IT was Benjamin Franklin who said nothing is certain in this world except death and taxes. More than two centuries later we’re still debating which is less popular. So it is a brave or desperate government that tries to ramp up taxes on the electorate. This week’s discussion paper on the subject from Nicola Sturgeon suggests the SNP administration is a bit of both.

It contains a blizzard of statistics on the levy supplying 30 per cent of Holyrood’s budget in 2018/19, but it boils down to one thing: income tax is going up. The immediate debate is about how much, and on whom the burden will fall. But there is almost no debate on how it should be spent.

Some 2.2m people currently pay the 20p basic rate between £11,500 and £43,000, another 346,000 adults pay the 40p higher rate, and 20,000 the 45p additional rate over £150,000. However 60 per cent of the revenue comes from the 10 per cent of folk paying the higher and additional rates. So while it might look obvious to squeeze the rich more, if they up sticks and take their cheques elsewhere there would be a disproportionate hit to the tax take.

To spread the pain more evenly, the paper crunches the tax plans put forward by parties at the 2016 Scottish election, filters them through four tests - protecting services, no tax rises for the low paid, fairness, and supporting the economy - and sketches out four alternative approaches to stimulate debate ahead of the Holyrood budget process. At this point, there are no hard and fast plans.

The four approaches are merely illustrative and intended to help the parties reach a consensus. As a minority, the SNP needs an ally to help it pass the budget, but this year is looking to get Labour and the LibDems on board as well as the Greens. The Tories are a lost cause.

The scenarios envisage three, four, five and six tax bands, and would raise an extra £80m to 290m.

The paper includes rates for each band, but all are subject to change. The six band model is the most progressive, and the only one to cut taxes on the poor, rather than merely freeze them. However it is also the most complex, and complexity means higher running costs. Ten of the 35 OECD countries have five-band systems, including Canada, Belgium, Norway and Spain, the paper notes, in a hint at the SNP’s thinking.

At the current personal allowance, people earning over £24,000 would typically pay more. If you earn £40,000 it would be up to £160 a year more. If you earn £60,000 up to £360 and if £120,000 up to £1610.

Launching the report, Ms Sturgeon called these “modest additional contributions”. She also defended her reputation for being “cautious” on tax. Too right, she was cautious, she said. Because any rise means money out of someone’s pocket. So how did we get here?

One school of thought is that she is going againt the SNP’s inherent caution because her party is under pressure from Jeremy Corbyn’s Labour, and she is veering left to hang on to votes. But I suspect she knows Mr Corbyn’s tax plans are but hazily understood by voters, and a public thirst for paying more tax is not the secret of his success.

Ms Sturgeon also has to respond to a Westminster spending squeeze - Holyrood’s budget is shrinking by £1bn over a decade - and the rising cost of an ageing population.

But there is another factor. What she and finance Derek Mackay called the “social contract” - the distinct Scotland-only policies introduced and protected by the SNP. The paper has a list: free university tuition fees, free personal care, free childcare, free school meals, free prescriptions, NHS eye test and concessionary travel.

But while the tax proposals are loose and flexible, the social contract is not. Its contents are inviolate. Indeed, tax rises are justified to “ensure their long term future”.

There is nothing wrong with a party wanting to defend its signature policies. But it is surely a bad sign when it has to plead with its opponents to pay for them, asking both for new ideas and for all concerned to break manifesto promises to reach a deal.

Ms Sturgeon says she wants a mature debate on tax, and that is a good thing. But she does not seem to have much appetite for debating the other side of the coin - where best to spend money in an age of austerity and uncertainty. The two ought to go hand-in-hand, but there is not much joined-up thinking on show.

At last month’s SNP conference, the First Minister announced an extra £400m a year for free childcare by 2021, something that would swallow all the extra tax money in one gulp. She also continues to stick rigidly to her plan to halve air departure tax, which would suck £190m out of the 2021 budget.

Also missing is a discussion about the tax rises floated this week not being enough to shield services from the spending squeeze, and cuts remaining inevitable. The SNP’s decision to ringfence health and police spending, means cuts to services outside these protected areas are likely to be savage.

As the Fraser of Allander Institute put it: “The notion that ‘difficult decisions’ on income tax can absolve us from difficult decisions on the spending side of the budget seems somewhat optimistic.

“We now know more about how the size of the budget envelope might be varied; but the debate on how large that budget envelope should be - and what is should fund - has barely begun.”

These are early days, of course. The tax powers now being flexed only kicked in earlier this year.

A cross-party consensus in the coming weeks may open the way to a deeper cooperation on future budgets, making more significant changes possible after that.

But for now, too many heads are in the sand for the mature debate on tax and spending we need.