Prescriptivism, in the form popularised by the late Professor Richard Hare, does not have many advocates these days, for many excellent reasons, the most obvious being that our actions and our beliefs are logically independent.

On the other hand, how our legislators love it. And what a shining example of moral inconsistency is provided by Holyrood MSPs, who have introduced some of the most authoritarian and illiberal legislation on tobacco anywhere in the world, but have nonetheless decided to invest a sizeable chunk (almost 3%) of their pension scheme in British American Tobacco and Imperial Tobacco.

Seldom can engineers have been so obviously hoist by their own petard. Or, I'm tempted to say, put that in your pipe and smoke it. But this is what comes of prescription.

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The historian AJP Taylor pointed out that "until August 1914, a sensible, law-abiding Englishman [for which read any Briton] could pass through life and hardly notice the existence of the state". For 98 years, politicians have been doing their utmost to reverse that. Every area of human activity is now thoroughly circumscribed by regulation, legislation and – wherever possible – taxation, including, as George Orwell foresaw, which modes of thought are permissible.

You and even, for the sake of argument, I may not think that is entirely a bad thing. Unless a fully consistent anarchist or libertarian (a vanishingly small group, even among those who profess those creeds), we accept that liberty and license are not the same.

In Mr Taylor's golden age, when passports, identity papers and – for the overwhelming majority of the population – even income tax were unknown, the Government nonetheless intervened in some areas. Most were presented as beneficial, a judgment most people then and now would share. They prevented the spread of diseases and the sale of food unfit for human consumption, provided education for children up to the age of 13 and pensions for those over 70, and imposed safety rules, and some restriction on working hours and provision for sick pay, in factories.

The difficulty with these modest prescriptions, and the innumerable additions to them since, is precisely that they were devised for our own good. The reason John Stuart Mill argued that restrictions on liberty should go no further than preventing harm to others was best explained by CS Lewis when he wrote that even robber barons were better than omnipotent moral busybodies, because "those that torment us for our own good will torment us without end, for they do so with the approval of their consciences".

Yet the danger even worse than the dictatorial certainties of prohibitionists and busybodies is that such legislation – precisely because it is well-intentioned – leads to moral idiocy, a position we arrived at some decades ago. It may be admirable to argue that the state can provide education, or welfare provision, or public health measures, but as those provisions are expanded, or applied into other areas, the false assumption arises that the state, and only the state, can or should provide such public goods.

Soon some are even arguing that charitable giving, or private education or health provision, or saving your own money, are actually wrong, because these matters are the preserve of the state. The other week I heard a sermon that argued that charitable food banks shouldn't be allowed, because they ought not to be necessary. Unless one advocates full-blown nationalisation of food and its distribution, that is a morally idiotic position that wrongly transfers practical, political questions about provision of services into the realm of ethics.

But while ethics are about the beliefs and correlated actions of individuals, the notion people should be responsible for their own conduct and its consequences is unpopular among legislators.

They take Cicero's celebrated declaration "salus populi suprema lex esto" – let the public good be the Supreme Law – omitting the first word of that sentence ("ollis", which directs the instruction at the consuls, not the people) to declare certain sorts of activity not merely potentially dangerous, or offering mixed benefits, or possibly unhealthy, but morally wrong.

Many firms – oil companies, arms manufacturers, parts of the financial sector, agriculture and fisheries, the drinks industry, pharmaceutical manufacturers, toy companies, fast food restaurants, the motor industry, energy suppliers – have had to deal with regulation justified on spurious ethical grounds.

Yet there is no ethical component to these activities; not even (except perhaps for universal conscientious objectors) arms manufacturing, if you approve of the cause the arms support. The deliberate confusion of consumer choices or public health, or energy, or foreign policy with ethical issues is morally illiterate.

It has been compounded by the notion of "ethical investment", which seldom picks on industries which might logically be thought immoral, such as state-owned cartels, or the alternative health industry, or the publishers of junk science and history, but reserves its hatred for companies which are about the most popular with consumers.

The ultimate bogeyman is the tobacco industry. Yet many people enjoy its legal product, even if, as the chief executive of ASH Scotland insists, it kills one in two smokers. I can remember when that figure was one in four, then one in three, then four in 10. The anti-tobacco lobby, however, has twisted epidemiological evidence for years – indeed, some of its senior figures are on the record as boasting of doing so, pro salus populi. Should you want chapter and verse, Christopher Snowden's excellent book Velvet Glove, Iron Fist lists many of the distortions perpetrated by tobacco prohibitionists.

But even if – as I don't doubt – smoking is bad for you, life has a 100% mortality rate, and if extending it for as many years as possible was a moral imperative, the evidence suggests our models should be the Chinese and Japanese. Unfortunately, they have higher rates of smoking. Of course they also have higher rates of fish eating, and of having more Japanese and Chinese ancestors than us.

A major technical objection to philosophical prescriptivism are dilemmas; where two opposed courses of action are both obligatory. ASH says any investment in tobacco "undermines public health". But as the Scottish Government knows, tobacco taxation underwrites public health. It's a dilemma.