Mary Whitehouse and the American moral majority wanted to stop broadcasters from “pumping filth into our homes”. Today’s student leaders are their successors. The president of the Cambridge Union simpers that a university is a “home” where students should feel comfortable and safe. It has never occurred to him that universities are not, or should not be, anything like a home. Higher education is meant to take students away from the prejudices and certainties of their childhood home, and challenge the ideas they learned from their parents. If students cannot handle the challenge without crying that they feel unsafe, they should not be at university in the first place. If universities refuse to challenge them, I wonder about their usefulness too.

We have gone from the principle that only speech that incites crime can be banned to the principle that speech that incites gross offence can be banned to the principle that speech that provokes discomfort can be banned. This is not so much a slippery slope as a precipitous drop.

Many want to take the plunge. A few weeks ago, 130 intellectuals wrote to the Observer to make the classic case for freedom of speech. They said that feminists critical of the sex industry and of some demands made by trans activists were being banned because the prevailing consensus was that the mere “presence of anyone said to hold those views is a threat to a protected minority group’s safety. You do not have to agree with the views that are being silenced to find these tactics illiberal and undemocratic.”

Who could possibly object to that, I thought.

Just about everyone, it turned out. Hundreds of other intellectuals replied in the next issue of the Observer. They made the counterfeit claim that being “no-platformed” by student groups was not an attack on free speech. They went on to confuse support for free speech with support for the speaker—the tactic of every grand dictator and little Hitler in history—and implied that standing up for open debate meant the letter’s signatories were indeed “transphobes” and “whorephobes”. Extreme though their reaction was, it was nothing when set against the reaction of online activists.

The indomitable gay rights campaigner Peter Tatchell is a hard man to frighten. He has fought homophobic vigilantes and Robert Mugabe’s security guards. But even Tatchell was unnerved by the 4,000 abusive Twitter messages he received for putting his name to the Observer letter. His abusers denounced him as a “homo”, “foreigner”, “misogynist”, “paedophile” and “nutter”. One correspondent informed him that “I would like to tweet about your murder you fucking parasite.” So much for the safety of those who seek to challenge “safe spaces”.

Electoral calculation ought to stop left-wingers allowing conservatives to own the inspiring idea of freedom of speech. If they could only see how they appear to others, they would understand that the people they are trying to convert tend to suspect those who would tell them what to say and how to say it. Many who should be open to radical arguments will turn away because they associate the Left with the silencing of contrary views and the imposition of orthodoxy. Above all, left-wingers need to grasp that speech codes and blacklists do not produce social change but a hypocritical observance of conventional pieties.