A PRIMARY function of a constitution is to define, delineate and regulate the institutional relationships through which democracy operates.

These include the relationships between the head of state and the prime minister; between the prime minister and the cabinet; between ministers and parliament; within parliament, between government and opposition, and, if there are two houses, between the upper and the lower house; between the central government and other levels of government; and between political actors and the permanent, independent, non-partisan institutions of the state, such as the judiciary, the civil service, the police and the military.

As regular readers will be aware, in the United Kingdom – which does not have a constitution in the sense understood across most of the rest of the world – these relationships are governed by convention, practice or tradition.

The problem is not just that these unwritten rules cannot be enforced, and are binding only so long as the political actors involved regard themselves as morally or expediently bound by them. It is also that the substance, or very existence, of these rules is contested.

Some of these relationships are regulated by statute. This is better than mere convention – at least the rules are declared and written down. However, ordinary laws can be amended by ordinary majorities, and therefore offer no protection against abuse of power of the manipulation of rules by incumbent majorities.

That might be acceptable, in practice if not in theory, when the people involved are good chaps with functioning moral compasses, a strong commitment to institutional propriety, and a sense of responsibility for their actions. Under current circumstances, it is madness to allow the dominant players to make up the rules as they go along.

PARLIAMENT sovereignty distils into the absolutism of a Government with a well whipped majority. All public institutions, the rules regulating them, and the relationships between them, as well all our rights as citizens, ultimately depend upon nothing more substantial than the self-restraint, the moderation and the goodwill of Boris Johnson and his knotty little crew.

But it shall not be so among Scots. The SNP has long promised that an independent Scotland will be founded upon a written constitution. If we are to tackle the problem at source, and are to prevent old ills from resurfacing in a new country, we hold them to that promise.

Scottish independence has never been about nation-building. The Scottish nation already exists. It has always been about state-building. If we want to build a better state, it must be placed upon a secure, stable, robust and legitimate constitutional foundation. Public power must be institutionalised, not personalised. The ground rules of a functioning democracy must be sensible, clear, enforceable and put beyond the reach of ordinary majorities.

The best – as well as the easiest – course of action would be to base the mechanics of a future Scottish Constitution quite closely upon existing rules. For the most part, this would be a matter of consolidating and codifying existing provisions – from the Scotland Acts to the European Convention on Human Rights – into the form of a proper written constitution.

Where existing provisions are insufficient, selective borrowing from other Westminster Model constitutions would be the safest and least controversial option. There are many Commonwealth examples of how to recognise the leader of the opposition, and how to constitutionally establish an independent electoral commission, a boundaries commission and so on. Ireland provides a near-to-home example of how to constitutionalise the parliamentary control of war-making and treaty-making powers.

Consistent with the need for stability and reassurance, the overriding concern should be to protect and conserve Scottish parliamentary democracy, and perhaps improve it in matters of detail, not to uproot it all. It should be possible to reach broad cross-party agreements on at least the bare bones of such a constitution.

THERE are other relationships, however, where scope exists for more innovative and bespoke constitutional provisions: in the relationship between private interests and public duties; between the sacred and the secular; between money and politics; between Central Belt, Highlands and Borders; between rich and poor; between the people and the land; between humanity and nature; and between past, present and future generations. In Scotland, many of these relationships have been deeply scarred and are in need of restoration.

A constitution cannot be, and should not try to be, a programme for government. The proper place for substantive policy commitments is in an election manifesto, not a constitution. Nevertheless, there may be certain provisions that are of sufficient importance, and command sufficiently widespread support, to justify being included.

The SNP’s 2002 draft constitution, for example, included rights to “conditions of work which are fair and which respect the dignity of the person” and rights to social security, housing, healthcare and education, as well as a right of access to “hills, mountains, waterways and open countryside”, while the Scottish Government’s 2014 draft interim constitution included provisions promoting a healthy environment, biodiversity and measures to tackle climate change, as well as a requirement for the sustainable use of natural resources.

None of this is necessarily prescriptive, but it does indicate the direction in which we should be thinking.

This column welcomes questions from readers

Ian Blackford MP, leader of the SNP Group at Westminster, is the guest on the TNT show at 7pm on Wednesday