Sixteen years ago, I sat in court 73 of the Royal Courts of Justice in London and felt the ground crumble beneath my feet. I was following the libel trial brought by David Irving, the Holocaust denier and “pro-Nazi polemicist” – to quote the judge’s eventual verdict – against Penguin Books, which had dared publish a text which told the truth about him.

I watched as Irving discarded the usual rules of evidence. The eyewitness testimony of survivors was dismissed as lies. Confessions by the guilty were waved away as fake. Inconvenient documents were written off as forgeries. All that was left was what he wanted to believe.

At the time, it struck me that Irving was threatening something greater even than the memory of the Holocaust: he was undermining the very idea of facts, history and truth. If every item of evidence could be rubbished as bogus, then how could anyone ever prove anything? How would we know that Henry VIII had six wives or that Napoleon fought at Waterloo?

Hence the queasy sensation the ground was falling away. As I wrote at the time: “If we start to doubt corroborated facts, how can we prevent ourselves being swallowed up in doubt, unable to trust anything we see? It might all be a conspiracy, a legend, a hoax. This is the bizarre, never-never world inhabited by David Irving. Now the court has to decide: is this our world too?”

That feeling returned to me this week, brought back by a screening of the film Denial, released next month, which dramatises the Irving trial of 2000. But it was also prompted by the reaction to events in Aleppo and, more widely, by the way 2016 has punched truth in the face, leaving it bruised and bleeding.

As Aleppo endured its final agonies, the simple act of circulating any account – a video, a photograph, a news report – would trigger an unnerving response. Someone, somewhere would reply that the photograph was doctored, the source was a stooge, the rescued child was not really a child or not really rescued.

Of course, we’re used to people taking different sides on conflicts far away, arguing bitterly over who is to blame. At its most extreme, it results in a newspaper like the Morning Star sinking so low that it hails the human devastation of Aleppo – where every hospital was bombed and where the slaughter of civilians became routine – not as a crime, but as a “liberation”.

But this is about more than assigning blame for this death or that bombing. This is about refusing to accept that the death or bombing occurred at all. This is about defenders of Bashar al-Assad, and his Russian and Iranian enablers, coming on television to say that what is happening on the ground is not happening, that it is all an illusion. The late US senator Daniel Patrick Moynihan used to say: “You’re entitled to your own opinion, but you’re not entitled to your own facts.” But that distinction seems to have broken down. Now people regard facts as very much like opinions: you can discard the ones you don’t like.

This problem is not confined to Syria. This week the CIA joined 17 other US intelligence agencies in concluding that Russia was behind the hacking of Democratic emails, adding its conclusion that Moscow had done so in order to tilt the US election towards Donald Trump. “Ridiculous,” said Trump, who has not looked at the CIA’s evidence and has refused to receive the daily intelligence briefing provided for all incoming presidents on the grounds that he is “like, a smart person”.

After Iraq and the weapons of mass destruction that never were, plenty are understandably wary of accepting the word of the intelligence agencies. But Trump’s scepticism – cynicism is a better word – operates on a different level. “Nobody really knows,” he says about the hacking charges, the very words he uses about climate change, in the face of a vast body of evidence. Recall that he also says that he won the US popular vote “if you deduct the millions of people who voted illegally”, a flagrantly false claim for which there is no evidence whatsoever.

We’ve been calling this “post-truth politics” but I now worry that the phrase is far too gentle, suggesting society has simply reached some new phase in its development. It lets off the guilty too lightly. What Trump is doing is not “engaging in post-truth politics”. He’s lying.

Worse still, Trump and those like him not only lie: they imply that the truth doesn’t matter, showing a blithe indifference to whether what they say is grounded in reality or evidence.

Back in 2000, such a posture left you isolated in that never-never world inhabited by Irving. Today you’ll have a US president, a British foreign secretary (never forget the £350m Brexit bus), as well as a ready army of fake news consumers to keep you company.

How has this happened so quickly? Technology has clearly played a part. Social media allows fact deniers to spread their anti-history fast and wide. Distrust in elites is also central. People are no longer prepared to take their leaders’ word on trust. Iraq poisoned that relationship, but its roots go deeper. In the US, Watergate broke public faith; some suspect the rot set in even earlier, with the Kennedy assassination.

But a crucial shift is surely the trend towards deeper and more bitter partisanship. Once people have aligned themselves with a tribe, studies show their first instinct will be to believe what favours their side and disbelieve what favours their opponent. One telling poll this week found Vladimir Putin’s approval ratings have shot up among US Republicans. They once hated him, but now their guy Trump is Putin’s buddy, they’re ready to see the Russian autocrat in a favourable light – and to ignore all evidence to the contrary.

This is making our public sphere a dizzying place. Without a common, agreed set of facts, we can hardly have any kind of public conversation at all. Writer David Roberts, who has a good claim to have coined the phrase “post-truth” [see footnote], says that these days: “There are no more referees. There are only players.”

We have no group of non-partisan arbiters, trusted to define at least the factual basis for our collective discussion. When actual judges enter the picture, as they have in the Brexit article 50 case, one side rushes to discredit them, branding them as biased, ideological partisans, no less tainted and untrustworthy than everyone else: enemies of the people.

What’s so odd about this is that we are happy to accept that there are facts, and judges of fact, in every other aspect of our lives. Philosopher Quassim Cassam notes if a car mechanic says your brakes have broken, you don’t denounce him as biased and drive on: you listen. If a doctor says you have a tumour, you don’t mock him as a member of the medical elite. We even accept expert judgment on reality TV: no one minds Mary Berry deciding who should win Bake Off.

Only in the political realm have we somehow drifted into a world in which no one can be trusted, not on questions of judgment, nor even on questions of fact. But we cannot live in such a world. Evidence, facts and reason are the building blocks of civilisation. Without them we plunge into darkness.

• This footnote was added on 20 December 2016. Oxford Dictionaries dates the first use of “post-truth”, with the implication that truth has become irrelevant, to a 1992 essay by the playwright Steve Tesich in The Nation magazine.