As so often, the late Christopher Hitchens put it well: “The struggle for a free intelligence has always been a struggle between the ironic and the literal mind.” Unlike rigid ideology and fundamentalism, irony – saying one thing while meaning another – helps us to recognise complexity, paradox, nuance and absurdity.

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It is one of the most important means of defending humanity, decency and pluralism that we have. It turns the frown of the fanatic into the wry smile of uncertainty. But, sorry to say, it is in poor shape. On social media, the inflections of face-to-face contact and the subtleties of humour are washed away in a storm of assertion and counter-assertion. Twitter is a boot stamping on the face of irony.

Comedians, for instance, face unprecedented scrutiny on the ludicrous grounds that they always mean what they say, and the even more ludicrous grounds that objectionable speech is violence (as opposed to an incitement to imminent violence, which is something very different).

As for politics, I cannot think of a time when the leaders of the two main parties were more stodgily wedded to the literal, so dull in their verbatim repetition of boilerplate language, so lacking in self-awareness when they sound silly. Theresa May set the bar high during the 2017 election campaign with her insistence that “nothing has changed” – immediately after the Conservatives had performed a screeching U-turn on social care.

Yet she outdid herself after her party lost its majority in a vote it was meant to win by a landslide, declaring in Downing Street that only the Tories had the “legitimacy and ability” to provide the “certainty” that Britain needed. Apparently satisfied by her own ridiculous argument, she concluded: “Now let’s get to work.”

Jeremy Corbyn, meanwhile, is to irony what Donald Trump is to feminism. The Labour leader, though routinely presented as a humble and saintly man, is not even slightly self-deprecating. His absolute certainty about absolutely everything means that he is often visibly exasperated by questions that he considers an impertinence – as he was during the furore over his laying of a wreath in Tunis four years ago.

His default position is that the debate, whatever it may be, is over, and that he is on the right side. Doubt, reflection and irony enter a hostile environment when they enter the world of Corbynism.

This is one of many reasons why his remarks in 2013 about “Zionists” were so preposterous, as well as being so vile. It was undoubtedly antisemitic of Corbyn to claim that “having lived in this country for a very long time, probably all their lives, they don’t understand English irony”.

Facebook Twitter Pinterest John McCain campaigning for the Republican nomination in 2008. ‘The death of McCain reminds me of an extremely unlikely conversation I once had with him about the pop group Snow Patrol.’ Photograph: Stan Honda/AFP/Getty Images

This, as Anthony Julius shows in his book Trials of the Diaspora, is a depressingly common rhetorical device in this country’s long history of antisemitism: “encouraging [Jews] to accept that they have certain talents, certain resources, the possession of which is not quite consistent with an ideal conception of what it is to be English … Jews are then thought to expose themselves as literal-minded, unhumorous people, incapable of taking a joke”.

That Corbyn believes that Jews (or the “Zionist” subset to which he refers) do not appreciate irony is – well – spectacularly ironic. It suggests that his acquaintance with Jewish humour is not, perhaps, as rich as might be expected of one who claims to speak to, and for, all people who suffer from oppression. It also suggests that he does not actually understand what irony is.

In this case, Corbyn was referring to an earlier speech by Manuel Hassassian, the Palestinian Authority representative to Britain, and the reaction to it. Hassassian had argued that the long suffering of the Palestinians had led him to question divine impartiality. “You know, I’m reaching the conclusion that the Jews are the children of God, the only children of God, and the Promised Land is being paid by God! I have started to believe this because nobody is stopping Israel building its messianic dream of Eretz Israel [the Land of Israel] to the point I believe that maybe God is on their side.”

In response to the objections of unnamed “Zionists” – apparently the blogger Richard Millett was among those he was referring to – Corbyn said triumphantly that “Manuel does understand English irony and uses it very, very effectively.” But Hassassian wasn’t being ironic at all. He was being hyperbolic: exaggerating his doubts about God’s allegiance to press home his point about the extent and duration of Palestinian suffering. In Corbyn’s tortured logic, the fact that the so-called Zionists didn’t grasp the irony – which, ironically, wasn’t there in the first place – showed only that, in spite of all the years they and their families had spent here, they didn’t truly understand Englishness.

In an age of post-truth populists and digital echo chambers, there has never been a greater need for irony

I find it depressing that so much energy is still being expended trying to explain away Corbyn’s remarks, or to deconstruct them to his advantage. They are wearyingly literal: these people have been here a long time but do not study history and still have not learned all our English traits.

It need not be so. There is nothing inevitable in the flight of irony from political life. Indeed, one of the privileges of writing about politicians over the years has been to hear them – usually, though not always, in private – acknowledging the nonsenses and contradictions of what they do, taking aim at themselves and their follies. The sad news of the death of the US Republican John McCain reminds me of an extremely unlikely conversation I once had with him about the pop group Snow Patrol, the absurdity of which was not lost on either of us.

I wish this approach to politics were not increasingly rare. But it is. In an age of post-truth populists, fundamentalist fervour, digital echo chambers and history-hugging ideology, there has never been a greater need for irony and the shared understanding and intelligence that it fosters. Its decline is disastrously timed. It is, one might say, ironic.

• Matthew d’Ancona is a Guardian columnist