Now hear this: once upon a time, a thoughtful doctor discovered that cholera was spread by filthy drinking water. This was usually lurking in contaminated wells.

At the time, the middle of the Victorian age, that deadly disease killed tens of thousands of people in this country. The clever doctor, whose name was John Snow, had long suspected that the disease could be spread through water.

In 1854, he showed by on-the-spot research that a severe outbreak of cholera in London’s Soho was directly linked to a particular pump. He chained up its handle. The epidemic faded away. Now here is the amazing bit: it then took nearly 30 years before the establishment admitted he was right.

The official view was that cholera was spread by ‘miasma’, through foul air. In Oxford, one college built high walls on either side of an especially smelly stream to keep the cholera from escaping.

They did this round about the same time that John Snow was taking the handle off that poisoned pump. I wonder how many people fell ill and died, thanks to the long delay in admitting the truth?

What has all this to do with modern Britain? I do not claim to have the courage or the scientific skill of John Snow. But for some years now, my friend Ross Grainger and I have been noting cases, here and abroad, where violent crimes have been committed by users of the drug marijuana.

Ross looked at wild, savage, pointless acts committed by petty criminals, or similar. I looked into the use of marijuana by the killers of (for example) Fusilier Lee Rigby and murders by the Bataclan and Charlie Hebdo killers in Paris, and by others in North Africa, Nice, the USA, Canada and Belgium.

In 2022 I made a written submission to the House of Commons select committee on this subject. Mr Grainger and I also provided detailed accounts of the evidence to Professor Dame Carol Black, when she was engaged on an inquiry into drugs and violence. If any of what we said made any difference, we do not know. Nobody seemed to be terribly interested.

The first stage of this argument, that marijuana is linked with mental illness, is now more or less established. This is thanks to the work of the eminent psychiatrist Professor Sir Robin Murray.

He said in 2015: ‘It is now well known that use of cannabis increases the risk of psychosis.’

He noted that doubters claim that the drug is not an important cause of mental illness. But his view was that we could prevent almost a quarter of psychosis cases if no one smoked high potency cannabis.

This was very much like the moment in 1950 when the Oxford scientist Sir Richard Doll concluded that there was a link between smoking and lung cancer.

In both cases there has been powerful resistance. Big Tobacco fought like a tiger to save itself. It claimed the link was unproven, it prevented bans on advertising and sale, long after it knew it was selling a horrible poison.

Marijuana too has a big lobby in the Western world.

Some of it comes from businessmen who hope to legalise the drug and make billions from it. I know from experience how utterly cynical they are. Some of it comes from the large number of people in our establishment who take marijuana, or used to, or let their children take it now.

Then there are those who really, really, want to believe that organised Islamic terrorism reaches into every corner of Britain and requires us to abandon our liberty to cope with it. And so, again and again, we ignore the obvious explanation for mad killings, such as the Calocane case where the culprit’s home ‘stank of weed’, and the Southport killings where the pattern of the murderer’s life completely fitted the horribly common one of the schoolchild who goes incurably crazy after starting on cannabis at the age of 11.

My opponents pretend to think I am trying to make excuses for these maniacs and killers when I point this out. This the most awful bilge. On the contrary. What I want, and what would save countless lives, is for the police to arrest, for the CPS to prosecute and for the courts to punish severely the undoubted crime of marijuana possession. Then, after a while, these killings would just stop.

You will not cure or control a disease if you do not know how it spreads or what it is.

Sour, bitter joke amid the misery 

What a miserable world is portrayed in The Cage, the new BBC drama series starring Sheridan Smith. Every family is in ruins. Everyone is hopelessly in debt. Evil rules the world unchallenged.

It is a post-Christian horror, and if it is true to life we might wonder if it was so clever to dump our national religion so completely.

But there was one very good, bitter joke, showing that every soppy attempt to give bad or sad things nice names will fail.

A crime syndicate is trying to recruit a far-from-clever young woman to take part in money-laundering. She cannot grasp the simple thing she is being asked to do. Eventually, the crime boss explodes in exasperation: ‘What is she? Special needs?’

Well, I laughed.

If I were the King, I would feel thoroughly used by President Donald Trump. Not only was Mr Trump able to bask in the golden glow of real monarchy before his voters, he got away with two outrageous claims – one that Charles supported his mad policy on Iran, and the other that the King disagreed with his own Prime Minister on the Iran war. If either of these things is true, His Majesty should have kept his mouth shut. 

He should stay out of such matters, especially abroad. If untrue, he cannot deny it. This is damaging. He should never have gone on the trip.