In psychology, there is something called the “illusory truth effect”.

Basically, it goes like this: if people hear the same false information repeated over and over, they often come to believe it.

Without being able, or wishing, to look into the minds of Boris Johnson and his adviser Dominic Cummings, one wonders whether or to what extent this phenomenon is at play when it comes to the Tory Brexit push and the British nationalist fervour it has unleashed.

We had the “Get Brexit Done” drive ahead of last December’s election. And then the claim that Brexit had been done on January 31 in some kind of final way.

In the most technical sense, Brexit had been done. However, in reality of course, little if anything changed on January 31. And, as we have seen since, it is anything but done, with the UK having for now been protected from Brexit’s huge ills by the transition period which runs out on December 31.

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Yet the electorate has been told by the Conservative Government repeatedly that Brexit has been done, with a picture painted for people that this has already happened in a meaningful and practical way. And it looks like many of the electorate believe that it has been done, in reality. Remainers who dare to highlight the Brexit folly are told that it has been done, and they should move on, and basically “get over it”, often discourteously.

It is not plain whether this is just a manifestation of the intolerance and entrenched views around Brexit, or whether the Leavers actually believe this is the case. If it is the latter, it is difficult to see how they could not know the Conservative Government remains locked in talks which could either result in a deal (albeit one which at best mitigates the damage to a small extent) or precipitate a chaotic no-deal departure from the European single market.

After all, Mr Johnson’s “oven-ready” Brexit deal, promised last autumn, has not materialised.

When it comes to repetition on the part of the Prime Minister and the arch-Brexiters, one big focus for them has been a relentless reiteration of the message that the UK will do just fine with a no-deal exit from the single market.

We heard this chat from Mr Johnson again at the weekend, even as he agreed an extension of the hugely protracted talks with the EU on a free trade agreement following a phone conversation with European Commission president Ursula von der Leyen.

This one-month extension, while obviously welcome given the Tories’ seeming indifference towards whether they leave with a deal or not, looks to put the crunch period back to later this month.

The UK and EU have emphasised that significant differences remain, after what appears to have been only limited progress in the talks last week.

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Major differences persist on the crucial level playing field issue. The EU, understandably, remains determined to ensure fair competition with the neighbouring UK, in terms of major issues such as state aid and standards. This continues to look like a reasonable request, given the UK is seeking a comprehensive trade deal with the biggest free trade bloc in the world. It cannot have the benefits of relatively free access to the huge single market without obligations.

And, of course, the Brexiters continue to bang the drum (loudly) on the emotive fisheries issue, which still looks extremely difficult to resolve amid the posturing. This is a classic issue for Brexiters with all the opportunities to assert sovereignty (which was never in question anyway) and shout loudly about trawlers from other countries taking the British fish.

Michel Barnier, the EU’s chief negotiator, on Friday cited “persistent serious divergences on matters of major importance for the European Union”.

He flagged the requirement for “solid, long-term guarantees of open and fair competition”.

Mr Barnier added: “Our new economic partnership must be underpinned by clear rules. These rules must be operational and credible. That requires effective enforcement mechanisms, in particular on state aid, and a commitment towards non-regression from social, fiscal, environmental and climate standards.”

The UK has, of course, made much of its wish to highlight its sovereignty, which never seemed in doubt as a member of the EU, during these negotiations.

But Mr Barnier said of the EU’s requirements: “This is possible while fully respecting the regulatory autonomy and sovereignty of both parties.”

He also cited the EU’s requirement for “a stable, sustainable and long-term agreement on fisheries, enabling the United Kingdom to further develop its fishing opportunities, while ensuring the sustainable use of resources and protecting the activities of European fishermen and women”.

For his part, UK chief negotiator David Frost said: “On the level playing field, including subsidy policy, we continue to seek an agreement that ensures our ability to set our own laws in the UK without constraints that go beyond those appropriate to a free trade agreement.

“There has been some limited progress here but the EU need to move further before an understanding can be reached. On fisheries the gap between us is unfortunately very large and, without further realism and flexibility from the EU, risks being impossible to bridge. These issues are fundamental to our future status as an independent country.”

Was the UK not independent before? Surely it was Lord Frost.

By late October, it will be little more than two months until the end of the transition period. This is a dismal situation for businesses and households, in terms of protracted uncertainty, given they are already having to deal with the huge economic fall-out from the coronavirus pandemic and second-wave woes and fears.

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It did not look as if it could become much more clear that it would have made sense to take the EU’s olive branch of extending the transition period beyond December 31, by the July 1 deadline. But the sense of such a move has become ever more evident by the day. That said, it looks to be too late for this now, with the UK facing an inevitably huge effect when Brexit becomes reality on December 31, whether it has a deal by then or not.

The Theresa May government forecasts show major damage to the UK economy from Brexit on any scenario in which the UK leaves the single market, something the arch-Leavers remain utterly determined to do for reasons of ideology which defy economic reason.

Investment banking giant Goldman Sachs said on Monday that a “thin” Brexit trade deal was likely.

A “thin” deal would obviously mitigate the damage to a lesser extent than a broad and deep, comprehensive one.

Goldman highlighted its “core view” that “a ‘thin’ zero-tariff/ zero-quota trade agreement will likely be struck by early November, and subsequently ratified by the end of December”.

However, it warned that “the risk of a breakdown in negotiations cannot be ruled out”.

And it flagged its continuing belief that “the perceived probability of ‘no deal’ will persist beyond the next European Council meeting in mid-October”.

Such a situation of protracted heightened worry is hardly a good one for millions of UK businesses and households already having to handle far too much uncertainty amid the coronavirus crisis, as the clock ticks ominously towards December 31.

In the context of the UK doing just fine without a deal, the Brexiters have seemed keen to cite what seems in their view to be almost unlimited potential to trade more with Commonwealth countries. Their notion of the importance these days in a global context of the good old British Commonwealth seems wildly outdated. The idea that it can fill the yawning gap left by giving up frictionless trade with the biggest free trade bloc in the world, and from losing free movement of people between the UK and EU countries and all the benefits that brings including immigration, is truly, truly incredible.

The UK has talked about how it wants a “Canada-style” deal with the EU. For its part, the EU has highlighted a basic geographical issue with this. Canada is not next to the EU.

Mr Johnson has talked in recent months about an “Australian-style arrangement” with the EU. This would, of course, amount to no kind of deal at all but rather a move to World Trade Organisation terms.

At the weekend, Mr Johnson was more explicit about an Australian-style outcome meaning WTO terms of trade with the EU. And he told the BBC that, while he did not want this result, “we can more than live with it”.

He added: “I think we can prosper mightily under those circumstances.”

These statements, while utterly baffling from any sort of reasoned economic perspective, are entirely in keeping with the message from the Brexit brigade throughout. Make a big noise, confidently, and ignore the realities.

Mr Johnson said in early September that leaving the European single market with no deal if the trade talks with the EU were to collapse would still be a “good outcome” for the UK. And remember that Mr Johnson in early July, after a familiar week of no significant progress in talks with the EU, signalled a belief on radio station LBC that a no-deal departure from the single market when the transition period ends would be a “very good option”.

“Prosper mightily” would, however, seem to take the biscuit.

However, it would also be fair to say the Johnson Government’s endless sloganeering has gone down a treat with patriotic Brexiters in the UK.

And it is difficult to believe the illusory truth effect is not playing its part.

Whatever the case, the crucial thing here is that something is not so just because you say it is. Even if you say it repeatedly. And even if people, perhaps including Mr Johnson himself, actually believe it.