There was a time when, even if we disagreed about the remedy, we did at least agree that the patient was sick. We might argue about the meaning of this or that event, but all sides usually accepted that the event in question had at least happened. No longer. A new and unsettling dimension has entered our collective, and global, conversation.

In the US James Comey, the former FBI director sacked by Donald Trump, released his memoirs and did a round of TV interviews. The FBI has not always enjoyed or deserved public trust – under its founder, the bigoted and brutal J Edgar Hoover, minorities and trade unionists had good reason to fear it – but in recent decades it came to be seen as one of the trusted arbiters of American life, a non-partisan referee whose word would be accepted. That would be truer still of the figure at the helm, the nation’s top law enforcement officer.

Now, of course it’s natural that Comey’s book would not be hailed as a definitive, objective account of recent, highly contested events. It was not sworn testimony, but an autobiography with all the self-flattery and exculpation associated with that genre. But what was striking was the casual dismissal even of the facts in the book. Trump and the Republican party instantly trashed the ex-director – himself a Republican – launching a website: lyincomey.com. You can argue about Comey’s judgment and his self-righteousness – which make parts of the book read like reflections of The Simpsons’ Ned Flanders – but his opponents go further, waving aside even that which is verifiably true.

The more egregious example is also the more serious. At a concert in Barcelona, the former Pink Floyd star Roger Waters accused the Syria Civil Defence, or White Helmets – the volunteers who pull survivors from the rubble of bomb attacks, and are widely credited with saving thousands of civilian lives – of being “a fake organisation that exists only to create propaganda for the jihadists and terrorists”.

That claim, which has been repeatedly debunked, was instantly applauded and spread by the same crowd of pro-Russia voices on the far left and far right who have served so dutifully as Assad’s online cheerleaders. To them, Waters was a hero for daring to speak an unpopular truth. For everyone else, a once admired musician had joined the ranks of conspiracist cranks and apologists for a murderous dictator.

The remark by Waters fitted a story the Assad denialists have been telling for a while: that the chemical attack in Douma was fake, staged by the White Helmets and their western friends. Russian state TV even aired footage that, it claimed, showed a film set where such atrocities are cooked up. (The pictures were, in fact, taken from the set of a Syrian feature film.)

The former Pink Floyd star Roger Waters accused the White Helmets of being “a fake organisation that exists only to create propaganda for the jihadists and terrorists”. Photograph: Pacific Press / Barcroft Images

Remember, these voices are not simply saying western air strikes were the wrong response to the killings in Douma. They are saying there were no killings in Douma. Or there were, but they didn’t involve chemical weapons. Or they did, but rebel groups were responsible. Or the whole thing was staged by Britain. The line changes with dizzying frequency.

The point is, whether it’s Comey or Syria, we are now in an era when the argument is no longer over our response to events, but the very existence of those events. It was the same story after the mass school shooting at Parkland, Florida, in February. A loud chorus of rightists was not content merely to denounce proposed gun control measures: they claimed the massacre had not happened at all, and that all those grieving parents and teenagers were “crisis actors”.

These are symptoms of a post-truth disease that’s come to be known as “tribal epistemology”, in which the truth or falsity of a statement depends on whether the person making it is deemed one of us or one of them. According to the writer David Roberts, “information is evaluated based not on conformity to common standards of evidence or correspondence to a common understanding of the world, but on whether it supports the tribe’s values and goals and is vouchsafed by tribal leaders. ‘Good for our side’ and ‘true’ begin to blur into one.”

Of course, we all suffer from confirmation bias, and all of us listen more to our friends than to our foes – but this goes further. Recall the Trump spokesperson Scottie Nell Hughes, who declared: “There’s no such thing, unfortunately, any more as facts.” For many Americans, Hughes explained, Trump’s tweets, regardless of the evidence, “are truth” simply because they come from him.

It fits that social media is the weapon of choice. Its algorithms are proven to favour virality over veracity, spreading false stories faster and wider than true ones. A mysterious pro-Assad tweeter, with no other traceable existence online, has nearly as many followers as the BBC’s Middle East editor. Meanwhile, the top story on Google News the morning after the US presidential election hailed Trump as the winner of the popular vote – even though he had lost it by nearly 3m votes. The tribe tells itself what it wants to hear.

The desire to stand with your fellow Republicans or Assad apologists is so great you’re ready to throw out not only your opponents’ arguments but their evidence too. And because trust in all the previously respected referees – from the media to human rights organisations to the FBI – has collapsed, you refuse to believe anything and anyone that contradicts you.

The danger this poses is clear. Comey’s moment will pass, but eventually his predecessor at the FBI, special counsel Robert Mueller, will deliver his report into the Trump campaign’s alleged collusion with Russia. Already Trump’s Fox News outriders are trashing Mueller – another Republican-appointed straight arrow whose honesty has never been credibly questioned – branding him biased, corrupt, even “the head of a crime family”. That will make it easier to ignore his findings, however damning. If Richard Nixon were in office now, even Watergate would not be Watergate: his own tribe would simply dismiss the hard, factual evidence against him.

If Nixon were in office now, Watergate would not be Watergate: his tribe would simply dismiss the hard evidence

As for Syria, Waters and the rest may seem like fringe voices, but such thinking eventually leaks into the mainstream. On yesterday’s Question Time, Emily Thornberry echoed the Russian claim that chemical weapons inspectors were being kept out of Douma by UN red tape and health and safety rules – when in fact it is Assad and Russia keeping them out: long enough, presumably, to ensure that by the time they’re granted access the crucial evidence will have been cleared away.

It makes for a chilling landscape, a world where atrocities are committed twice over – once when they are done, and again when they are denied. Not decades later, but even as the dead are still being buried. The great division of our time may not, after all, be between left and right or open and closed, but something more fundamental still: between what is true and what is false.

• Jonathan Freedland is a Guardian columnist