IT is now broadly accepted that an independent Scotland should be founded upon a written constitution as its supreme and fundamental law. This has been SNP policy for a long time. It is widely endorsed by other parts of the Yes movement and by the general public.

Despite this, the Scottish Government and the SNP leadership refuse to be drawn on the specifics. After 14 years in office, no progress has been made. They have still not committed to developing a draft constitution before the next independence referendum.

I would suggest three reasons for this lack of urgency. The first is that they see a written constitution as “nice to have”, not an essential foundation. They think that Scotland could rumble along fine without one for a while, just as the UK (until recently, at least) has done.

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This is rooted in a misunderstanding about what a constitution is and does. A constitution is not just an opportunity to make things better. It is, primordially, intended to stop things getting worse. The constitution is not a go-faster stripe to be stuck on the side; it is where the rubber of statehood meets the road. Get it wrong and we go skidding into the ditch.

The independence movement has plenty of high-minded idealism. It needs more hard-headed realism. New states are extremely vulnerable. They have to move fast to ensure their institutional stability and legitimacy. Delay at that point is fatal. We do not want to have to do constitution-building at the same time as adopting a new currency, creating an army and opening embassies. We do not want investors, or potential allies, wondering what sort of state Scotland is going to be. We do not want others imposing terms on us in return for recognition. We need to get these things nailed down beforehand.

Threats are not only external. I understand the SNP’s desire to be unfettered during the transition process, but having such a constitutionally unanchored state is dangerous. In many new states, national independence leaders have quickly turned into corrupt despots.

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Perhaps we think we are better than that. Maybe so, but the Unionists do not. Fear of an “SNP one party state”, with “Alex Salmond’s head on the coins” was a jibe that could be powerfully deployed by the No campaign in 2014. People thought voting Yes was voting for the SNP to be in power forever. A robust constitutional commitment disarms that entirely.

This brings us to the second reason: the SNP leadership see the constitution as an electoral risk. It could divide support. They would rather keep quiet, so that the tough questions can be kicked down the road.

Aside from being cowardly and dishonest, this strategy of silence will backfire. To hold a referendum, without having agreed how the new state is to be organised, the principles on which it will be based, or how civil liberties and democratic standards are to be guaranteed, is asking people to sign a blank cheque. Why would people fall for that?

Thirdly, there is uncertainty about the process. The core issue is inclusion: How can a constitution be drafted for an independent Scotland, in an inclusive, consensual and democratically legitimate way, when half the population opposes independence?

The water has been muddied by amateurs prattling about blockchain constitutions, “crowdsourcing” and other nonsense. Let’s skip the gimmicks and stick to what works.

A statutory foundation is important. The first step is for the Scottish Parliament to pass a Constitutional Transition Act (CTA), setting out the timing and sequencing of the process.

The CTA would establish a Constitutional Convention and define its composition. It should, to the greatest extent possible, be reflective of Scottish society. The Scottish Constitutional Convention, which designed devolution, is an instructive example. The Conservatives were never part of it. The SNP walked out. Even so, the presence of all pro-devolution parties and civil society groups enabled a ‘sufficient consensus’ to be reached.

The CTA should also prescribe the Constitutional Convention’s terms of reference, setting out certain democratic principles that the new constitution would have to embody.

The CTA should make provision for an effective secretariat and expert advisory group to support the Constitution Convention’s work. It should require the Constitutional Convention to conduct public consultations. It should provide for the draft to be subject to approval by the Scottish Parliament, before being put to the people in the independence referendum.

If passed, there would be a mandate for independence on those specific constitutional terms. The new constitution would then come into effect on independence day.

The new constitution, while it has to be robust and complete, does not have to be final. There must be provision for periodic review (for example, by a Citizens’ Assembly) and subsequent amendment after independence.

We can and should start now. There is no excuse for delay.

Julie Hepburn of the Social justice and Fairness Commission is Wednesday’s guest on the TNT show live at 7pm