hard practicalities, rather than ideology, with regard to developing EU relations. What Keynes (or someone else) would call the changed facts.

Of these, the biggest is the question of continuing trade with England, after independence. That has to be addressed, particularly in the light of the Northern Ireland problems.

One SNP Minister told me this week that government and party work on such issues was “powering ahead”. Supporters of independence will certainly hope so, in that they would need precise, credible answers to place before the voters.

Further, the issue has to be addressed in a context where, just as with the currency, it would be in the political interest of UK Conservatives to talk up the challenges, rather than offer to resolve them.

Right now, the Tories are torn on whether to engage in full-scale conflict over independence-related topics. Instinct says “go for it”. Strategy says “stay off SNP territory, stick to the line that indyref2 isn’t going to happen”.

Self-evidently, there would be no such constraint as and when an independence referendum campaign is in progress.

Equally, I am struck by the level of nuance on the EU from the SNP. Understandably, there is rhetoric aplenty about Scotland being torn from the EU against her will.

But look at this promise from Nicola Sturgeon in her November conference speech. She said Scotland would “rejoin the European family of nations”.

That could mean full EU membership, which remains the core objective. But it could also encompass Norway-style membership of the European Economic Area.

That might certainly serve in the interim because Scotland could not formally apply to rejoin the EU until independence was attained, and those negotiations would take time.

More generally, I believe there is more to come on the European question in this fascinating debate about Scotland. Facts and minds might yet change still further.

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