How you think is as important as what you think. If you believe you can ban your way to victory by mounting heresy hunts against all who veer from the true faith, you will not only deserve to lose by some airy moral reckoning. You will lose whether you deserve to or not. As losing is no longer a trivial event in the age of Brexit and Trump, it is worth understanding the consequences of going beyond the old liberal principle that only demagogues who incite violence should be banned.

The moral arguments against censorship are so old I can recite them in my sleep. The practical case against a “liberal” movement that reaches for the censor’s red pen like a drunk reaching for a bottle deserves more attention.

People who call themselves progressives don’t worry enough about unintended consequences because they lack the broadness of mind to see themselves as others see them. They see no reason to treasure free debate. No argument will persuade Donald Trump or Nigel Farage to hold up their hands and admit they are wrong. Their dedicated supporters, meanwhile, are no more likely to change their minds than fanatical believers in any other political ideology or religious creed. These are good points that are beside the point, because they are based on a deep ignorance of how debates work.

You don’t argue to convert your opponents. You argue to persuade the undecided audience watching on in silence, as it judges which side is worthy of support. I doubt that waverers nod their heads in approval when universities, of all places, do not allow speakers to appear on platforms, or when the state capitalists of Virgin Rail refuse to stock the Daily Mail. Look at them, and maybe look at yourself too. It’s not a compelling sight.

The alt-right wants to provoke liberals into showing they are repressive... Why play the part it has allotted you?

For all their bombast, censors give every appearance of being dictatorial neurotics, who are so frightened of their opponents that they cannot find the strength to take them on in the open. I can’t imagine many saying, “I’ll side with the people who tell me what I can and can’t think.” I find it equally hard to picture readers turning away from the Mail because Sir Richard Branson and “alternative” comedians who haven’t had an alternative thought since Blair’s second term tell them to.

“Liberals” still do not understand that when they censor they are falling into their enemy’s trap. The alt-right is as much a satirical as a political movement: more South Park than The West Wing. It is at its happiest trolling liberal culture rather than governing, which is why Brexit and the Trump administration are so shambolic. The alt-right wants to and needs to provoke liberals into showing they are repressive, so it cast itself in the role of transgressive rebel. Why play the part it has allotted you?

We are in a contradictory culture. On the one hand, “liberals” rightly say that sexists, racists and homophobes are preposterous bigots. On the other, they run away from the chance to confront them. If you can’t beat a bigot in argument, you shouldn’t ban them but step aside and make way for people who can. It’s not as if they have impressive cases that stand up to scrutiny.

As pertinently for those wondering how a pornographic thug like Trump or such transparent charlatans as Johnson and Farage can win, if you don’t debate them, you will never learn how to defeat them. You won’t feel the ripple in the audience as you make a good case or telling jibe. You won’t learn which shots hit home and which miss the mark. When the battle is finally joined, you will enter it unarmed, then look around in bewilderment when you are defeated.

In the hours after Trump’s victory, the American author Walter Mosley said Democrats had put on “the blinders of superiority” when they assumed he must surely lose. There is no more effective way for the superior to blind themselves to the world around them than by refusing to argue with it.

Again, I am not making a moral point – we can save John Milton and George Orwell for another day. As a matter of practical politics, you had better be very sure that you will win before pandering to inquisitorial desires. So much of what passes for “liberal” debate just assumes that liberals have already won and possess the power to decide what is read and said. They don’t fret that reactionaries in office will use the arguments in favour of censorship that liberals have nurtured to restrict their freedom to speak. Nor do they question whether their repression will work.

Harvard’s Stephen Pinker recently listed true but “politically incorrect” assertions that have driven American students rightwards when they discovered US campuses rarely discuss them. His unpalatable propositions included: capitalism is preferable to communism – no one would prefer to live in North rather than South Korea, after all; most of the world’s suicide terrorists are Islamists; and different ethnic groups commit violent crime at different rates.

Pinker said that if only universities had the courage to face awkward facts they could make perfectly good rejoinders against the apparent justifications for racism and anarcho-capitalism. The most successful capitalist societies have strong welfare states rather than unregulated markets, for instance. Most American terrorists are white supremacists. Ethnicity isn’t destiny and the propensity of a group to commit crimes changes over time.

Inevitably, creepy American leftists cut his explanation out when they edited a video of his talk to present him as a fascist. They should have thought harder about the failure of US campuses to impose their taboos in a setting where liberals have power. It is a warning that authoritarian liberalism is an impossible project.

Let’s try a thought experiment. Even if you were to suppress the rightwing press and rightwing social media, as so many “liberals” appear to want to do, you would not ban rightwing ideas, merely win them more converts by investing them with a dissident glamour. What’s next? Vet candidates for office to make sure they conform to your desires? Stop your opponents voting?

The motivation behind much modern censorship is essentially religious: an affirmation of the urge to parade your righteousness. It is an egocentric and frivolous emotion to indulge at a time when the stakes could not be higher, and every opponent of the populist status quo ought to be concentrating on winning converts rather than driving them into the arms of their grateful opponents.