‘You can’t change sex – biologically, that is impossible.’ That, by most people’s standards, is a simple observable truth. But by the standards of campus activists, it is tantamount to hate speech, deserving of merciless retribution.

The quote above is from Selina Todd, a professor of modern history at the University of Oxford. And for daring to express what most other people in this country would take to be common sense, she has been marginalised and threatened.

Today the Telegraph reveals that she has been assigned two bodyguards to accompany her to all of her lectures for the rest of the year, after threats were made against her and circulated online.

Todd has for months been treated by trans activists as if she is some kind of bigoted thug who needs to be chased off campus. She has been labelled a ‘transphobe’ and subjected to complaints insisting that it would be ‘impossible’ for her to teach trans students.

And for what? She doesn’t believe people can change their sex, via surgery or ‘self-identification’. And like many feminist academics, she has raised concerns about how proposed changes to the Gender Recognition Act may affect women’s rights and women’s spaces.

She has also caught flak for her research. Todd, who focuses on women’s history, argues that women who posed as men in the past ‘were often lesbians seeking to protect themselves, or because they want[ed] to do jobs that were only available to men’.

Last year, she was the subject of a formal complaint, backed by a Facebook petition, purely on the basis of trans-sceptical things she had tweeted and retweeted on Twitter.

Todd’s is not an isolated case. Trans-sceptical academics have increasingly become pariahs on British campuses, and face remarkable intolerance and harassment.

Kathleen Stock, a professor of philosophy at the University of Sussex, has been fighting off a near-constant campaign against her. Sussex students have made several formal complaints, and she says she has faced ‘hostility’ from colleagues.

‘It is quite a strange situation to work somewhere where people make it clear that they loathe you’, she recently told Times Higher Education.

Last week the University of East Anglia postponed – translation: cancelled – a planned seminar with Stock because of threats of protest. She was told it was called off in order to respect ‘the views of members of the transgender community’ and because allowing the talk to go ahead ‘raised issues of academic freedom’. ‘Security and health and safety issues’ were also cited.

Stock – like Todd – has never been accused of discriminating against, or even being rude to, a trans student. She says she happily uses trans students’ preferred pronouns and names and doesn’t want to push her views on anyone. Her only ‘crime’ is calmly to disagree with the idea that biological men can become biological women and vice-versa.

But reasonable debate on these issues is increasingly being made impossible by intolerant mobs of activists. Rosa Freedman, a trans-sceptical law professor at the University of Reading, has revealed that a male student once confronted her outside the students’ union, declaring her a ‘transphobic Nazi who should get raped’.

This is one of the more stark expressions of the misogyny of the campaign against these (largely female) academics. The slur ‘TERF’ (meaning trans-exclusionary radical feminist), so often spat at these heretics, increasingly feels like a byword for ‘bitch’.

How did we get here? How did we find ourselves in a situation where academics need bodyguards for expressing the wrong kinds of views?

Trans activists and their allies have become increasingly intolerant of late. But the more immediate problem is that they’re pushing at an open door. University authorities appear cowardly and defensive in the face of such threats, terrified of appearing ‘transphobic’, while these academics’ colleagues either join in the witch-hunt or keep their heads down.

It is this cowardice that risks ushering in intolerant mob rule on campus, of the sort that has cropped up in recent years at US colleges. Academics, whether they agree with Todd or Stock or Freedman or not, must show them some solidarity.

If they allow the boundaries of acceptable thought to be policed by these crazed ideologues, it won’t be long before they themselves are accused of wrongthink.