It appears to be impossible to have a rational debate about drugs. David Nutt, the head of the government's advisory council on drugs, argued that alcohol and tobacco were more dangerous than some drugs which are currently illegal. That point seems so obvious as to be barely worth stating. Prof Nutt also said that it was silly to upgrade cannabis, from class C on the illegal drugs register to class B. The maximum penalty for using a class B substance is five years in prison. Does anyone believe that any judge would ever pass such a sentence for smoking marijuana? So what is the point of pretending to buttress a law that is already widely flouted with even more pains and penalties which will never be enforced?

Before taking such an absurd decision, ministers might have considered the experience of the Black Code in the early 19th century. By mandating ferocious penalties for trivial offences, it brought the law into disrepute and undermined the penal justice system, until Sir Robert Peel – no softie – replaced the savage and inspissated nonsense with a sensible criminal code. But an examination of precedents would require thought, reading and a knowledge of history. Under this government, those are class A crimes.

Instead, the Professor was sacked, which has annoyed some of his colleagues, who see no point in continuing to assist a government which has no interest in reasoned debate. This does not mean that politicians must always accept expert advice. Sir Christopher Kelly and Sir Thomas Legg should be treated with much more scepticism than they are likely to receive. A minister is perfectly entitled to say that he had received some advice from Professor so-and-so, for whom he had considerable respect – and that on this occasion, he respectfully disagreed. But there is no point in asking academics to serve on a committee if their intellects are to be subjected to a three-line whip.

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Moreover, drugs policy is in urgent need of hard thinking. Our present arrangements are a mess. So let us start with fundamentals. Until the 1960s, our legal system was overshadowed by pre-libertarian theories of the state, which criminalised breaches of Christian morality and started from the assumption that governments were entitled to regulate the private behaviour of adults. As that has all gone over the past few decades, what theory of the state now permits governments to prohibit adults from taking drugs? There is only one intellectually respectable answer to that question: none.

This does not mean that those who wish to retain prohibition are bereft of arguments. Their counterblast might run along the following lines. "Intellectual respectability be damned. You are talking as if the drugs question could be resolved in an academic seminar. Go a few miles from intellectually respectable London to disintegrating London, where the wreckage of David Cameron's broken society is outward and visible, where so many forces are already at work to accelerate social breakdown – and then tell me that you would like to add to the problem by legalising drugs".

That is what many judges and policemen believe, based on their experience of trying to hold society together, and it is a powerful case. It is also a pragmatic one – none the worse for that – and as such, open to challenge on evidential grounds. The evidence does seem to suggest that the present policy is failing. Drugs are readily available, while drug-users mug and burgle to sustain their habit. In her forthcoming study of underclass youth, Harriet Sergeant depicts the allure of drug dealing: its corrupting effect on the de-socialised young. If no one who wants drugs has to go without them, while the illicit trade is worth hundreds of millions of pounds, it is hard to see why legalisation would make things worse.

There is a further point. The drug menace is not only impairing the quality of life in British cities. It is wrecking countries. Trinidad, Guyana, Jamaica: those really are broken societies. Colombia and Mexico have had dreadful difficulties. Admittedly, this arises far more from the lucrative American market than from the much smaller British one. But if we British concluded that the current war on drugs could not be won, we would be doing the world a favour.

This is how legalisation could work. Allow adults (photo ID necessary) to buy limited supplies of their chosen poison from licensed and regulated outlets. Ban all advertising. Tax the stuff as highly as is possible without creating a black market. Announce an amnesty for all drug crimes, in the hope that the skilled operators would take the chance to go legit.

Increase the penalties for illicit drug-trafficking, to include impoverishment. Anyone involved in selling drugs to children would lose all his assets, however acquired, and would not leave prison if there was any suggestion that he had some cash stashed away. Step up police operations, hoping to catch new dealers while they were still inexperienced. Employ the SAS to eliminate foreign traffickers who were trying to supply the British criminals who remained in business.

The aim of these measures would not be the promotion of universal hippydom: still less, to bring the decadence of the late Roman Empire to the streets of South London. The intention is to reduce drug-related crime and to make it easier to deal with the criminal underclass. There might also be a fall in drug consumption, especially among children, whose supplies would be significantly interrupted. That said, there would be a price.

We can surely assume that there are some young adults who might be curious about drugs, but who do not like the idea of searching out dealers in insalubrious parts of town. They are also reluctant to run the risk of being arrested. There may not be many such persons: there must be some. After legalisation, the restraints are removed. So they try the stuff, and one or two of them turn out to have addictive personalities and turn into druggies.

Although there are those who insist that anyone who might become a druggie already has, legalisation is bound to create some new addicts, whose lives might be wrecked. This is not a pleasant thought. Then again, the individuals concerned would be adults, unlike many of those who are destroyed under the current arrangements. Adults are entitled to make their own choices.

One hundred and fifty years ago, John Stuart Mill published On Liberty. The passage of time has not diminished its radicalism. Mill realised that the desire to interfere with others' freedoms has deep roots in the human psyche. If it is denied one outlet, it will find another. Fifty years ago, homosexuals were persecuted. Recently, some half-witted police force wanted to persecute a woman who complained about the excesses of homosexual demonstrators. The rights to free speech and free expression can never be taken for granted, especially under this government. It might seem absurd to cite drug-taking in the same context as those dignified, noble freedoms. But freedom is freedom.

Mill could also remind us that you do not arrive at truth by suppressing opinions, even if they are unpopular. Admittedly, this government has hardly been successful in suppressing Prof Nutt, but it is now time for the opposite approach: a Royal Commission on drugs, to review all aspects of current policy, from philosophy to policing. David Nutt should certainly be a member.