Usually, before someone breaks the law, they have to think the law is looking the other way. They assess their chances of being caught, the likely benefits they'll reap, and whether their victim will notice.
So with car insurance scams an innocent driver gets rammed from behind by a fraudster who reckons no witness will refute him, goes on to claim compensation for whiplash, and the victim is everyone who uses the same insurance company and has to pay a little more.
With burglaries, a house is watched or happened across while dark and empty, someone weighs up the chances of finding a laptop they can get £50 for which will pay for a few drugs, and hey presto that's the back window smashed.
Phone hacking is - or was, since I doubt anyone's daft enough to be doing it any more - no different. As in white collar crimes like embezzlement, bribery, or insider dealing people do what they think they can get away with.
If they keep getting away with it, the temptation and frequency grows until it becomes brazen and so obvious the bank fails, the Ponzi scheme collapses or your newspaper shuts down.
In all cases the crime is the sole responsibility of the criminal. But when banks fail we ask what the regulator was doing, when the house is burgled we wonder if the culprit is already known to police.
Since the News of the World was closed two years ago, one tiny aspect in the abhorrent phone-hacking scandal has been quiet. That is why, despite the duplicity of a few criminals listening to voicemails on a near-industrial scale and despite the fact police were told about it in 2002, they did not stop them.
It is something which means you can't simply put the whole scandal down to the Press Complaints Commission being toothless, or a need for state control of the media. The criminals told the state what they were up to, and the state let them carry on.
In any crime, at any time, the public has a right to know who decided that and why. But especially so in this case, since that decision was one of the main causative factors of quite a lot of worrying things.
In the past two years journalists of all types have been traduced and vilified by a judge-led inquiry into the culture, ethics and practices of a Press involving 30,000 employees and brought into disrepute by, at best, a handful of their number.
You may love or loathe journalists, but we have only the rights and freedoms of ordinary citizens. If our right to question or publish within the reasonable limits of the law is infringed, then so is that of everyone with a Facebook or Twitter account.
Attack the Press and you attack everyone - even those who do not read their newspapers. Every one of us benefits from those in power being afraid of scrutiny, and if the scrutiny is removed every one of us pays the price.
At the last count there have been 59 arrests and 14 journalists charged, and even if against all probability they are all guilty they represent just 0.04% of the industry.
The current cost of that is £19.5million, predicted to rise to £40million by the end. That's £2,857,142.86 per person charged, without counting prosecution or defence costs of the trials.
There are more police hunting bad journalists than there are child rapists. You might not like journalists complaining about that, but you'd hate paedophiles crowing about it far more.
Of course an investigation was needed. It was needed in April 2002 when representatives of the News of the World contacted detectives investigating the disappearance of schoolgirl Milly Dowler.
These representatives told officers they had listened to 13-year-old Milly's voicemail. It's been said they met officers in person at Staines police station, and that they told them over the phone. They said there were messages which appeared to suggest she was alive and working at a Midlands clothing factory.
So certain were they that a team of reporters and photographers had been sent to stake out the factory - something which, even for a newspaper with massive resources, is not done lightly. They hassled the employment agency which left the messages, as well.
Unfortunately it was a simple wrong number. Unbeknown to anyone, Milly had been grabbed off the street and killed on her way home from school by a local man called Levi Bellfield, who had dumped her body in woods where she wasn't found for six months.
He lived 50 yards from where Milly had last been seen and drove a red car like one seen acting suspiciously in the area when she disappeared. He had a history of alleged rapes, drugging and assault.
But police failed to pick him up. They knocked ten times at his door but when there was no answer did not chase it up with the landlord until long after he'd moved out and the flat had been steam-cleaned and redecorated. They briefed journalists their main suspect was Milly's father after they found extreme pornography and bondage gear at the family home, and for a short while their attention was diverted to a Midlands clothing factory.
The NOTW voicemails were, in context, little more than a brief distraction from a criminal inquiry which was already fatally-flawed. It didn't help.
Before police finally caught Bellfield he murdered two women in hammer attacks and attempted to kill a third. He was not convicted of Milly's murder until 2011 - just a month before the NOTW closed, and not before his defence brief had dragged Bob Dowler through the public wringer over his sexual tastes.
In the nine years between that day and Milly's disappearance, her family suffered unspeakable grief. Not only did their daughter and sister die, she was murdered. Not only that but she was dumped and undiscovered for six months. Worse, when they rang her mobile they thought she was alive.
The act of listening to messages on Milly's voicemail caused them to be deleted. Both police and the NOTW listened to them, but not at the right times to be responsible for the deletion which gave her family false hope. The mobile was never found - perhaps Bellfield destroyed it. Perhaps he listened to her messages first, too.
The Dowler family have had all that to overcome, while also quite rightly being given centre stage among those calling for a change in the way the Press operates. I am not sure, in their shoes, whether I'd still have the strength to get out of bed.
And in all of the phone-hacking scandal there are so many strands that it's near-impossible to say one is the principle cause. You can't pin it to one person, or one day - it was a perfect storm where things collided to be as bad as possible.
Just one example is the bad bit of luck that Hugh Grant's car broke down on a motorway just as seedy ex-NOTW hack Paul McMullan was driving past with a camera in his glovebox - a fluke which meant he papped the actor, who later secretly recorded 'Mucky' telling tall tales about his red-top days, and wound up on Newsnight representing my entire trade despite the fact he hadn't been in it for donkeys' years.
Little threads like that combined with shocking revelations and unbearable human tragedy to produce something which, when woven together, looked very much like a fishing net. You could use it to catch a few big fish, and you could also trawl it to scoop up every innocent sprat.
It's impossible to unravel all those things, and even if you tried most of the net would still be there. But all those threads lead back in time to the day that police knew NOTW were hacking phones, and didn't nick anyone.
They did not, as far as we know, tell them off, speak to the editor, point out the error of their ways or ask the Crown Prosecution Service if it was worth a charge.
We don't know what they did, because as the Independent Police Complaints Commission has finally and belatedly revealed today every single officer involved has conveniently forgotten about it.
The report found that despite this hacking being known about "at all levels" in the murder inquiry, "no action was taken to investigate it despite an indication that a crime had potentially been committed".
It went on: "We have not been able to uncover any evidence, in documentation or witness statements, of why and by whom that decision was made: former senior officers, in particular, appear to have been afflicted by a form of collective amnesia."
Let's leave journalism aside for a moment and imagine what we'd make of this if it was any other crime. Organs being stored without relatives' approval, for instance, a cover-up into 96 deaths at a football match, perhaps, or a GP who was touching up his patients.
There'd be a bloody outcry, wouldn't there? The police knew, and did nothing, for nine years? It was exposed only by journalists ferreting around and leaked information, and the ensuing scandal means a hospital has shut down, a city has been defamed, or hundreds of people abused?
Parliament. Inquiries. Politicians demanding answers. We'd get the lot.
This time around, with hundreds of innocent journalists sacked by a company terrified of what people who used to work for it were accused of, thousands of people hacked and an industry defiled by both public disdain and our own rotten apples, we won't get any such thing.
And if there is any part of this entire shambles which deserves millions of pounds worth of investigating, a judge-led inquiry and some collars being felt it is the failure of Surrey Police to do their job.
Their failure to catch Milly's killer, their failure to stop him killing again, their failure to act on a crime not only perpetrated under their very noses but with the evidence presented to them afterwards in a big red bow, and their failure to extract digit from backside at any point between now and 2002.
Today all they've done is apologise to the Dowlers. They should actually be visiting every house in the country to grovel on the doormat.
Surrey Police know which detectives spoke to the NOTW, they know what they were told and by whom, and nary a copper has been nicked. From the IPCC statement it sounds like those concerned have retired on a nice, fat pension.
Collective amnesia? My fat arse it is. Collective back-covering more like, of exactly the kind which caused public disgust at the NOTW.
Surrey Police's failings led to the death of two women and traumatic injuries to a third. They led to a newspaper getting the impression phone-hacking was fine. They led to thousands of people, from Sienna Miller to families of 7/7 victims, getting their privacy trampled, to the most successful English language newspaper in the world shutting down, to hundreds being sacked, dozens arrested, to a £40million bill, to fears of secret justice and unknown arrests, and an 18-month inquiry the lasting legacy of which is a dog's breakfast of a Press regulator with a side order of shagging barristers.
Crimes are down to the criminals who commit them, and in the absence of any trials we don't yet have anyone to legally blame.
But crimes which the police know about and do nothing to stop are things we can discipline, sack, name and shame officers for. If we don't, we're telling the police that when they do wrong they won't get caught and no-one will notice.
And if they look the other way for phone-hacking, what are they going to do when your house is burgled, your car is rammed, and your boss has his fingers in the till?
There is no way to avert the things that have gone wrong, from bringing Milly back to staying the hands of the hackers before they picked up the phone. But if we have learned anything it should be, at the very least, to remember.
And bang up the amnesiacs responsible.