Julie Etchingham: “Does the truth matter in this election?”

Boris Johnson: “I think it does.”

The studio audience, after a moment’s pause to take this in, burst out laughing, as if the Prime Minister had said something which coming from him, sounded preposterous.

In the polling after Tuesday’s debate, Jeremy Corbyn was seen as more trustworthy than Boris Johnson, though by a margin of only five percentage points.

Could this be Johnson’s Achilles’ heel? Downing Street knows he has a trust problem – but draws comfort from the knowledge that all politicians have a trust problem.

There are, however, considerable variations between politicians, and great leaders are often admired in part because they are seen to be straight with people. Winston Churchill made no secret in 1940 of the severity of the defeat Britain and her allies had suffered, or of the grievous sacrifices which would be demanded on the path to eventual victory.

Clement Attlee, who beat Churchill at the polls in 1945, communicated complete honesty of purpose, He could commend socialism to the British people because he himself was so evidently brave, patriotic and unselfseeking.

Churchill’s peacetime successor, Sir Anthony Eden, made matters worse, after the Suez debacle, by lying to the House of Commons, where in December 1956 he denied having colluded with the Israelis.

Margaret Thatcher was generally thought of as sincere. That is one of the things her critics held against her. She actually believed in things, and clung tenaciously to her beliefs, though on tactics she showed greater flexibility than has commonly been realised.

Even her supporters could find her directness of speech disconcerting. As I remarked in my brief life of her, “Her conversation rendered the standard English methods of of evasion – jokes, paradoxes, understatement, any number of ironical devices which enable one to avoid commitment – unusable.”

Johnson is a master of such methods of evasion. In order to avoid answering a serious inquiry on an inconvenient topic, he will launch into a riff on some extraneous matter which is so entertaining that the questioner may not object to being thrown off the scent.

This technique has served him well with newspaper interviewers, who are grateful for vivid copy, even if it has nothing much to do with the question put.

On live television, the refusal on occasion to give straight answers can become more obvious, and makes some viewers very angry. Peter Oborne watched the ITV debate and demanded: “Is the BBC going to call out Johnson as a liar?”

Oborne went on: “He’s lied to the British people about the NHS tonight. That’s a pretty dark thing to do.”

This topic has long excited Oborne’s ire. In 2006 he published a book called The Rise of Political Lying, in which chapter four is entitled “The Lies, Falsehoods, Deceits, Evasions and Artfulness of Tony Blair”.

It probably indicates some sort of weakness in my own character that I cannot share Oborne’s indignation.

Certain formalities have to be observed. In the Commons, each member has to treat the others as honourable. Otherwise debate would become impossible. One cannot have an argument with an opponent one dismisses as a liar.

But that is exactly why the shout of “liar” should not be lightly uttered. Donald Trump’s opponents did themselves enormous harm by condemning him as a liar, for that gave them an excuse to stop asking why what he said was so appealing to, for example, American car workers who feared their jobs were going abroad.

Trump’s turpitude distracted his opponents from the task of examining his proposals. By dismissing him as a disgusting human being, Hillary Clinton fell into the trap of accusing his supporters of being “a basket of deplorables”, a description which made them even less inclined to listen to her.

Dismiss Johnson as a liar if you wish, but very soon you will find yourself uninterested in grasping why his message on Brexit appeals to millions of voters who do not think of themselves as Conservatives. Character assassination displaces comprehension.

The wider public know that literal truth is seldom to be expected from politicians. The art of persuasion, even the art of telling the truth, is more mysterious than that. Dickens told the truth by exaggerating it. So too, with cynical but romantic wit, did Disraeli. So too, with tremendous moral seriousness, did Gladstone.

It was disreputable of the Conservative press office to rebrand one of its Twitter accounts as a fact-checking service during the ITV debate, and it was also stupid, not just because the subterfuge was sure to be discovered, but because such debates cannot be settled by an appeal to the facts.

The judgments involved in deciding whether or not to trust someone – Prince Andrew, for example – are far more complicated, and entail trying to reconcile a swirling mass of often inconsistent considerations, moral, historical, psychological and so forth.

Voters understand this better than fulminating pundits do. Here is a woman in West Bromwich, quoted in a Vox pub piece published a fortnight ago:

“If I was going to vote, I’d vote for Boris Johnson because he’s a fool.

“I don’t care that he’s lied and cheated because that is his way and I support Boris.

“I will definitely vote for Boris, liar, cheat and fool! And for Brexit! I want to get out.”

In other words, voters can allow – or not – for a candidate’s frailties, and may prefer to be led by a Prime Minister who does not pose as a pillar of rectitude.