In a civilised society, we recognise that a distinction must be made between what people think and what people do. We insist that while it is very often legitimate to punish people for their actions - particularly their violent actions - it is unacceptable to punish them for their thoughts and their fantasies, however perverse they might be.
We look back at the incarceration of the Marquis de Sade for writing filthy books, and we balk. A civilised society punishes crime, but never thoughtcrime, never the crime of fantasising about things.
Well, Britain, it seems, is heading away from civilisation. For today, David Cameron will announce that it will become a crime in England and Wales to possess certain forms of extreme porn, including "rape porn" - that is, porn made by consenting adults who are pretending to be in a rape scenario.
In short, the PM is making it a crime to have certain, warped sexual fantasies, to get excited, in your head, by things that the vast majority of us find disgusting. He's making the enjoyment of certain fantasy images and fantasy words into a thoughtcrime, a crime in which no assault or battery or other physical transgression has occurred, yet where individuals can still be arrested, basically for deriving mental and physical pleasure from perversion.
The vast majority of us can agree that "rape porn" is disgusting, as is a great deal of porn. Many of us also agree with Cameron's assertion that we live in an era saturated by sexualised imagery.
But when it comes to state intervention into pornography, the properly liberal approach is to ask: is the porn consensual or non-consensual? If it's the latter, as all child porn automatically is, then it is of course a police matter, and a pressing one at that - the circumstances in which the porn was made, and the identities of both the makers of it and the victims of it, must be thoroughly investigated.
But if the porn is consensual, then there should be no state clampdown, on the basis that it ought to be none of the state's business how people have sex, whether they film it, whether other adults watch it, and what those other adults think while they watch it.
There are three reasons we should be concerned by Cameron's criminalisation of certain kinds of extreme porn. First, it is fuelled by the much discredited "media effects" theory - the idea that media images and words lead directly to real-world mayhem.
Cameron explicitly says that extreme porn can "pollute minds and cause crime". Cause crime? What about the thing that stands between the viewing of extreme porn and the commission of a crime - that is, the viewer? Does he not possess willpower and agency? "Media effects" theory lessens the moral responsibility of the criminal in favour of effectively saying "the media made him do it" - a weird argument for a supposedly conservative PM like Cameron to push.
The second problem with Cam's clampdown is the slippery slope. If media depictions of pretend rape can be banned on the basis that they cause real rape, what about media depictions of other forms of pretend violence? Pretend assault? Pretend burglary? Would extremely violent horror films be next for the chop? After all, some of them depict violence, sometimes sexual violence, that looks shockingly real.
What about Lars von Trier's Antichrist, which depicts graphic sexual violence against a penis? Or the French arthouse/porn crossover Baise Moi, which has disturbingly graphic rape scenes with a feminist twist? If we accept the idea that media images lead directly to copycat behaviour, then we throw the door open to all sorts of censorious state meddling into fictional or fantasy-fuelled material, entertainment and literature.
And thirdly, there's that enlightened principle which Cameron seems to have forgotten, which is that people's minds, people's fantasy lives, should be no-go zones for the state. To punish people for enjoying fantasy material is to open the door to the creation of thoughtcrime and fantasycrime. The following is going to sound terrible, but that's because the truth is sometimes terrible: if a man rapes a woman, he should have the full force of the law thrown at him; if a man fantasises about raping a woman, nothing should be done to him.