Picture the scene. You’ve joined a political party so that you can hang out with people who share your worldview. But to your horror you discover that there are people in the party who think differently on certain issues. They may agree with you on 95% of the party manifesto, but on a few important questions they hold a different opinion to you. What to do? You could ignore them, you could argue with them in an attempt to convince them to change their mind, or you could work with them to find common ground and a compromise position. Or you could try to kick them, and those who think like them, out of your party.

Fortunately, the latter is rare on most issues. Political parties (with the possible exception of the Korean Worker’s Party) can only exist by accommodating a broad church. So it is that Labour can accommodate Remainers and Brexiteers, the Lib Dems can include people who think gay sex is sinful, and the Greens can be home to people fiercely pro and anti nuclear power.

Yet on one issue, the inability to allow any difference of opinion has become commonplace in UK politics – the issue of sex and gender.The target for silencing is anyone with so-called gender critical views: the view that biological sex exists and sometimes matters more than self-declared gender. Similar views are held by the majority of the British public and just a few years ago would have been entirely uncontroversial. But now, in many parts of the political left, gender critical people (or TERFS as they are often pejoratively called)  are on the receiving end of sustained hatred aimed at forcing them, and their opinions, out of politics.

To take such a stance against people simply because of a disagreement might seem intolerant. But in fact, those who favour such tactics say they are being extremely tolerant. And to support this surprising claim they offer no less an authority than the great 20th century philosopher Karl Popper. Any time an activist is accused of being intolerant, say for hurling violent threats at feminists on twitter, they’ll retort with “Popper’s Paradox of Tolerance”. More specifically, they’ll probably tweet this graphic:

Now I might be being unfair here, but I suspect most of the people quoting the paradox only know of Popper’s work through this cartoon. It’s therefore not surprising that they completely misunderstand Popper’s philosophy. I’ve been lucky enough to study Philosophy of Science where Popper was, of course, a key figure. It’s only through understanding Popper’s broader world view that you can appreciate why using his work to justify silencing gender critical views is such a gross distortion.

The key to Popper’s philosophy is the fallibility of human thought. However convinced we are that something is true (in science or politics), we should always recognise that it may be false. Indeed the more certain we are of a truth, the more we must open our minds to the possibility we are mistaken.

Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem which it was intended to solve.

Karl Popper — Objective Knowledge: An Evolutionary Approach (1972)

Take Newton’s theory of gravity. The force of gravity can be witnessed at every moment, and Newton’s equations worked perfectly to send men to the moon and back. Yet Einstein’s theory of general relativity showed that Newton’s theory was wrong. Newton’s theory gives a result that’s close enough for most practical purposes, but in extreme conditions (close to a black hole for instance), it gives the wrong answer.

We now know, from accurate observations, that Einstein’s predictions are confirmed whereas Newton’s are disproved. So is Einstein correct? Not necessarily. All we can say is that his theory is the best explanation of gravity we currently have.

So, scientists should constantly ask themselves “What if I am wrong?”. Not only should they be open to the possibility that they are mistaken — they should actively try to disprove their own theories. The mark of science is that its theories are capable of being falsified by making testable predictions. Having made those predictions, scientists should use experiments to see if their theory can be falsified.

Popper applied the same principle to politics — that you should always consider your own fallibility. However passionately you believe something you should ask yourself, “What if I am wrong and my opponents are right?” Just as scientific theories should be tested by experiment, so political ideas should be tested through rational debate. You should be constantly willing to engage with your political opponents, approaching each interaction with a genuine openness to changing your mind. That’s why freedom of speech and thought are so vital to Popper. Far from being a problem, those who disagree with you provide a vital service. They allow you to test your ideas, potentially finding a flaw and convincing you to think again.

However, Popper recognised that there’s a problem with unlimited freedom of speech. If an ideology comes along that refuses to engage in rational debate, suppresses those who have any contrary view and answers any verbal challenge with violence, its advocates may succeed in stopping all debate. This intolerant faction may destroy that precious freedom of speech. “If we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.”

This is the paradox — to preserve tolerance we must be intolerant of those who are themselves intolerant. Of course, the crucial question then becomes “when is it acceptable to be intolerant”? Many Twitter commenters think that Popper is justifying intolerance against any views you find reprehensible — but that’s far from his intention. We can’t justify intolerance against others simply because we believe that they are profoundly wrong or even harmful. Evil ideas are still best defeated through argument — their lies exposed by evidence. The paradox of intolerance only justifies intolerance against those who cannot be argued against by any other means: those who themselves reject the very notion of rational debate and suppress criticism through violence. Those who “answer arguments by use of their fists” as Popper put it.

So, what does this mean for the current discussions around sex and gender? Does either side justify intolerance? Try to put out of your mind which side you think is correct or morally right – that’s not the question. The issue is whether one side has actively attempted to make rational debate impossible? To my knowledge only one side has actively tried to prevent discussion. There are many examples of trans rights activists shutting down any debate. As a fellow filmmaker, I was particularly struck by the experiences of Olly Lambert when he was making a film about gender medicine and discovered a complete refusal to discuss the issue from one side only — the like of which he’d encountered in no other issue.

What about physical violence? There are many instances of women attempting to have peaceful meetings to discuss issues of sex and gender facing mobs of black-clad protesters, with the clear intent to intimidate and in some cases even assault. If there are cases where Gender Critical protesters have attempted to intimidate trans rights groups I’ve not seen any evidence for it.

It’s obvious when you look at the social media posting of the trans-rights activists that they believe with an almost religious zeal that they are doing the right thing. But as Popper would argue, that’s when the potential to do wrong is most acute. “The attempt to make heaven on earth invariably produces hell.”

Karl Popper’s philosophy is not, of course, infallible — and he would be the first to say so. But if you believe he has something useful to say about human thought then keep a watchful eye against any attempt to shut down discussion — particularly on your own side — and be intolerant of it. And if you are ever tempted to silence someone — always ask yourself, “What if they are right and I am wrong?” It’s a question we could all do with posing to ourselves more often.