MANY people in Scotland today are supportive of the idea of a written constitution. In the words of the late Professor Neil MacCormick, lead author of the SNP’s 2002 draft constitution, a written constitution incorporating justiciable human rights is “part of the common stock of democratic thought in Scotland today” and integral to the SNP’s strategic aim of transforming Scotland into a “normal European democracy”.

Unlimited parliamentary sovereignty always sat awkwardly with a people whose national culture has since the Reformation been shaped by principles and practices of strictly constitutional government in the ecclesiastical sphere. If the Barrier Act could constrain a General Assembly, why could not a constitution, in the same way, constrain parliament?

In both cases, the purpose of such constitutional constraint is to prevent the current majority in representative bodies from making unilateral, over-hasty, and perhaps irrevocable, change to the fundamentals. It is reckless not to have this in place.

Scotland might have put up these theoretical absurdities, were it not for the practical consequences of being on the receiving end of unconstrained parliamentary sovereignty.

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The devolution settlement, which was an attempt to establish a form of quasi-federalism despite the absence of a written constitution, was always based on a mere convention (the so-called Sewel Convention), which was that Westminster would not normally legislate on devolved matters without the consent of the Scottish Parliament.

That convention was incorporated into statute in 2016, but it has now been set aside and parliamentary sovereignty means there is nothing we can do about it. The saying that “power devolved is power retained” has always been true, but it was never so evident.

In a system based on self-restraint and moderation, where institutional propriety was maintained by the frown and the tut, the shameless way in which this UK Government treats Scotland is dangerously destabilising.

If the United Kingdom had a proper federal constitution – if Scotland had the position of a Canadian province, or even an Indian state – none of this would be possible. Whatever the extent of powers conferred upon Scotland by that federal constitution, the status of those powers would be secure; only a constitutional amendment, not the unilateral imposition of a Tory majority, could change the arrangement.

So long as the basic problem of parliamentary sovereignty persists, no amount of “Devo Max” can guarantee Scotland’s autonomy, nor reassure us that powers given will not be taken back. Without a written constitution, everything hangs on politicians’ promises.

We see the same thing with the Human Rights Act. This act was fatally and deliberately weakened by the desire to preserve parliamentary sovereignty intact. It falls short of the enforceable human rights provisions found, for example, in most other Commonwealth countries.

Nevertheless, it was an important step forward, which is now under threat. As long as parliamentary sovereignty persists, we have no guarantee, no reassurance worth counting on, that our rights will not be violated. What they say is “trust us”. Well, I don’t.

All the theoretical arguments against written constitutions fail when exposed to this simple reality: that without constrains upon the power of the incumbent majority, all our rights – our right to vote, our right to regular elections, our right to democracy itself – are upheld only by political promises and by the self-restraint of the ruling majority currently in power.

Conservative academic lawyers might wring hands over the power of judges under a written constitution. They might point to the extreme politicisation of the US Supreme Court as a salutary warning. It is indeed a warning – not against written constitutions as such, but against a bizarre judicial appointments process that is not replicated elsewhere. Most Commonwealth constitutions have a nonpartisan commission to advise on judicial appointments. A similar commission has been proposed in every recent draft constitution, both for Scotland and the UK.

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Besides, it is not the judges who threaten your liberty, or seek to undermine devolution, or to strip us of our rights. The danger lies in populist government that is willing to breach international law, demonise immigrants, and trample roughshod over both parliaments.

For many in the Scottish Government, if not in the wider pro-independence movement, the constitution is seen as a problem. They fear that constitution, which would have to wrestle with certain issues – like church-state relations and the role of the Crown – could divide their base. Independence first, is the motto. Let’s sort the constitution out afterwards.

This is a grave mistake. Those broadly sympathetic to the SNP might trust those currently in power not to abuse it. But to the moderate Unionist or wavering voter, who might not trust Holyrood or the SNP any more than Westminster and the Tories, there is a pressing need for the reassurance and guarantees that only a written constitution can offer.

Far from being a political problem, an unfeigned, enthusiastic and detailed commitment to a written constitution, if properly presented, may be the secret to unlocking the votes of the “cautious centre”.

Robin McAlpine is the next guest on the TNT show, at 7pm on Wednesday