It is a basic rule in politics that a promise made without a price tag attached is no promise at all.
Parties putting forward election manifestos must show that they have worked out the detail of a policy; that they have established a figure for how much delivery of that detail would cost; and that they have a plan for paying for the policy. Those parties which fail this basic test of credibility find themselves accused of making promises they can't then afford to keep or of storing up big tax rises they won't tell us about.
What is true of the process of winning permission to govern for a few years is even more so when seeking permission to dissolve a 300 year-old union. The nationalists risk further-undermining the already-thin credibility of their case by failing to impose the kind of basic spending-discipline expected in any election campaign.
Take the example this week of shipbuilding. Shipyard workers at the three yards which build for the Royal Navy have been saying for months that their jobs depend on being part of the UK. Their argument is incontrovertible: you can't sustain the same number of yards and jobs building ships for a small nation like Scotland as you can building ships for a country 10 times the size. The numbers just don't add-up.
In a BBC interview on Friday, Deputy First Minister Nicola Sturgeon stuck to their line: that the workers are wrong and Scotland would continue to build warships for the UK despite leaving the UK.
However by the start of this week they did not seem so sure. The yards would be kept open, not through UK contracts, but by building frigates for a separate Scottish Navy. So claimed the Minister working on SNP defence plans, Keith Brown.
Presumably these ships are needed to enforce Alex Salmond's threatened blockade of the North Sea against non-existent Spanish trawlers.
In any event, the Minister claimed a defence budget of £2.5billion would be used to keep the yards open. To put that £2.5billion budget in context: the type 45 destroyers that sustained jobs at the Clyde yards for years cost around £1billion each to build. Even the more modest estimates for the 13 new Type 26 frigates has them costing £250-£350million each. Little wonder the promise that the SNP Government would keep the yards open were dismissed as "complete fantasy" by the workers who build frigates for a living.
This make-it-up-as-they-go-along approach to policy making is far from a one-off. Policy promises which are completely lacking in detail and cost are coming thick and fast from the Scottish Government.
A few days ago John Swinney pledged increases in state pensions. Then Alex Salmond pledged a job or training for every unemployed young person as a right. On Sunday Children's Minister Aileen Campbell pledged an extension of free nursery education or childcare to all one and two year olds.
Few would argue with the intention behind the promise. But for any of these promises to be credible they needed both policy and price-tag. Neither was presented in each case.
The only detailed or costed independence policy we have from the nationalists is for a 3% undercutting of the UK corporation tax rate. A Reaganomics race-to-the-bottom in the taxes paid by companies like Google, Starbucks or Amazon. Ironically, by taking even more money out of the budget, this policy simply serves to make the challenge of paying for other policy promises even more difficult.
I know from own time working in Government that policymaking is frustratingly hard. Change is difficult. Most policy is complex. In the few examples where policy is simple tend to be expensive. Like diets that promise you can eat as much as you like and still lose weight, the nationalist promise of effort-free and cost-free change is too good to be true.