We live in a world in which many people have a dubious connection to reality. Asserting that something is true even when the evidence exists to contradict it has become a point of honour for scores of prominent people. They see themselves not as outliers but as brave adopters of a lonely position.

This was what I faced in the High Court of Justice when, as depicted in the film Denial, I was sued for libel by David Irving, a man who not only espoused Holocaust denial but who helped craft many of its central arguments. Ultimately, Irving had ¡°misstated historical evidence; adopted positions which run counter to the weight of the evidence; given credence to unreliable evidence and disregarded or dismissed credible evidence¡±.

There is, of course, nothing wrong with taking a minority position. Galileo was persecuted by the Church for asserting, based on observation, that the Earth revolves around the Sun. After being forced to recant, he is said to have protested: ¡°And yet it moves.¡± Whether he really did so is open to debate. That the Earth moves is not.

A in The New Yorker succinctly describes the situation. The host of a television quiz show called Facts Don¡¯t Matter tells one of the contestants: ¡°I¡¯m sorry, Jeannie, your answer was correct, but Kevin shouted his incorrect answer over yours, so he gets the points.¡±

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Consider the claim that Muslims in New Jersey danced in celebration on 9/11. No less a figure than Donald Trump asserts that this happened and that he saw it ¨C despite the fact that no footage has ever been found and that police officials have repeatedly and . During the presidential campaign, : ¡°I watched [as] thousands and thousands of people were cheering as [the World Trade Center] was coming down ¡­Now, I know they don¡¯t like to talk about it, but it was well covered at the time.¡± It was not. Trump might have seen of Arabs dancing, but the dancers were .

Trump is not the only culprit, however. During the Brexit campaign, Michael Gove, on being challenged that ¡°experts¡± found his conclusions untenable, : ¡°People in this country have had enough of experts.¡± The fact that Gove was once secretary of state for education made his comment stupefying. Then there are those who still insist that vaccines cause autism, even though The Lancet, which first published the claim in 1998, .

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But how does one fight such people? A few weeks ago I encountered this problem first-hand at my local gym. Someone said something absurd and I declared: ¡°That has as much validity as Muslims dancing in New Jersey. Never happened.¡± A man working out next to me said: ¡°Oh yes it did.¡± When I asked for his evidence he quickly responded: ¡°There is film footage. Fox News found it. I saw it.¡± An intrigued spectator immediately began working his smartphone. In a moment, he announced: ¡°Breitbart News says it doesn¡¯t exist.¡± Our informant may have known that if he had cited more mainstream sources, the man would have dismissed them as the untrustworthy ¡°liberal media¡±. Breitbart News is a far-Right news platform whose former executive chair, Stephen Bannon, is now Trump¡¯s chief strategist.

Yet still the man didn¡¯t back down, and I am not sure he could ever be convinced. He is not interested in evidence unless it comports with his pre-existing beliefs ¨C which, however irrational, make sense to him. He is the classic adherent to conspiracy theories: he has made up his mind and any information that challenges his foregone conclusions must be false. Overturning that with a rational explanation is virtually impossible ¨C especially when the paranoia that often comes with this mindset is laced with hatred, anger and vindictiveness.

The question is whether, by taking such people on, we are giving them precisely what they need to survive: attention. This was the dilemma I faced when I began to write about deniers. It is why, as depicted in Denial, many people insisted that I settle with Irving. Eventually, however, I concluded that giving him some precious oxygen was the price I had to pay to reach those who might be swayed by him. After all, whereas we might once have dismissed some of these folks as tinfoil hat wearers, today they use social media to find other followers.

I am not arguing there should be an orthodoxy about Holocaust history or any other topic. But there are differences between facts, opinions and lies. In the academy, it is considered rather unenlightened to argue that not everything is open to debate. But, as Galileo demonstrated, there are facts ¨C and scholars, teachers and all those who are disturbed by recent developments must be unafraid to proclaim that. It is not the conspiracy theorists at whom we should be aiming. It is the person in the gym working out next to them.

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Deborah Lipstadt is the Dorot professor of modern Jewish and Holocaust studies at Emory University in Atlanta, Georgia. Denial is currently showing in cinemas across the UK.