FOR most of the 20th century being on the political left meant supporting a set of principles dating from the European Enlightenment. Humanism, progress, freedom of speech, scientific objectivity, tolerance. Somehow in the 21st century, the left seems to be choosing a different course.

Last week, many supposed left-wingers openly celebrated Edinburgh University's “cancelling” of David Hume, arguably the greatest philosopher of the Enlightenment.

Hume's name is to be erased from the university precincts because of a 1758 footnote which suggested he thought non-white races were incapable of becoming civilised. The David Hume Tower will now be called “40 George Square”. This is a bit like cancelling the Parthenon because the Greeks held slaves and calling the Acropolis by its Athens post code.

Robert Burns is accused of racial iniquity because he considered working on a slave plantation in the West Indies – a career move which was common amongst his contemporaries. Yet his poetic works, from A Man's a Man to The Slave's Lament, should demonstrate to anyone sensible that Burns was no racist bigot, though like Hume he was a product of his times.

This silly inquisition of historical figures has become an obsession. Even Sartre and De Beauvoir are sin-binned along with Einstein and Bertrand Russell. Many of these self-righteous accusers believe themselves to be followers of Karl Marx without understanding his debt to philosophers like Hume, Kant and Hegel, who would all fail the woke test of moral probity.

Marx himself wrote far worse than any of Hume's footnotes. In On The Jewish Question the father of communism said: “What is the worldly religion of the Jew? Huckstering. What is his worldly God? Money. Money is the jealous god of Israel in face of which no other god may exist”. It can surely only be a matter of time before Marx himself is cancelled by the witch-finders of the left.

By comparison, the old rogue Henry Dundas, he of the maligned pillar in St Andrews Square, comes out as rather benign, woke even. The former Scottish Secretary was an abolitionist, albeit a gradualist, and made his name as a lawyer by defending a runaway slave, Joseph Knight, in a case that established that slavery was unlawful in Scotland.

Unthinking denunciation has led to repeated historical blunders. Take the campaign against singing Rule Britannia at the last night of the Proms. A BBC Songs of Praise producer, Cat Lewis, compared this to celebrating the Holocaust. Yet Rule Britannia has nothing to do with slavery or race. It’s a unionist anthem, written by a Scottish poet, James Thomson, in 1740 to celebrate the Glorious Revolution and the end of absolutist tyranny.

Many of the Twitter left's obsessions are even more risible. The boycotting of “Tory” Yorkshire Tea, “decolonising” the school curriculum, accusing people of “cultural appropriation” if they wear dreadlocks or fancy dress.

One of the saddest aspects of the contemporary left is an inability to laugh at itself.

During the December general election, Labour's Equalities spokeswoman, Dawn Butler, announced that “children are born without a sex”. She believes that sex is “assigned” to babies at birth, presumably by transphobic midwives, and not observed. Biological sex is thus a matter of personal choice not scientific reality.

The Scottish Government seems to agree and will advise people to “self-identify” their sex in the (delayed) national census, thus rendering the most important collection of social statistics unreliable.

Sexual liberation and the abolition of laws against homosexuality are of course among the greatest achievements of Enlightenment thinking. Trans people are direct beneficiaries of humanism, individualism and intellectual freedom. But the gender left seems to prefer dogmatism. Ms Butler went on to say that “to talk about penises and vaginas doesn't help because then you're saying that a transwoman isn't a woman”.

To suggest that a transgender woman is not literally female is, in her view, “transphobic”. She went further and said that it is a “hate crime”.

This has nothing to do with “trans rights” since no one is suggesting that transgenderism should be made illegal or should lead to discrimination. Yet, feminist writers like JK Rowling, Julie Blindel and Beatrix Campbell have been vilified for arguing that sex is real, and that if saying so is transphobic then so are textbooks on human reproduction.

Hate crime is more than an empty threat too. Under the Scottish Government's Hate Crime Bill, such remarks could very well lead to prosecution for “stirring up hatred”, if enough people claim they feel abused or threatened by them. It doesn't matter that you had no intention of abusing or threatening anyone. Thus has a supposedly left-wing leader, Nicola Sturgeon, adopted a policy that even the Scottish Police Federation say will damage freedom of speech.

So why has the left departed from liberal values? Well, possibly because most of the great campaigns of the liberal era have been won. Abortion is legal, as is homosexuality, same sex marriage and no-fault divorce. Public attitudes towards sexual minorities, including transwomen, have changed out of all recognition. From being feared and demonised, gay and trans people are now celebrated in the media for their creativity and vision.

Women are legally equal under the law and even the gender pay gap is disappearing at the BBC. Similarly, no one thinks black people should be discriminated against, apart from a handful of knuckle-dragging racists. The British Social Attitudes Survey repeatedly confirms that British people are more liberal and open minded than at any time in history.

Perhaps the left is just a victim of it own success. Youthful radicals, devoid of targets, have taken to attacking statues, biology and what is called “white privilege”, the intersectional doctrine that all white people are racist even if they aren't.

The left used to celebrate being “colour blind” to race. Martin Luther King said that people should be judged “on the content of their character rather than the colour of their skin”. Now race writers like Reni Eddo-Lodge proudly say they are “no longer talking to white people about race”, thus dismissing them because of the colour of their skin.

Environmentalism came in from the cold many years ago and anthropogenic climate change is now a political consensus endorsed by Conservatives like Boris Johnson. Polls show that voters now see global warming as a real and present danger. Oil companies are turning into renewable energy corporations because they realise the fossil fuel economy is dying.

This consensus doesn't leave a lot of room for environmental radicals, which is probably why Extinction Rebellion has been getting into difficulties. Disrupting the green London underground and preventing newspapers being published are actions so counter-productive you'd be forgiven for thinking they were the work of agent provocateurs.

And let's not even start on the drive to “defund the police” which has largely remained a US phenomenon, despite determined efforts to import to Britain the idea that you can have justice without law-enforcement.

Perhaps this “infantile leftism”, as Lenin called it, is just a phase, magnified by social media. Perhaps Sir Keir Starmer will halt the left's flight from enlightenment. He’ll have to if Labour is to win power again.

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