“In practice,” Evan Davis writes, “we evidently are quite happy to believe untruths.” Davis is stating what is, perhaps, the most indisputable fact regarding what has been trumpeted as the rise of a new kind of “post-truth” politics. Shrewdly, he describes the belief that we a living in a post-truth era as “an expression of frustration and anguish from a liberal class discombobulated by the political disruptions of 2016”. A catch-all term used by today’s liberals to describe upheavals that confounded their most basic beliefs, “post-truth” politics is like “populism” in implying that these unexpected shifts occurred because reason had been subverted. Duped by demagogues deploying new information technologies, voters disregarded argument and evidence in favour of manipulated emotion and fake news. The idea of truth was lost in a morass of relativism, and the politicians who controlled government for decades were abruptly dislodged from power.

It’s an appealingly simple tale, which many liberals are more than happy to believe. But if we have entered a post-truth era, when did this epoch begin? Matthew d’Ancona is highly specific as to the date: “2016 was the year that definitively launched the era of ‘Post-Truth’.” We have inhabited this new world for not much more than a year, but its dominant characteristic is all too clear. There has been “a crash in the value of truth, comparable to the collapse of a currency or a stock”. It’s not that politicians lie more than they did in the past. “Political lies, spin and falsehood are emphatically not the same as Post-Truth. What is new is not the mendacity of politicians but the public’s response to it. Lying is regarded as the norm, even in democracies.” But if there is such a thing as the post-truth era, it didn’t start last year with Brexit and Trump. It began with the Iraq war, which D’Ancona barely mentions. More than any other single event, it was this stupendous exercise in disinformation and denial that convinced the public that indifference to truth had become the norm in politics.

In Britain, it wasn’t only the dodgy dossier and the misuse of intelligence in the run-up to the war that produced toxic mistrust of government. It was the refusal of the chief architect of the war to acknowledge the disaster that followed. The consequences – the break up of the state of Iraq and a quantum leap in the power of Islamism – are irreducible facts. Yet Tony Blair remains adamant that the war was essentially benign in its effects. This may seem strange, but for Blair facts are irrelevant. As he put it in his speech to the Labour party conference in September 2004: “I only know what I believe.” This statement was a manifesto for post-truth politics. If each of us knows only what he or she believes, only the subjective certainty that Blair describes in the speech as “instinct” has any importance. When he insisted that Iraq was the ground on which the war against terrorism was being won, he was not lying. Judging by his behaviour, he lacks the ability to tell the difference between truth and falsehood to be capable of old-fashioned mendacity. Instead he was creating a form of hyper-reality – an imaginary world that, he could not help believing, was coming into being as a result of the war.

Tony Blair’s a press conference, responding to the Chilcot report on the Iraq war, July 2016. Photograph: Stefan Rousseau/PA

Blair’s assertion that the world is fashioned from our beliefs was echoed by an anonymous aide of George W Bush (widely thought to be Karl Rove) when in an interview reported in October 2004 he dismissed the “reality-based community” – “people who believe solutions emerge from your judicious study of discernible reality” – as no longer important in politics. “That’s not the way the world really works any more … We’re an empire now, and when we act, we create our own reality.” If the post-truth era is defined by the conviction that reality is constructed through the exercise of power, this is when it began to shape democratic politics.

The view that we create our own realities is, of course, the core of relativism. D’Ancona believes the ground was prepared for a post-truth age when postmodern philosophers propagated a way of thinking in which the gap between the real and the imaginary disappeared, abandoning “the central value of the Enlightenment” – the affirmation that truth is independent of opinion, and supremely worth pursuing. He overlooks the fact that modern relativism begins in Enlightenment thinkers. To mention only a few, Montesquieu argued that values are culturally variable, Marx insisted on the class origins of moralities and worldviews, while Karl Mannheim suggested that much of human knowledge consists of ideological perspectives formed in particular social situations. The postmodern assertion that reality is socially constructed is a footnote to the Enlightenment rather than a departure from it.

While the notion that we entered a new age 12 months ago is far-fetched, D’Ancona’s punchy polemic asks questions that are urgently topical and undeniably important. He is right that the internet and social media have transformed communication, giving disinformation and deception in markets and politics added potency. Davis considers how this has happened, and in a rich and probing analysis of the use and misuse of the media suggests the answer can be found in the economics of information. What seems like an irrational message may contain information of a subtle and tacit kind to which people respond. Even when what is being communicated has little or no cognitive content, there are rational explanations as to why such messages can be so effective. Distinguishing between post-truth, post-fact, nonsense and gibberish, Evans’s ambition seems to be to develop a general theory of bullshit.

George W Bush (left) with his White House adviser Karl Rove, 2007. Photograph: Jim Bourg/Reuters

One of the implications of his analysis is that bullshit can be found across the political spectrum. He devotes remarkably little attention to the fact, but liberals engage in bullshit as much as populists. While leave campaigners may have exaggerated Britain’s financial contribution to the EU, as he shows at length, remainers launched “Project Fear” – a naked appeal to the emotions deploying unverifiable figures about the economic consequences of leaving the EU that were plucked from the air. Few voters can have imagined they were making their decision on the basis of brute facts. In the event counter-productive, Project Fear was an example of incompetent bullshit – a larger category of discourse than Davis seems to think. Most voters made their decision on the basis of what was most important to them – in other words, they were guided by their values.

Davis says little about ethics. Perhaps, as an economist, he feels he has no business pontificating on the subject. Or maybe, like many liberals, he thinks what ethics demands is self-evident. Populism is, among other things, the return to politics of issues that had been depoliticised on the basis that only one view of them could possibly be right or rational. In the febrile liberalism of recent times, anyone who questions continued large-scale immigration must be ignorant of its economic benefits or else racist. Similarly, anyone who voted leave because they wanted greater self-government for Britain could only be an atavistic nationalist, blindly resisting the onward march to supranational government.

For these liberals the referendum wasn’t a democratic debate like any other, with both sides using emotive language and disputable arguments, but a clash between truth and the forces of darkness. When leave prevailed, such liberals suffered something like a collective nervous breakdown. How could their obviously superior rationality have failed to persuade? Either much of the population was incurably ignorant, or the debate had somehow been rigged. Over the course of the last year, the latter view has gained the upper hand. There was a time, not so long ago, when no self-respecting liberal would go anywhere near conspiracy theory. As Richard Hofstadter described in his seminal essay “The Paranoid Style in American Politics”, which first appeared in 1964, delusions of conspiracy thrived mainly in the fever swamps of the far right. Today the belief that politics is being controlled by dark forces is shaping the liberal mind. There is no consensus as to the identity of the conspirators – some have pointed to sinister big data companies, others identify Russia as the hidden hand. Even so, the clear implication is that without the intervention of covert actors the Brexit referendum and the US presidential election would have ended differently.

Contrary to those who think 'peak populism' has come and gone, despair will continue to fuel extremism in Europe

Political movements use all the resources they can command to achieve their goals, including new technologies, and no one who knows anything about Putin’s Russia can doubt that it will use any means at its disposal to subvert democratic politics. But this does not mean the historic shifts of the last year were engineered by cyber-leaks or psychometric profiling. Whatever Trump’s relations with Russia may be, it was decades of neglect in the American rustbelt – symbolised by Clinton failing to make a single visit to Wisconsin during her campaign – that stirred many voters in these crucial states to vote for Trump or stay at home. Emmanuel Macron’s victory occurred despite attempts to hack the French election. But a third of those who voted opted for Marine Le Pen, while more than one in 10 went to the ballot box only to submit a spoilt or blank vote. Contrary to those who think “peak populism” has come and gone, the despair of large numbers who are economically marginalised will continue to fuel extremism in Europe. Brexit is different from these electoral revolts in a number of ways, but it wasn’t a digital conspiracy that produced a landslide for leave in Sunderland.

It is droll to find liberals aping the paranoid mindset of the far right. To their credit, neither D’Ancona nor Davis endorses conspiracy theories. But such fantasies have something in common with the belief that we live in a time when politics has morphed into a wholly new mode. Talk of post-truth politics absolves liberals from responsibility for their defeats and failures, while sustaining the comforting illusion that the supposed normality of the recent past can be retrieved if only they fight harder for their certainties.

John Gray’s most recent book is Gray’s Anatomy: Selected Writings, published by Penguin.

• Post Truth: The New War on Truth … by Matthew d’Ancona is published by Ebury. To order a copy for £6.79 (RRP £7.99), or to order Post-Truth: Why We Have Reached Peak Bullshit ... by Evan Davis (Little, Brown, £20) for £17, go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.