Bad ideas blunt the sharpest minds, and Boris Johnson’s intellect was hardly an instrument of surgical precision before it rubbed up against a hard Brexit. The prime minister is no fool, but his talents are ill-suited to crafting a new relationship with the EU. Johnson’s cleverness is rhetorical; his unique talent is for lifting spirits while lowering expectations. His upbeat bombast is laced with self-deprecation conveyed in his artfully tousled appearance and the elongated ums and ahs that signal improvisation, although the lines are scripted.

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The whole act is a wink inviting the audience in on a joke, the butt of which are “doomsters and gloomsters” who try to hold Johnson to his word. He persuades his fans to take their satisfaction purely from the experience of being persuaded. To be seduced by him is to forgive him in advance for underachieving.

The EU is unsusceptible to that charm. The negotiations that began in Brussels this week will codify legal obligations across multiple issues, and Johnson’s inflated verbiage is not a meaningful currency in that commerce. It can be an impediment to progress when it undermines trust. Continental leaders take note when the prime minister tells manufacturers that they will encounter “no forms, no checks, no barriers of any kind” for the transit of goods between Northern Ireland and mainland Britain. Ministers blithely dismiss the prospect of a customs border in the Irish Sea.

The truth is that Northern Ireland will end up in a different regulatory space to the rest of the UK. That was a concession Johnson made last year to allow the UK to jettison EU rules without reimposing a combustible border on the island of Ireland.

In a recent speech Johnson’s chief negotiator, David Frost, set out a fundamentalist line on deviation from continental rules. The capacity to do so, he argued, was the essence of “independence” and “the point of Brexit”. When that ethos is pursued in practice, forms, checks and some kind of barrier to invigilate the boundary between EU and British jurisdictions are inevitable. A border in the Irish Sea is not a matter of conjecture. It is described in a treaty that Johnson signed and waved in triumph. But his enthusiasm was for the idea of a deal, not the real thing. He likes deals for their instant retail value in domestic politics, believing that problems in the small print can be blustered away. That is not how the trade negotiations work in Brussels, where the deal is the small print.

Facebook Twitter Pinterest ‘Johnson’s chief negotiator, David Frost, set out a fundamentalist line on deviation from continental rules. The capacity to do so, he argued, was the essence of ‘independence’.’ Frost, left, with the EU’s chief Brexit negotiator, Michel Barnier. Photograph: Olivier Hoslet/AP

Johnson’s cavalier disregard for the facts of Brexit is where his character and Eurosceptic ideology are fused. It flows from the arrogant cultural assumption that Britain is pursuing its manifest destiny away from Europe, and that Brussels, by taking the legalistic approach, fails to see things in their true, epic perspective. That argument was the core of Frost’s speech, which was Johnsonian in the way it camouflaged flabby thinking in historical dress.

Smuggled between references to Edmund Burke and Charles de Gaulle were some strange ideas. (The heroic emancipation narrative implies that Britain loves independence more than other EU nations – or that they are not smart enough to grasp the abject nature of their colonial submission by Brussels). The Johnson-Frost doctrine rejects the Treasury view that disrupting trade between neighbours makes them poorer. Costs are outweighed by “other factors” intrinsic to the “complex and adaptive” nature of the modern economy, which in its unfathomable genius generates responses “we do not foresee” and “solutions we did not expect”. This hints at the view, championed in Downing Street by Dominic Cummings, that fretting about EU markets is for analogue scaredy cats who care too much about gravity and not enough about the weightless digital future. Farmers bleat about borders, but the 21st century belongs to countries that master artificial intelligence.

The thesis is immune to evidence and too nebulous to be disproved in the time available for Brexit negotiations. It allows believers to write off any short-term disruption against notional gains down the line. If the prime minister believes this stuff – if he prefers pristine sovereignty in a hypothetical economy to defence of the real one – he has little incentive to compromise. And if compromise feels embarrassing, he might prefer to obstruct talks and blame EU intransigence when they collapse.

There is a less pessimistic view: Johnson’s priority with Brexit since the election has been to keep it out of the news. (He promised voters he could make it go away, and he wants that illusion to be complete. To that end, he needs any deal.) He will be relaxed with compromise because he thinks he can spin base metal into gold. Most Tory MPs will not sweat over the detail, and the 80-seat majority is a cushion against any rebels who do.

Downing Street also expects to force concessions out of Brussels through brinkmanship, spooking the EU by threatening to walk away. In the authorised Tory version of history, that is what happened last year. Theresa May had a bad deal because she was afraid of no deal; Johnson wasn’t, so he got a better one. In reality, Johnson’s first offer of revised terms for Northern Ireland was laughed out of Brussels so he defaulted to a model that he had previously rejected.

It is not the prime minister’s style to admit retreat, so he now speaks of the withdrawal agreement in fictionalised terms, free from costs or Irish borders, as if it is the deal he wanted and not the one he got. This is the Johnson method: sign anything, then sell it as everything; make any economic sacrifice on the altar of wishful thinking; compromise for a deal then disown the downside. That last tenet is infuriating for the EU side, which needs to know that Britain after Brexit is still a country that honours treaties. It cannot afford to be the rogue kind that doesn’t, but Johnson seems to think he can afford the political luxury of refusing to care.

• Rafael Behr is a Guardian columnist