Years ago, a well-known journalist of my acquaintance told me how, after he was arrested for being drunk and disorderly, he had subsequently been released on the grounds that he was “known but not wanted”.

How we laughed!

But turning a blind eye while trying to keep things in perspective isn’t always the answer. It is one thing to let a tired and emotional reporter out of police custody on the basis that he is not a danger to society. Most of us would agree that the punishment in this case – a night in the cells – and the offence – ­ unseemly behaviour in the streets of London following an altercation – were more or less in balance, with no realistic threat to life and limb. It is quite another to conclude that a religious extremist, who may very possibly be planning to murder his fellow citizens, should be left to his own devices – in this case literally – because he has not thus far committed the offence.

This is what happened in Paris at the weekend – and not for the first time. The Islamist looney who started knifing people at random in the Place d’Opéra was known to the police and suspected of being up to no good. He was on their “watch list”. But because he hadn’t actually murdered anyone yet, he was left to continue on his sadly un-merry way.

It happens here, too. How often do we hear after a terrorist outrage that the perpetrator was “known” to the authorities? Knowledge is supposed to be power (though don’t believe that for a second). When dealing with likely or potential killers, the opposite is often true. In all too many cases, knowledge means backing off.

Any yet, and yet … few of us, I imagine, would welcome the emergence of a police state, in which stop-and-search and “sus” laws became normalised, so that anyone thought by the police to be no better than they ought to be were locked up at Her Majesty’s pleasure. On what charge? And for how long? Would internment without trial sort out the wheat from the chaff in a way that would satisfy most of us? Is a judicious tweak of police powers all that is necessary to keep justice on the right side of the moral balance sheet? Come to that, would it help if habeas corpus stepped back a tad or two so that it doesn’t get in the way of putting the bad guys out of circulation until a judge or a police inspector rules that they no longer pose a plausible threat?

Probably. Except that … hmmm …

I don’t have the answer to any of this – though I think I know what Orwell and Kafka would say.

The Tom Cruise movie Minority Report, a piece of sci-fi nonsense released the year after 9/11, turns on the ability of some semi-human creatures to correctly identify crimes and criminals before the offences are carried out. Cruise, a member of the PreCrime unit, spends the first half of the action tracking down and arresting people who will in future commit murder or rob a bank, only to discover (who would have thought it?) that it is not quite as simple as that. What about free will versus determinism? If everything is set in stone, what choice do we have? And what if a person, once alerted to their future, adjusts their behaviour so that the crime is no longer committed? What, in that case, are they in prison for?

But you see the problem.

At street-level, where the police sometimes still operate, there will always be individuals and groups out there who, if left unmolested, will carry out the most dreadful crimes, including rape and murder. In many cases, the officers concerned know exactly who these people are, so that they are not in the least surprised when events pan out as forecast.

What should they have done? If the authorities in Paris had previously picked up and held the man suspected of last weekend’s knife attacks, the horror he unleashed would have been prevented. The same applies in the UK, where it has become almost routine for MI5 and the Met to tell us after a terrorist incident that those responsible were already on file, making them, post-crime, that much easier to process.

At a more visceral level, in the U.S., we see what happens when the police confront citizens, usually black men, who it is thought are pre-guilty by virtue of their skin colour. They get shot on an almost daily basis, and the excuse, invariably, is that the officer felt his life to be in danger, regardless of the facts as they subsequently emerge.

I don’t blame the French or British intelligence agencies or police for their reticence to act. I don’t even blame the relevant justice departments – though I might still, on occasion, blame the lawyers. But there is a conundrum here that needs urgently to be resolved. People’s lives are at stake, sometimes numbered in the hundreds. Freedom is fundamental in western society. None of us wants to take up residence in “1984”, and the twenty-first century should not become Brave New World. Waterboarding and rubber truncheons must not become an acceptable accompaniment to questioning a suspect down your local nick. At the same time, we want our streets to be safe, not only for us, but for our families and friends and everyone else just trying to get on with their lives.

If the police honestly believe that someone they know is bent on murder and mayhem, don’t they have a duty to bring that person in for questioning, with a trained psychologist sitting alongside the good and bad cops? Shouldn’t there be a process that then keeps those they judge to be psychopaths out of circulation, at least until they can be further assessed? Isn’t that the least we should expect?

Internment didn’t solve the Northern Ireland problem. Guantanamo Bay has become a source of lasting shame to successive U.S. administrations. And any system that allows people to be deprived of their liberty on the word of someone in uniform or working undercover for the state is clearly open to abuse. But letting known madmen and villains walk the streets until they are ready, sometimes literally, to explode doesn’t exactly fill the rest of us with confidence. We, too, have our rights, not least the right to live.

Tomorrow (Tuesday), the Queen is scheduled to have tea and scones with President Erdogan of Turkey. She is an unelected democrat. He is an elected autocrat who has locked up thousands of his citizens because he suspects them of plotting against him. I know which one I would trust to do the right thing in a crisis. With this in mind, is it time for a royal commission?