WESTMINSTER may be sovereign, but Holyrood is kept on a tighter constitutional leash. While MPs can make or unmake any law, the Scottish Parliament must respect three basic constitutional rules. If Holyrood legislation “relates to reserved matters”, it will run into trouble in the courts. Even if every MSP in the chamber voted to expel nuclear weapons from Scottish soil, for example, Trident would remain a reserved matter. Even if Holyrood passed a Bill for Scotland to remain in the EU – under the Scotland Act, this would have no legal heft.

In passing laws, our MSPs must also respect fundamental rights. Any Holyrood legislation which would legalise inhuman and degrading treatment, for example, or rob people of their property, would be null and void. The same goes for EU law. Scotland’s participation in the common European market is stitched through the Scotland Act. We must respect EU competition law. We can’t discriminate against EU nationals or European companies or their goods and services.

It is against this legal backdrop that Ken Macintosh made his statement yesterday, decreeing that key sections of the Scottish Government’s Brexit Continuity Bill falls outside Holyrood’s legislative competence. On introducing any Bill to the Scottish Parliament, the Scotland Act requires the Presiding Officer to state whether, in their view, the law falls within or outwith legislative competence. This analysis is based on independent legal advice. Ken Macintosh’s tells him that the Scottish Government’s new Bill may be problematic.

Why? What does the legislation do? Essentially, John Swinney wants to lay the groundwork for what happens if a negotiated solution fails with the UK government. His proposals would establish a devolved framework for dealing with EU law after Brexit, mirroring David Davis’ legislation which is grinding its way slowly through Westminster. But John Swinney faces a problem. The Scotland Act says he can’t act in defiance of EU law.

But with a bit of creative lawyering, the Scottish Government has devised a cunning workaround. Holyrood may not be able to footer with EU law under the current dispensation, but when the UK leaves the EU, these competencies will suddenly expand. The Bill sets out a framework for devolved trimming of EU regulations – but provides these won’t kick in till exit day.

The essence of Ken Macintosh’s objection to this is that John Swinney’s canny workaround is a bit previous. He concludes Holyrood can’t “exercise competence prior to that competence being transferred” to Edinburgh. In determining legislative competence, his advice says, we need to consider the here and now.

Here’s the nub of it: in his view, Swinney’s wheeze to postpone “the exercise of powers until a future date” doesn’t fix Holyrood’s competence problem. On the face of the Scotland Act, EU law still can’t be altered. But Ken doesn’t get the last word. As constitutional law professor Aileen McHarg noted yesterday, the purpose of the Presiding Officer’s statement “is to provide independent advice to the Parliament; it is not intended to prevent Bills being discussed by the Parliament.” Or being passed, for that matter. But the opportunities for litigation are legion. UK law officers, or a punter with money to burn, can refer the issue to the UK Supreme Court for an authoritative decision of the best constitutional interpretation.

Here, law and politics crash into one another. The Lord Advocate thinks Nicola Sturgeon’s administration at least has an arguable case the Bill is – as lawyers say – intra vires. The Speaker of the Welsh Assembly has looked favourably on a parallel proposal in the Senedd. Holyrood’s presiding officer has taken a different view.

What to make of this? The key lesson is this. Law isn’t always a big book of rules. It isn’t mathematics, with buttressing latin phrases. Two plus two, constitutionally, does not necessarily equal four. As the PO’s statement of reasons recognises, these are debatable lands, constitutionally, the issues are “complex and novel.” The Scottish Government are right to live boldly within the devolution settlement. But they – and their supporters – shouldn’t kid themselves. This week’s developments only sluices more mud into a very sticky constitutional quagmire.