The great French writer Voltaire famously said: 'I disapprove of what you say and would defend to the death your right to say it'. In this way, he encapsulated what it meant to be an enlightened human being — someone prepared to consider all points of view.

But in recent years the principle of freedom of speech, sacred since Voltaire's 18th century, has been lost, and this is surely one of the most sinister features of our times. It is as if we are entering a new Dark Age of Intolerance.

The irony is that this intolerance has come about as a result of what were initially good intentions. One of the things which makes me happy as I grow older is the thought that during my lifetime we have all tried to become a kinder society.

In recent years the principle of freedom of speech has been lost, and this is surely one of the most sinister features of our times

When I was a boy and a young man, for example, racist jokes were the norm on radio and TV. Now they would be unthinkable. Mockery of homosexuals, and the equation of being gay with being limp-wristed and camp, were absolute norms of comedy when I was growing up. Now no longer.

Such jokes have gone the way of boarding-houses which used to put 'NO BLACKS. NO DOGS. NO IRISH in the window'. Obviously, all civilised people feel pleased by this.

But somehow those initial good intentions — to be kinder to and more tolerant of others — have morphed into a political correctness that has had the very opposite effect.

Two notorious recent examples of this concerned the treatment of a Christian baker in Northern Ireland, and some Christian bed and breakfast owners in Berkshire. The baker had not wanted to make a wedding cake for a gay couple who were getting married. The B&B owners had refused to let a gay couple share the same room in their establishment. In each case they were successfully sued for unlawful discrimination.

Now, a gay activist would no doubt say this was a good thing, arguing that the baker and bed and breakfast owners' behaviour was comparable to the racism of the past. Yet this is surely getting things wholly out of proportion.

The baker was not persecuting homosexuals, as Hitler did. He was not saying they should be put in prison, as all Home Secretaries in Britain did until the Sixties. He was merely saying that, as a Christian, he thought marriage should be between a man and a woman, and that two chaps tying the knot were doing something rather different, which is contrary to traditional Christian teaching.

Whatever you think about this matter, the Northern Irish baker and the B&B couple were merely holding on to Christian beliefs.

I don't happen to share their views myself, and think that if two people are rash enough to promise to live together for the rest of their lives, good luck to them, whether they are gay, straight, trans or anything else. But surely you can understand both sides of this dilemma, can't you?

Well, the answer, more and more in our intolerant society, is 'No'. My concern here is not about the rights and wrongs of gay marriage, transgender rights, our colonial history, or any of the other emotive issues that are subject to endless debate in the modern age.

It is about freedom of thought and speech; freedom to disagree in a liberal society; freedom to have thoughts which are different from the current orthodoxy.

What began as our very decent desire not to be nasty to those of a different ethnicity, or sexual proclivity, from ourselves, has turned into a world as intolerant as monkish Christianity in the days of the Dark Ages, when any freedom of thought is questioned.

Tim Farron, leader of the Lib Dems during the General Election, was asked repeatedly about his views on gay marriage. As a fairly old-fashioned Christian, he did not believe it was possible — marriage should be between a man and a woman.

As the leader of a modern political party, he knew that it would be political death to admit this. He was finally forced to resign.

This was a signal to the world that if you want to succeed in modern politics, it is simply not allowed to hold views which, until a very short time ago, were the consensus among the great majority of people in the Western world.

I use the words 'not allowed' advisedly. What is sinister about living in the new Dark Ages, however, is that it is by no means clear who is doing the allowing and not allowing. In Mao's China, it was obvious: thought crimes were ideas which contradicted the supreme leader.

In Britain today, however, it seems an army of self-appointed censors — from internet trolls to angry students, lobby groups, town hall officials, craven politicians and lawyers and Establishment figures, as well as a host of other sanctimonious and often bilious busy-bodies — have taken it upon themselves to police what we can and cannot think or say.

Not believing in abortion, like not believing in gay marriage, is now, unquestionably, a thought crime. It was hardly surprising that the Tory MP Jacob Rees-Mogg recently said he did not believe in abortion, because he is a man of conviction as well as a Roman Catholic, and this is the teaching of his Church. Yet his view was treated with incredulity and disdain by everyone from trolls and women's groups to the higher echelons of the political Establishment.

As in the case of abortion, debate is no longer allowed on transgender issues. There was a BBC2 Horizon Programme last Tuesday night called Being Transgender. The close-up shots of transgender surgery in a Californian hospital will not easily leave the mind.

We met a number of nice people who had decided for one reason or another that they were not the gender which they had once supposed. They were all undergoing some form of transformative medical treatment, either taking hormones or having surgery.

What made the programme strange as a piece of journalism was the fact that it did not contain one dissenting voice. Not one psychiatrist or doctor who said they doubted the wisdom of some of these procedures, especially in the very young.

Still less was there anyone like the redoubtable feminist and academic Dr Germaine Greer who once expressed her view that a man did not become a woman just because he had undergone transgender surgery — and was, as a result, decried from the rooftops with everything from petitions launched to stop her from speaking at university campuses to death threats.

Dr Greer had also been bold enough to say 'a great many women' shared her view, which is obviously true — a great many women do not think that transgender people have really changed sex. What has changed is that it is no longer permitted to say so.

A friend of mine who likes bathing in the women's pond on Hampstead Heath in London says that at least one person now uses the female changing rooms who is obviously in a stage of transition from man to woman, and is simply a hairy man wearing lipstick.

However uncomfortable this makes the women feel, they know that they cannot say anything.

There was an ugly incident lately at Hyde Park's Speakers' Corner, which used to be the place where anyone could go and stand on a soap-box and hold any opinion they liked.

Speakers' Corner was a symbol of British Freedom of Speech. As a schoolboy, I had a Jewish friend whose grandfather used to take us there to listen to people proclaiming that the earth was flat, preachers praising Hitler, Stalin, and others saying whatever they liked. It was the freedom to do so, said the old man who had escaped Hitler's Germany, which made the very air of Britain so refreshing to him.

What would he have thought had he witnessed the scene earlier this month at Speakers' Corner when a 60-year-old woman called Maria was smacked in the face, allegedly by a transgender fanatic, while listening to a talk on planned reforms to the Gender Recognition Act. Reforms which would allow men to 'self-identify' as female, and enter women's changing rooms or refuges unchallenged.

For Maria, as for the intimidated women of Hampstead swimming pool, and for Germaine Greer, it is by no means clear that transgender people have changed their sex.

Transgender activists have labelled women like Maria TERFS — Trans-Exclusionary Radical Feminists. When news of the assault on her reached the internet — ie instantaneously — the trolls began baying, like the bloodthirsty mob during the guillotine-executions of the French Revolution. 'Burn in a fire, TERF'. 'I want to f*** some TERFS up, they are no better than fascists'.

The use of the word 'fascist' is commonplace in our new Dark Age for anyone with whom you happen to disagree. You hear it all the time in the Brexit arguments which rage all around us and which I dread. As it happens, I voted Remain. But I do not regard Brexiteers as 'fascists', and many of their arguments — wanting to reclaim the power to make our own laws and control our own borders — are evidently sensible.

Yet I have lost count of the number of times I have heard Remainers say that Brexiteers are fascists. As a matter of historical fact, many of the keenest supporters of a united European superstate were actual fascists.

The only British politician who campaigned on the ticket of Europe A Nation during the Fifties was Sir Oswald Mosley who was leader of the British Union of Fascists. But then, today's PC censors don't let facts get in their way of bigotry.

Branding anyone you disagree with a fascist; hitting people in the face; tweeting and blogging abuse behind the cowardly anonymity of the internet — these are the ugly weapons used to stifle any sort of debate. And it is often in the very places where ideas should be exchanged and examined that the bigotry is at its worst: our universities.

This week on the Radio 4's Today programme, we heard James Caspian, a quietly-spoken, kindly psychotherapist, describing what has become a cause celebre at Bath Spa University.

He has been working for some years with people who for one reason or another have begun the process of gender-transition, and then come to regret it.

Caspian is evidently not a judgmental man. He wanted to write a thesis on this subject from a sympathetic and dispassionate point of view.

What makes people feel so uncomfortable with their own apparent gender that they wish to undergo painful and invasive surgery to change it? What makes people then come to reassess their first idea? These are surely legitimate questions about a subject many of us can't quite comprehend.

I have two friends who started out as men, and decided in mid-life that they were really women, or wanted to become women, which is what they have done. I do not really understand what has happened to them, even though they have tried to explain it to me.

Surely a man like James Caspian, who has worked with transgender men and women, should be encouraged by a university to explain this area of medicine or psychology?

But no. The university, having initially approved of his idea for a thesis, then turned down his application. 'The fundamental reason given was that it might cause criticism of the research on social media, and criticism of the research would be criticism of the university,' he told Radio 4 listeners. 'They also added it's better not to offend people.'

This is all of a piece with students at Oxford wanting to pull down the statue of 19th century imperialist Cecil Rhodes from his old college, Oriel, on the grounds that he was racist.

Rather than having a reasoned debate weighing the evils of racist colonialism against Rhodes's benevolence, the student at the forefront of the movement — who had actually accepted a £40,000 Rhodes scholarship funded by the fortune the colonialist gave to Oxford — wanted to pull down the statue.

This is the same attitude of mind as that which led monks in the Dark Ages to destroy the statues of pagan gods and goddesses, or the Taliban to do the same to age-old Buddhist artefacts.

Reason, debate, seeing more than one side to an argument, surely these are the foundations of all that has fashioned the great values of the West since the Enlightenment started in the 18th century with an explosion of new ideas in science, philosophy, literature, and modern rational thought that ushered in the Age of Reason.

Realising that human actions and ideas are often mixtures of good and bad — isn't this what it means to have a grown-up mind? Surely we should be allowed to discuss matters without being accused of thought crime?

In universities, as at Speakers' Corner and in the public at large, there used to be the robust sense that sticks and stones may break our bones but words can never hurt us. Now, the 'hurt-feelings' card is regularly played to stifle any debate.

Little by little, we are allowing the Dark Ages of intolerance to come again. We should not be letting this happen.

We should be able to say: 'We disapprove of your views — on Europe, on Transgender Issues, on Islam, on absolutely anything, but we defend to the death your right to express them'.