A MANIFESTO, going by the name, ought to be a list of things easily seen, clearly understood, or otherwise obvious. For movements in fine art, film-making or literature, that means general principles, rather than a detailed description of forthcoming work; but that hasn’t been fashionable in politics since the Communist Manifesto’s stirring but vague: “Workers of the world, unite!”

Nowadays, it’s the other way around. The manifesto, for modern political parties, resembles a catalogue of things they would like to see. So, to take a random example, converting one million homes from gas to zero emissions energy by 2030, one item in the Scottish Liberal Democrat manifesto announced yesterday.

This may be less rhetorically memorable than Marx and Engels, but it’s more specific. The voters are therefore entitled to regard it, and the offerings of the other parties, as a shopping list and, like all shopping lists, assess whether they would like those particular items, and ask how much they’ll cost.

What’s more, cynical though it may seem, since no political party of any sort ever elected anywhere has delivered every single manifesto promise – it would be fairer to the parties, and more honest, to call them “ambitions” – voters are entitled to take a sceptical approach when considering whether proposed measures are a) desirable; b) affordable; c) likely to happen.

All parties offer policies they think will gain them support: when it comes to something like Brexit or independence, or even issues such as voting reform or nationalisation, voters may already have firm views on which they favour, or be open to persuasion. Party positions are likely to be clear.

When it comes to spending priorities, manifesto claims are more likely to be both blatantly populist and challenged by political opponents on the grounds of affordability and credibility. The more of them there are, the easier it is to present them as unrealistic.

The SNP’s proposals – which supporters will no doubt characterise as ambitious and offering real improvements – are certainly wide-ranging enough to have been greeted with incredulity by opposition parties. To promise a 20 per cent rise in NHS spending over the parliament, free dentistry, pilot schemes on universal basic income and a four-day week, free laptops and bicycles for schoolchildren, increased education, mental health and other public spending, while at the same time ruling out any rise in income tax, is to invite the question of where the money is coming from.

Other parties will have similarly ambitious and superficially attractive policies that deserve equally close scrutiny: the Tories, as well as spending with abandon across the UK, are promising 3,000 more teachers and extravagant transport projects; Labour aims to guarantee a job for every young Scot, as do the LibDems; the Greens want to spend £7.5 billion and create 100,000 jobs. Any or all of these plans may well be judged undesirable, costly or unrealistic.

Nor are they voters’ sole criteria: the drubbing Labour received at the last General Election may have been due to people thinking its manifesto expensive and fanciful, but other factors, such as Brexit and Jeremy Corbyn’s personal qualities, may have been more important. Individual members of the electorate may prioritise independence or the Union, a party’s general stance on, for example, the economy, or their judgment of its leaders’ competence and integrity.

But the fact that the SNP has been in government for 14 years is bound to place its manifesto plans under closer scrutiny. Any party’s record in government should be a consideration: has it delivered on previous promises? Have matters, in any given sphere, improved or deteriorated?

According to the Institute for Fiscal Studies, Holyrood is already spending £1.30 for every £1 of public spending in England and Wales, almost all of it provided by the Barnett formula. If a still more expansive and expensive range of policies is now desirable and possible, why haven’t they already been introduced; and if there is a good reason why, what has changed? Voters may not decide on these issues alone, but they deserve to hear an honest assessment of them.