THE British “constitution”, if such a thing can be said to exist, consists of the law of the constitution and the conventions of the constitution. The law of the constitution is to be found in Acts of Parliament and in judicial decisions. The conventions of the constitution are to be found in the traditional body of political practice that determines the acceptable use of public power.

Consider the analogy of how bills become Acts of Parliament. As a matter of law, no Act of Parliament can be enacted without royal assent. As a matter of convention, however, the King is normally required to give assent to bills which have been passed by both Houses of Parliament.

There is some debate about whether, in ­certain circumstances, assent may be ­withheld on ­ministerial advice, but these are rare ­exceptions, and in fact, no such exception has occurred in the United Kingdom (although it has arisen, on technicalities, in some ­Commonwealth ­countries).

So, by law, royal assent is necessary, but by convention, it is compulsory. What looks like a power of the monarchy actually becomes a duty. To refuse to do that duty, by unilaterally ­withholding assent, would be unconstitutional.

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It is here that we run into difficulties because the convention is ultimately only enforced by the good sense and good will of the Palace. This is, of course, an inadequate arrangement.

It would be better to codify both the law and the conventions of the constitution into a ­proper written constitution that is clear, ­legitimate and enforceable. The Constitution of Malta, for ­example, requires the Head of State to grant ­assent “without delay”. In Malaysia, the ­constitution is even stricter: it requires the Head of State to grant assent to bills within 30 days, and if he does not do so, the bill is enacted anyway at the end of that period.

Until we get such a proper Scottish constitution, however, this unwieldy fragile amalgam of law and convention is all there is. We need to work within it and work it to our advantage.

So, back to independence. As a matter of constitutional law, it has always been accepted that to be lawful, Scottish independence must be ­approved by an Act of the Westminster ­Parliament.

As a matter of constitutional convention, however, Westminster has a duty to grant ­independence when Scotland so requests.

This gets to the heart of the conventional ­understanding of the nature of the Union, on both sides of the border. It has long been held that the Union is voluntary and consensual and that if Scotland indicated a desire to do so, the Union could be brought to an agreed end.

Westminster has passed dozens of such ­Independence Acts over the past century. This was the standard procedure in many ­countries becoming independent from British rule: there was no need for a referendum, just the ­election of a pro-independence majority to the ­legislature and a clear indication that there was a mandate for independence.

The current British Government, in an ­unwelcome deviation from established ­traditions of Unionism, is trying to change that. We cannot let them. We must do all we can to insist upon and defend our conventional, if not our legal, rights.

We cannot become independent without Westminster’s say-so, fine. But equally, they have an obligation, by convention, to say so when we say so.

The question arises: when does that power to grant independence become a duty? When must a Scottish Independence Act (or similar) be enacted?

We can argue about what demonstrates a clear mandate for independence. But the ­argument must be about the modalities, not the principle. The principle is set out in the Claim of Right: the sovereign right of the people of Scotland to determine the form of government best suited to our needs.

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The Claim of Right is not law, but it is a good statement of the convention of democratic ­consent at the heart of the Union. Cling to it. Proclaim it. Insist upon it. Refuse to engage with the British Government or with Scottish Unionists on any other basis.

In the few cases where the British ­Government failed to honour a clear mandate for ­independence, the situation escalated. ­Tempers frayed and relationships soured.

Such escalation is not in anyone’s ­interest. ­Independence has always been a ­democratic, constitutionalist, peaceful, civic cause, ­appealing to moderation and good sense. The worst outrage was for Jim Murphy to get egged. The movement is peaceful precisely because the basis of the Union is democratic and ­consensual. We should all want to keep it that way.

So, keep the heid! There is nothing the UK Government wants more than to divide and ­inflame the Scottish independence ­movement. The unity, cohesion and discipline of the ­movement must be maintained. Unionists are still political opponents, not enemies. They must be persuaded, not defeated.

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