THEY call it post-truth politics. Donald Trump is its most grotesque embodiment. But it is gaining a foothold here, as we saw from the hysterical reaction of pro-Brexit politicians to the High Court ruling that Parliament must be consulted as Britain leaves the European Union.

Politicians have always lied. But post-truth politics is something new. It was first defined in the 1990s, when US conservatives became alarmed that laws to reduce carbon emissions would hit them in the pocket. The way to fight back was to question the underlying science. The strategy was laid bare by a leaked memo to President George W. Bush, which suggested that public opinion would harden, once people came to believe that the science was settled.

Throughout the presidential campaign, Mr Trump told lie after lie, beginning with the claim that he started his business empire with a “small loan” from his father, when in fact he inherited $40 million. His falsehoods were too many to catalogue here. When the professional fact-checkers PolitiFact scrutinised his speeches, they found that 70 per cent of his factual statements were “mostly false”, “false”, or “pants-on-fire” untruths. Washington Post checkers also examined the speeches, and found much of the remaining 30 per cent also untrue.

There is, however, more to post-truth than a lack of factual accuracy. It also involves insult and innuendo, scares and smears, paranoia, and the psychology of conspiracy. The response of our populist press to the High Court ruling on the process of Brexit reveals that post-truth politics has taken root here, too.

The Daily Mail was particularly egregious, in a front page that set out — using the headline “Enemies of the People” — mugshots of the three judges who had ruled that Parliament must have a say on the mechanism of Brexit. You might have thought that the system of checks and balances between the executive, the legislature, and the judiciary embodied in the British constitution was a vital part of the national sovereignty that Brexiteers voted to restore. The Mail, however, preferred a diatribe about how “the will of the people” had been flouted by judges who — it informed its readers in a classic dog-whistle smear — included a gay Jew, a committed Europhile, and a “pal” of Tony Blair’s.

Mr Trump has for months been up to similar tricks in the United States, with slurs on Mexicans, Muslims, and menstruating women. Often he acknowledged that he was merely insinuating something by using the preparatory phrase: “A lot of people are saying . . .”. (He aired no fewer than 58 conspiracy theories in his campaign.) But many times he told barefaced lies with brass-necked audacity. Then, when his accuracy was questioned, he replied: “Well, they would say that, wouldn’t they?” Mostly, his audience roared with approving laughter.

What was at work here was an advertising-industry model of communication, in which the important thing is not what you say, but how you make people feel. So long as something feels true, that is enough. The American comedian Steven Colbert even coined a phrase, “truthiness”, to describe ideas that “feel right” or “should be true”. British politicians should be wary of following our sillier newspapers into this dangerously debased territory.

Paul Vallely is Visiting Professor in Public Ethics and Media at the University of Chester. www.paulvallely.com