How does an honest person deal with an opponent who is a liar? Donald Trump raises this problem in acute form, and on neither side of the Atlantic have his critics yet shown the slightest sign of working out how to respond to his outrageous behaviour without playing into his hands.

The American media yesterday continued, with characteristic naivety, to condemn Trump himself, and his spokesman, Sean Spicer, for telling lies and making “false claims”. British liberals such as Sunny Hundal took to Twitter to criticise the BBC for being less trenchant than the New York Times in its reporting of Trump, and therefore showing itself “incapable of dealing with this era”. Nick Cohen said the serious press and the BBC “cannot be impartial between truth and lies”.

For the first, quite natural answer to the problem is to expose the other side’s mendacity, and wait for justice to take its course. Fact-checking is regarded by serious American publications as a professional obligation of the most sacred kind, and they duly applied this rigorous discipline to every word uttered by Trump.

Their approach cannot be said to have been a complete success. For example, on 26 September, after the first presidential debate, the New York Times published a piece called “The Lies Trump Told”, listing no fewer than 26 lies, and ending with the words: “He lied about calling women ‘pigs’. He lied about calling women ‘dogs’. He lied about calling women ‘slobs’.”

You could click on any of these lies in order to obtain the evidence the paper offered for it. But although this may have convinced a few readers that Trump was an even more disgusting and untrustworthy person than they already thought, it fell a long way short of stopping the candidate in his tracks. In the unlikely event of Trump’s supporters turning to the New York Times for enlightenment, they would have noticed that it loathed him, and would have discounted what it said.

In the face of an opponent as cavalier as Trump, fact-checking is like trying to deal with a football hooligan by asking him to be a bit quieter, and then objecting in a prosy tone to his use of foul language. The hooligan takes a mischievous delight in winding you up, and is so quick-witted and shameless about saying the very things which distress you most, that you are likely to lose your temper, and become drawn into a shouting match.

The American elections and British EU referendum degenerated into the most ignoble and repetitive exchanges of insults in living memory. They demonstrated that accusing your opponent of lying is worse than useless.

For this implies that you consider yourself so high-minded, and your opponent so low-minded, that you cannot even begin to talk to each other. You are ruling out debate. As a liberal, you believe in the value of reasoned argument, and expect it will lead over time to the spread of liberal values. But Trump is in your eyes so despicable that you renounce your weapon of choice.

As an expression of moral outrage, the denunciation of an infuriating and unscrupulous opponent is understandable. But from a political point of view, it exposes you to great perils. It means you are in no fit state to understand why your opponent appeals to large numbers of voters.

Angry condemnation prevents you from detecting those parts of the other side’s case which are so persuasive you should consider adopting them, and also those which are so weak you ought to be able to destroy them. If you are not careful, you will convey the impression that your opponent’s supporters are liars too.

Hillary Clinton is a highly professional politician, but on 9 September she allowed her tongue to run away with her at the LGBT Hillary Gala held in New York City:

“You know, to just be grossly generalistic, you could put half of Trump’s supporters into what I call the basket of deplorables. Right? The racist, sexist, homophobic, xenophobic, Islamaphobic – you name it. And unfortunately there are people like that. And he has lifted them up. He has given voice to their websites that used to only have 11,000 people – now 11 million. He tweets and retweets their offensive hateful mean-spirited rhetoric. Now, some of those folks – they are irredeemable, but thankfully they are not America.”

The following day Clinton expressed regret for saying “half” of Trump’s supporters, but the damage was done, and the “basket of deplorables” phrase will haunt her to her dying day.

The second drawback of shouting “liar” is just as bad. You don’t just ignore your opponents’ case: you fail to make your own arguments too. Moral superiority supplants the need to persuade people. Brandishing your moral club, you require the immoral members of society to fall into line.

If you are not careful, you start to sound like a canting, self-righteous liberal, so convinced of your own rectitude that you expect unquestioning obedience, and will not listen to those who defy your authority. The Pharisee is an unattractive figure, and you and your family and friends know you are not like that. But how does anyone else know it?

Your opponents are delighted to be called liars, or denounced in other ways, for this draws attention to what they are saying. As Arron Banks, who funded Leave.EU, happily avowed when interviewed by Martin Fletcher for the New Statesman, that campaign learned from Trump “how to use and abuse the mainstream media with their phoney outrage”. Trump gained prodigious quantities of free publicity by saying things his opponents found so repugnant they could not resist denouncing him.

And what scope this gave the Trump team for counter-attack. Clinton could be denounced as a hypocrite: a wealthy, greedy, privileged insider with an intolerable sense of entitlement, who owed her position to her husband and was just pretending to be on the side of ordinary Americans.

Some incomprehensible story about her emails was dragged up, and because it was incomprehensible, she could not kill it off. She was now in a tricky position, so she and her allies produced further evidence that Trump was a truly dreadful person. The number of women who testified that he had behaved abominably towards them multiplied.

Yet this did not finish him off. For part of the problem with Trump is that although he is an habitual and frequent liar, he is also deeply sincere.

Trump’s faults give him an authenticity which is part of his appeal. His supporters see a man who is not pretending to be more perfect than they are. They also see someone who is not constrained by the fear of being called racist, so can express the worries they have about China and Mexico.

He practices a greater freedom of speech than an uptight liberal would dream of using. He becomes, if not the perfect protest candidate, at least a very attractive one, for his moral vulgarity inflicts extreme pain on the liberal establishment, and undermines its power to determine the terms of the debate.

For the avoidance of doubt, I should say I find various of Trump’s remarks repulsive. I am not trying, out of sheer perversity, to portray him as a noble person. In his inaugural address, he behaved like a braggart. But calling him a liar did not destroy him, and it was naive to imagine it would.

Here we reach the heart of the matter, which is that all politics involves a degree of deception. Whether one is virtuous or corrupt, there is no getting away from putting on some kind of an act. This point is made with learning and subtlety by David Runciman in Political Hypocrisy (2008), in which he surveys what thinkers including Hobbes, Mandeville, the American Founding Fathers, Bentham, the Victorians and Orwell, have had to say on this subject.

We tend to think of the Italians and French – pre-eminently Machiavelli and Rousseau – as the masters of pulling aside the mask of virtue and showing what lies behind it. But Runciman reminds us of the highly sophisticated tradition of such writing in English, which is generally neglected by English-speaking liberals.

He observes that “democratic politics appears to have a tendency to produce complicated, and compromised, choices…not between truth and lies, or sincerity and hypocrisy, but between politicians who are sincere but untruthful and those who are honest but hypocritical”. In 2016, Trump was the sincere but untruthful candidate, Clinton the honest but hypocritical one.

When I spoke after the campaign to Runciman, he observed that Clinton’s truth-telling “makes the public want to throw up”. For “the type of hypocrisy that really turns the public off is double standards”, and she used “one kind of talk in public”, and quite another in her leaked emails, which were “devastating” because they revealed “a whole secret world of backscratching”.

Trump, on the other hand, “is authentic in the way that he does not moderate himself for a public audience”. As for his lies, people can forgive those, for they expect all politicians to lie, and consider them as bad as each other.

Runciman pointed out that “you can be authentically cavalier about the truth – people know what you’re doing”. It is wrong, he added, to see politics as a “binary” choice between truth and lies: there is “a huge grey area” which cannot be categorised in that way.

Voters are nothing like as stupid, or as literal-minded, as some political commentators, and fact-checkers, assume them to be. They can see a politician’s faults, but decide that he or she is the right, or least bad, person to vote for.

Idealists yearn for a politics in which telling the truth, and pursuing it with unwearying fidelity, is all that is required. The Liberal Democrats have often adopted this high-minded pose, which is why their betrayal on tuition fees did them such terrible damage.

Tony Blair managed for a rather longer period to convince people that he was an honest man. In The Rise of Political Lying (2005), which is a furious denunciation of Blair’s mendacity, Peter Oborne observes how, at the beginning of Blair’s leadership, everyone was prepared to give him the benefit of the doubt:

“Books on New Labour always lazily state that the abolition of Clause Four was announced in…the inaugural party conference speech in 1994, the same one in which he made his dramatic pledges about truthfulness and honesty. But that is not the case. What actually happened was rather different. Towards the end of his speech Tony Blair embarked on a purple passage, ending with the proclamation: ‘Let us say what we mean, and mean what we say.’

“Almost at once he declared that he and John Prescott were to propose a new statement of the objectives of the Labour Party to ‘take its place in our constitution for the next century’. Tony Blair received a standing ovation and the great majority of delegates walked out of the conference hall without the faintest idea that anything out of the ordinary had happened…

“Tony Blair never announced that this innocuous sounding statement of aims meant the end of Clause Four. He left that task to his spin doctors, who briefed the press and others immediately after the speech. Far from saying what he meant, as he brazenly claimed, Blair quite brilliantly pulled the wool over everyone’s eyes. This evasiveness was deliberate, a tactic to avoid a hostile reaction from the conference floor… Very few of us in the hall cared to reflect on this at the time.”

Blair’s adviser, Philip Gould, relates in The Unfinished Revolution (1998) how the change was sold on the day of the speech to key Labour figures:

“That morning saw a continual flow of shadow Cabinet members coming to Tony’s room to be informed of what he intended to do. I had to be kept hidden; the change to Clause IV was combustible enough without my involvement in it being too apparent. Once, I hid in a wardrobe.”

There’s straight politics for you: a key adviser concealed in a wardrobe like an illicit lover. One can argue, if one wishes, that this trickery was justified by results, though it has left Labour in the hands of Jeremy Corbyn: the activists’ revenge for being ignored by every leader since Neil Kinnock took over in 1983 from Michael Foot.

But for the purposes of this argument, it is sufficient to agree that Blair was nothing like as honest as he claimed to be. He too was an actor, which is another word for hypocrite, and for a long time, there was among most of his audience a willing suspension of disbelief in his performance.

Then came Iraq, and with it the question of whether or not Saddam Hussein had weapons of mass destruction. How wretched that this great question of war and peace, involving the widest and most difficult judgments, was reduced to a merely factual inquiry. To understand any serious question, the facts are never enough, especially when one considers that deciding which facts are relevant cannot itself be an exclusively factual judgment.

Before the 1979 election, Labour claimed the Conservatives planned to double VAT if they won. Margaret Thatcher assured voters, “we will not double it.” Within months of her becoming Prime Minister, VAT was raised from eight per cent to 15 per cent. She had been, at the very least, disingenuous.

Journalism, I ought here to agree, is just as hypocritical as politics. The great pundit makes a show of being more far-sighted, noble and unprejudiced than his or her colleagues will tell you is actually the case.

The unknown leader-writer (a role I played for several years) pretends to olympian knowledge and insight. I remember writing a leader about the elections in Portugal on the basis of a short telephone conversation after lunch with one person who appeared to have some knowledge of the subject: two conversations would have been confusing.

The over-worked reporter gets together such facts as can be obtained in the short time available, and presents them as if they are the most exciting and significant aspects of the story.

The prudent reader is well aware that the newspaper is incapable of giving the complete and unvarnished truth, and turns to the most enjoyable bit of it, which is the sports section or the crossword.

And yet there are always some journalists and readers who fall for what one might call the Fact-Checking Fallacy: the belief that as long as the facts have been exhaustively checked, one will arrive at the truth.

An inverted form of this fallacy was detectable in the liberal reaction to Trump. Because quite a few of Trump’s facts were lies, it was assumed that drawing the point to public attention would infallibly destroy him.

But the public turns out to take a more realistic and less literal-minded view of facts. It knows that drawing a clear line between justified and disgraceful falsehood is impossible, for circumstances alter cases.

My local Co-op offers “Irresistible Tomato and Lentil Soup”, at a price of £2.59 for 600 grams. But although I have without difficulty resisted buying this soup, which appears to me to be over-priced, I do not dismiss its sellers as liars. I understand the spirit in which they are puffing their soup.

On Monday 13 May 1940, three days after becoming Prime Minister, Winston Churchill declared in the House of Commons: “I have nothing to offer but blood, toil, tears and sweat.” That again was not literally true – after all, he was offering extensive experience both of war and of high office, as well as astonishing oratorical gifts – but everyone understood the meaning of his words, and felt they were a true statement of the spirit in which he undertook his commission, and of the desperate position in which the nation found itself.

On 8 March 1994, a Conservative minister, William Waldegrave, told a select committee that “in exceptional circumstances it is necessary to say something that is untrue to the House of Commons. The House of Commons understands that and accepts that.”

Waldegrave offered the example of Jim Callaghan in 1967 as Chancellor of the Exchequer, presiding over a fixed exchange rate, planning to devalue, but unable to say, in answer to a direct question, that he was going to do so, for if he had said that, devaluation would have occurred instantaneously.

All this was true, but Waldegrave, who has written about this incident in his memoir, A Different Kind of Weather, was incredibly foolish to say so. Labour at once raised a storm of moralistic protest, with Callaghan insisting he had never told an untruth, and the party’s leader, John Smith, declaring: “We don’t really need Mr Waldegrave to tell us the Tories don’t tell the truth. We know they don’t tell the truth. And that is why no one will ever believe their promises again.”

These protests were hypocritical, but in a sense they were also justified. For the Commons works on the understanding that ministers cannot lie to it. If that pious doctrine were ever to be abolished, there too debate would become impossible.

In the Chamber, one is forced to engage with the other side’s arguments, and cannot take refuge in the cheap cop-out of condemning them as liars. To pretend that your opponents are honourable is not just a quaint survival from an earlier age: it is much more useful than to point out that they are scoundrels.

Democracy demands that various repetitive hypocrisies, in which voters as well as candidates are implicated, are observed. At elections, we expect our politicians to promise, as forcefully and convincingly as they can, that they have the solution to our problems. The pressure we place on them to over-promise is almost irresistible.

We then blame them, a short time afterwards, for breaking their promises. The saviour serves also as the scapegoat, and Trump is unlikely to be exempt from this cycle.

But meanwhile, the best approach to Trump, and even the best way to embarrass him, is to take him seriously. We should examine his plans for more infrastructure, or less immigration, or a revival of American manufacturing, and agree or disagree with him on a case by case basis.

Simply to dismiss him as a liar is to underestimate him. To treat that as an adequate verdict on the new President is frivolous.