In 1964, US Supreme Court Justice Potter Stewart struggled to define hardcore pornography, eventually settling on his infamous test: “I know it when I see it.” Over 60 years later, the Labour Party has come up with its own addendum: we know it’s disgusting, so we’re banning it.

From November, new offenses will criminalize the possession and publication of certain pornographic material depicting strangulation, incest and adults pretending to be children. Ministers will no doubt congratulate themselves on another victory for safeguarding, but banning something unpopular is easy. Drawing a clear line between what is criminal and what isn’t, enforcing it fairly, and dealing with the inevitable mistakes are considerably harder.

Consider a property such as Game of Thrones. Millions of people have watched the television series filled with incest, sexual violence and the exploitation of its young characters. George R. R. Martin’s books explore this territory in even more graphic detail. Nobody seriously expects Labour to send officials into bookshops to confiscate fantasy novels, because we understand that fiction can depict terrible things without endorsing them, let alone committing them.

So why should pornography be treated differently? The obvious answer is that pornography can involve real people, and that any cases of abuse and exploitation must be taken extremely seriously. But the law should distinguish between consent, exploitation and actual harm, rather than focusing simply on what a scene depicts. Two consenting adults performing taboo acts are not automatically abusing one another just because the material portrays something that the viewer finds disturbing.

Once the state begins prohibiting material because of the acts it depicts, rather than the circumstances in which it was produced, the boundary between a criminal offense and a fictional or consensual portrayal becomes much harder to defend. If that distinction cannot be drawn clearly, how can anyone know what the law permits before they risk breaking it?

Nobody needs to defend the material itself to challenge the law. Free speech means protecting expression that is disturbing, transgressive and deeply unpopular too. Otherwise, it becomes a principle we invoke only when the speech in question happens to be respectable. If the Government wants to criminalize particular material, it should have to explain why the material warrants it. Otherwise, it comes off as arbitrary, even capricious.

This is what makes Labour’s approach to the internet so maddening. The Online Safety Act was sold on the fanciful promise that ministers could make the internet “safer” by forcing platforms to police it more aggressively. The predictable outcome is that companies will resort to removing lawful material rather than risk falling foul of the rules. Now comes another ban, with yet another promise that the state can legislate its way out of a complicated social problem.

When restrictions are presented as necessary to protect children or prevent harm, questioning the powers they create often leads to social ostracization. We are encouraged to judge a law by the material it targets, rather than the powers it gives the state or the precedent it sets.

Labour must understand that safeguarding and civil liberties are not mutually exclusive. Criminal law requires more than righteous indignation. The Government may know what it dislikes when it sees it, but that doesn’t mean it knows how to write a law, enforce it fairly, or stop it reaching further than intended. When the punishment involves prison time, “we know it when we see it” is nowhere near good enough.