WHAT is the primary purpose of, and thus the moral justification for, a government? This is a question that since the time of Plato has occupied great minds. And though I was not as attentive as I ought to have been during the late Dudley Knowles’s lectures, I cannot remember him saying that any of them concluded that it was to prohibit free poppadoms and prawn crackers with a carry-out meal.

Or not, as the Scottish Government has over-ruled that bit of the consultation paper on its strategy to tackle unhealthy eating and drinking. The paper said that “targeted” foods (including poppadoms and prawn crackers) should be banned from promotions such as multi-buy deals and as add-ons to main meals. After a lot of people thought this a ridiculous idea, a government spokesman declared they would be classed as part of a main meal, and thus exempt from the proposals.

Perhaps the supposed health impact of fatty, salty or sugary foods is immediately nullified when they are classed as an intrinsic part of a meal, while they are stuffed full of calories and poison when wolfed down as a snack before the main event. Or perhaps the Government is making it up as it goes along, and performing an instant U-turn when people point out that its proposals are illogical, tyrannical and impertinent.

You may remember that one of David Cameron’s first acts was to set up a “nudge unit” (it was officially called the Behavioural Insights Team), which was given National Lottery money to persuade us to insulate our lofts and pay our vehicle excise duty by direct debit. I can only assume that Nicola Sturgeon has secretly set up some kind of bloody great shove unit, since the chief interest of the Holyrood Parliament seems to be to tell us what to do.

Standards of education may be falling, hospital waiting lists growing, pay and productivity stagnating and public spending going up but Ms Sturgeon and her sidekicks, such as Joe Fitzpatrick, the Minister for Public Health, Sport and Wellbeing, are convinced of the need for “decisive action” on crisps and ginger.

“By restricting at the point of purchase the promotion and marketing of foods that have next to no nutritional value, it is reasonable to expect less of these foods will be purchased, improving over time our diet-related health,” he explained. Though Mr Fitzpatrick may think it reasonable, the facts don’t bear him out.

There are few areas as contentious, as riddled with contradictions, and as dominated by special interests as the debate on public health. Those special interests are not merely the drinks companies, tobacco manufacturers, “Big Pharma” and the food industry, but nutritionists, epidemiologists and activists, usually funded by the public purse, who have issued conflicting and contradictory advice. From the 1930s, nutritionists, led by Lord Boyd Orr, argued that people should eat more milk, eggs, butter and meat, and cut down on potatoes and bread; in the 1980s, they argued that dairy and other fatty foods were the problem, and that people should eat more grains and cereals; nowadays, the primary enemies are sugar, salt and bacon.

The reality is that studies that purport to show, for example, that bacon causes cancer, and which lead to hysterical headlines, usually have only very minor indicators. But they are seized upon as “targets for action”. What’s more, the evidence is ignored if it’s not helpful: the constantly changing drinking guidelines, for example, are based on data that shows that those who have one or two drinks (but no more) every day are healthier than non-drinkers. Yet to acknowledge that would undermine the general message that overall alcohol consumption should be lowered – even though it is, in historical terms, at one of its lowest levels ever.

Even if scientists could agree on the perfect diet (they mostly do, but only in the vaguest terms, and only as a generalisation for the population as a whole) there remains the problem that it works only if people follow it. This is where the activists and politicians, crying with Cicero salus populi suprema lex esto (let the welfare of the people be the supreme law), jump in.

They take contradictory and half-understood dietary science and impose it with contradictory and half-understood methods for changing people’s behaviour. The quantity of salt and sugar in today’s processed foods, for example, is in part because of the reduction in their fat content which governments previously imposed on manufacturers. Many of such tactics have been useless or counter-productive. The recent change to the recipe for Coco-Pops, for example, was designed to cut down the sugar content, to fight the “obesity epidemic”. The new recipe has 30 per cent less sugar, but is one calorie lower than the old one, which you may not regard as a stunning victory in the war against fat.

Anti-smoking activists, ignoring the fact that vaping is the single biggest contributor to the recent reduction in smoking, want it banned, on no scientific evidence. Meanwhile, they remain convinced of the effectiveness of plain packaging, although smoking increased in Australia after the measure was brought in there.

A round table discussion conducted by Scottish Grocer found that minimum unit pricing for alcohol had not reduced, but in many cases actually increased, sales. “Gin sales are up 90 per cent year on year,” said Brian Eagle-Brown of The Retail Data Partnership (TRDP).

The sugary drinks tax isn’t doing any better. James Loker, also from TRDP, said that “essentially sugary drinks are doing as well as they always have, and in some cases actually better”. The only drinks where sales were down, he concluded, were those, such as Irn Bru and Ribena, that had changed their recipes to include less sugar.

On the whole, people know what they should do: eat a balanced diet, take more exercise (walking more is the number one marker of continued good health), don’t smoke and don’t drink too much. But if they won’t, the Government is on a hiding to nothing trying to make them. Even if it improved a few people’s health, it would not justify the imposition on everyone else.