‘ARGUABLE” is one of these lawyerly words which sends ordinary folk running for pitchforks. But legally “arguable” is precisely what the case for a second independence referendum without Westminster’s consent would be. Like it, dislike it – there’s no getting around it. If Holyrood passes referendum legislation without Westminster’s consent – this can only end up in court.

In the public imagination, law can sometimes seem like a big book of rules. It’s easy to assume these rules could and should be applied like mathematics – 2 + 2 = 4. But as any lawyer worth their salt will tell you, law often isn’t like that. It can be greyer. Vaguer. Arguable both ways. That’s why we have an adversarial legal process, that’s why we have appeal courts, and that’s what makes a hypothetical indyref case so difficult to call.

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So if it’s an arguable case, what are the arguments? Here, we need to go back to devolution’s founding documents. We have a reserved powers model under the Scotland Act 1998. Instead of setting out everything Holyrood can do, instead, the Act lists all the powers Holyrood can’t exercise. It’s a curious list: foreign affairs and control of nuclear weapons, treason and time, outer space and Antarctica.

Missing from this list of reserved matters are “referendums” – but right at the start of schedule 5, “the Union of the Kingdoms of Scotland and England” is identified as one of the reserved aspects of the constitution. So how are we to resolve the tension? On the one hand, referendums are devolved – but on the other, the Union is explicitly reserved. Would an independence referendum be inside or outside Holyrood’s powers?

Section 29 provides a framework for resolving this question – but no easy answers. It says that if an Act of the Scottish Parliament “relates to a reserved matter”, it is outside the legislative competence of MSPs.

In deciding whether a statute “relates to a reserved matter,” the Scotland Act says we need to consider the legislation’s “purpose”, having regard to “its effect in all the circumstances.” So what would be the “purpose and effect” of a second Scottish Referendum Act? Discuss.

After all, there are different ways you can describe this. The purpose of any referendum could be characterised as canvassing public opinion on a contested constitutional issue. In terms of the legal “effect” of such a referendum – you might point out that even a majority “yes” vote would not unilaterally dissolve the United Kingdom.

All true – but you might argue, all rather artificial. The UK Government will argue that a second referendum isn’t just a state sponsored opinion poll. Back in the real world, they’ll say, in real political terms – the “purpose and effect” of this poll is to break-up Britain and dissolve the Union. Don’t believe us? Just ask SNP politicians what they think the effect of a Yes vote would be.

But there are problems with this argument too. After all, it assumes the outcome of a second referendum would be a vote in favour of independence. It seems to suggest the legislative competence of a second poll should be assessed solely on the basis of one of the two potential outcomes.

It isn’t obvious that legislation authorising a second poll should be read in such a way. To illustrate the point, take an example from BBC Scotland’s ongoing indyref documentary – broadcast on Tuesday night. The documentary revealed that in 2007, arch-unionist Jim Murphy tried to persuade Gordon Brown that Westminster should hold an early referendum on independence to shoot the SNP fox. Brown was unpersuaded. The early poll never happened. But it would stretch credulity to suggest the “purpose and effect” of Jim Murphy’s plan to hold an independence referendum was the break-up of Britain.

As Brexit vividly illustrated, referendums can have results which surprise the people who organise them. The same goes for court cases in the murky regions of constitutional law.