The harder Brexit becomes, the less economically plausible it is for Scotland to break away and rejoin the European Union. The costs rise to prohibitive levels. Such is the Brexit paradox for Scottish independence.

Nicola Sturgeon missed a trick earlier this year. She should have told her Westminster troops to abstain on Theresa May’s deal and let it limp over the line, since it created unique circumstances for secession on tolerable terms, or at least as tolerable as could be hoped for post-Brexit.

This is not obvious to those who see the Scottish drama chiefly in terms of emotion and identity politics. The relevant point is that Mrs May’s Brexit package removed the risk of a hard economic border on the Tweed for an independent Scotland: specifically, in the words of a leaked EU briefing note, it “required” a customs union as the basis of the future UK-EU relationship.

It would have let Scotland leave the UK on terms that preserved intimate trade linkage and supply chains with its hegemonic market - England. There would have been no need for rules-of-origin documents and extra customs clearance at the Scottish land border.

If it is such a calamity for Britain to leave the EU customs union - as the Scottish National Party tells us - then it must logically be a greater calamity for Scotland to leave the UK union since the same problems exist and are greatly magnified. “The links between Scotland and the UK are much deeper, so the pain for Scotland would be commensurately larger,” says Sir Andrew Large, ex-deputy governor of the Bank of England.