It used to be much easier. There was a common understanding that a university was a public institution dedicated to teaching, research and the free exchange of ideas. Freedom to argue and freedom to be challenged were indispensable parts of a university’s DNA. Freedom of speech was the precondition of the discovery of truth. As such, the university was a quintessentially liberal institution, a cornerstone of an Enlightenment society.

Today, such liberalism is in across-the-board retreat. The idea of a public realm is shrivelling. Those who want to create, say, public space in new developments in our cities, uphold the idea of public service broadcasting or defend the concept of the university as a citadel of argument are portrayed as enemies of individual choice, statist interveners, or, worse, tolerant of evil opinions.

This disdain for notions of publicness has created a vacuum occupied by the rise of a libertarian individualism that indulges belief over reason. Non-falsifiable belief systems used only to be the hallmark of ideological communism or religious zealotry. Now, ideas, especially on the right but also to a degree on the left are less and less tested in a public realm by debate, with evidence marshalled to justify them. Instead, they are asserted as valid because the holder believes them.

This new individualism, alongside the decline of the public, has provoked a mounting tide of, at best, siloed thinking, impervious to criticism, and, at worst, the indulgence of rank prejudice. Thus, on the right, if I feel that Britain is being swamped by immigrants, climate change is bogus, maleness is being overwhelmed by “femininazi” women or the welfare system is transfixed by cheating, then, whatever the facts, my feeling is valid because I hold it. That suffices without proof or evidence. In any case, there will always be some article in a rightwing paper to justify it.

British conservatism, once pragmatic and sympathetic to the liberal ideal of a public realm, is becoming populated with ever-wilder ideas – and more willing to try to put them into practice. Meanwhile, on the left, there is a parallel if more subtle closure. As reform of the existing economic order to deliver personal enfranchisement seems ever more unachievable, so the fight for change has migrated to cultural issues on which the advocate can be more certain not only of the rightness of the cause, but of the possibility of victory.

For them there can be no doubt, for example, that Israel is so wholly in the wrong that it warrants an intellectual boycott or that any departure from the new consensus on sex or gender deserves total censure. Indeed, in universities there is a growing willingness by student censors to withdraw any speaking opportunity for those who express any contrary opinions, especially if they might upset those who hear them, creating so-called “safe spaces”. Thus the Germaine Greer row at Cardiff University after student complaints that she was “transphobic”, and also Warwick University’s student union’s recent attempted ban on Iranian human rights campaigner Maryam Namazie. She had spoken out against sharia law, which allegedly might have troubled Muslims.

We should remain exactly what we are: a tolerant society that makes no demand on us but to obey the rule of law

Universities as liberal public institutions are increasingly being undermined. The proposed new counter-extremism bill now lays the responsibility on civil society in general, and universities in particular, to identify and inform on individuals who engage in non-violent, non-criminal but “extremist” activity. What is extremism? The guidance to universities following the Counter-Terrorism and Security Act tells us it is “vocal or active opposition to fundamental British values including democracy, the rule of law, individual liberty and the mutual respect and tolerance of different faiths and beliefs”.

But as Ken MacDonald, warden of Wadham College at Oxford argues, literally understood this would mean that he – or I, as principal of Hertford College, across the road – would have to keep the government informed on a regular basis. We’d have to report that our colleges had, say, hosted lecturers teaching Marxist theory that the rule of law is a class construct to enforce bourgeois supremacy; or note any atheist logician who deconstructed the foundations of religious belief. Both would have challenged our values as the government defines them, but in a non-violent, non-criminal way. As MacDonald continues: “To the response that these forms of speech are not the ones the government wishes to target, we might ask: who is to say?”

But the former spooks and security agents cooking up these proposals for home secretary, Theresa May, to speed into law to appease the untested and unchallenged prejudices of her party are no more than the flip-side of students wanting to censure free speech to create “safe spaces”. Too few are being sufficiently challenged to provide evidence, or willing to accept the legitimacy of challenge, over what they propose or think. We have all the law we need to deal with incitements to murder, hatred and terrorism. We mustn’t and shouldn’t turn universities into giant “safe spaces”, self-censoring themselves over what can be discussed because it might contravene some government definition of what might trigger terrorism – and on top to censor speakers who might give offence to some interest group.

David Cameron argues that to engage with extremism it’s time to stop being “a passively tolerant society that makes no more demand on us than to obey the law”. He is right to want to engage with noxious extremism, but not to discard the foundations of a liberal society. Rather, we should remain exactly what we are: a tolerant society that makes no demand on us but to obey the rule of law. But we should proselytise what that means, argue our heads off for it and insist we keep and nurture the forums in which that argument can happen.

It becomes ever more obvious that the war against terrorism launched post 9/11 has been a dreadful wrong turning. Imagine if, instead, our response had been non-violent – asserting our values, respecting and entrenching liberty and the rule of law and arguing ceaselessly back to the militant Islamicists. The best defence against the non-violent Islamic preacher inciting jihadism or the transphobe is evidence, fact and argument. Once we stop talking – and once universities close down free expression and free debate – we have lost all that is precious. Defending the Enlightenment legacy and the idea of the public realm has never been more urgent.