OTHERS were calling him ‘scum” and “traitor”. But as the BBC’s James Cook was being harangued outside the Tory leadership hustings in Perth earlier this month, one protestor had a simple question for him.

What, the TV man was asked, did he know about the Claim of Right? A video of Mr Cook replying, amid a volley of cursing, went viral on social media and has sparked scores of stories and hand-wringing columns about “nasty nats”. Don’t worry: this is not another of those.

Maybe, like many journalists, I am so used to being called names that I mentally edit them out when I hear them. So what really struck me about the footage from Perth was not so much the abuse of reporters, but that of history, and of constitutional law.

Why was Mr Cook being asked about the 17th century Claim of Right? Well, because there are Scottish nationalists who think this 1689 document offers them some kind of “get-out" clause for the Union, a short-cut past the legal and political complexities standing in the way of independence.

There are even those who have got it in their heads that the Claim – essentially a legal justification for the Glorious Revolution north of the border – was some kind of ‘pre-nup' for Scotland’s coming 1707 ‘marriage’ with England.

This, as you have probably guessed, is white-hot historical and constitutional nonsense.

Yet it is also a genuinely fascinating development on the ragged fringes of the Yes movement. The theory sounds outlandish – because it is – but I think it is worth thinking about why it is false, where it came from and where it might lead. So please bear with.

Supporters of the idea that the Claim of Right can be used to “kill the Union” coalesce around a group called Salvo. Its banners were waved in Perth.

Adherents talk of “excavating” a secret but somehow still functioning early modern Scottish constitution and using it to achieve independence.

“Our constitution based on the Claim of Right still empowers us to reject any law, any policy or any government which goes against the expressed will of the people,” Salvo says on it website.

The group clearly anticipates academic pushback. The Establishment, it adds, “will call in learned historians and legal experts to rubbish” its claims.

This new Claim of Right theory comes in two parts.

The first first is that the Scottish people – rather than parliament – were sovereign before the Union.

This is an old Scottish exceptionalist trope, and one that has been regularly rehearsed even by mainstream pro-independence politicians over the years. The claim is – at best – contested; more aspirational, I suspect, than historical.

The second part of the theory is that this mythologised pre-Union popular sovereignty is enshrined in Scots constitutional doctrine and can be invoked to end the union.

I doubt a Herald columnist counts as the Establishment but I did call history and law experts and they did indeed, as Salvo predicted, rubbish the group’s claims.

Allan Kennedy teaches history at Dundee University. Early modern Scotland – his speciality – was no democracy, he stressed. And the Claim of Right, he added, was not – is not – a meaningful doctrine of popular sovereignty.

So what was it? This was a document dotting the i's and crossing the t’s on the formal transfer of the Scottish Crown to the protestant William and Mary from the Catholic James VII.

And it was also a defence of a contractual monarchy – that the powers of kings were not absolute – and a robust rejection of what Scotland’s Presbyterian elite back then saw as political “popery”.

Mr Kennedy was not impressed. “Trying to read a 17th century text through 21st century eyes, without understanding the ideas that animated people, the problems they were engaging with, or the different meanings they attached to apparently-familiar words,” he told me, “risks producing an understanding of the past that is not only misleading, but flatly wrong.”

Later, on Twitter, he was blunter: the historical analysis on Salvo’s website was “stratospherically stupid”.

But what of the Claim of Right today? Is it still relevant? Yes, said Aileen McHarg, professor of public law at Durham University, but not in the way Salvo suggests. The 1689 document was cited in big Brexit cases.

But Prof McHarg does not buy the Claim of Right as enshrining a doctrine of popular sovereignty, a right to rebel agains unjust rulers, in to the present day.

“It is a mistake to see Scots constitutional law as something preserved in aspic since before the Union, ready to be rediscovered and reactivated,” she said. “The Claim of Right does not purport to lay down a set of trigger conditions, or establish a procedure by which power may be taken back by ‘the people’; nor does any other legal document.”

So that is the expert take. Me? I think a small element of pro-independence opinion is constructing an alternative legal and historical reality. This is not healthy.

These “Sovereign Scots” – they use this term – remind me of various populist groups which flourished during the pandemic.

There are "Sovereign Citizens” around the world who mythologise and misrepresent landmark legal documents – such as Magna Carta in England – to place themselves and their actions above the pesky actual laws of the land.

Many nationalists, and unionists too, are frustrated by Scotland’s legal and political impasse.

Bogus law will not unstick the UK. But it might serve as an albeit flimsy justification for a new wave extra-parliamentary politics. So I think we can expect to hear more nincompoopery about the Claim of Right on our streets.