“The moment you doubt whether you can fly,” J M Barrie once wrote, “You cease for ever to be able to do it.” Elsewhere in the same book he was blunter, still: “Whenever a child says, ‘I don’t believe in fairies’, there’s a little fairy somewhere that falls right down dead.”

I would never have expected that Peter Pan would become one of the key political texts of the twenty-first century, if I’m honest. But predictions are not my strongpoint, and over the last few years, what one might term the Tinkerbell Theory of Politics has played an increasingly prominent role in national debate. The doubters’ lack of faith, we are told, is one of the biggest barriers to flight for everything from Jeremy Corbyn’s poll ratings to Brexit. Because we don’t believe, they can’t achieve.

It was in run up to the Scottish referendum that I first spotted Tinkerbell in the wild. Reports suggesting that RBS would consider relocating from Edinburgh, should independence lead to a significant rise in business costs – a statement of the bloody obvious, I’d have thought – were dismissed by then-First Minister Alex Salmond as merely “talking down Scotland”. Over the next few months, the same phrase was deployed by the SNP and its outriders whenever anyone questioned the Yes campaign’s optimistic estimates of future North Sea oil revenues.

The implications of all this were pretty clear: any practical problems apparently arising from independence were mere phantasms. The real threat to Scotland was the erosion of animal spirits caused by the faithlessness of unpatriotic unionists, who’d happily slaughter every fairy in the land before they risked an independent Scotland.

All this seemed pretty obnoxious to me, but at the time of the referendum it also all seemed to be a reassuringly long way away. Little did I realise that Salmond and co were just ahead of their time, because today, Tinkerbell-ism is bloody inescapable.

On Monday, Sir John Major made a wonkish speech laying out his concerns about Brexit. He talked about the threat to the Northern Ireland peace process, the way it would isolate Britain diplomatically, the difficulty of negotiating highly complicated trade deals on the timetable imposed by Article 50. He wanted, he said, to “warn against an over-optimism that – if unachieved – will sow further distrust between politics and the public, at a time when trust needs to be re-built”.

And how did Britain’s foreign secretary respond? “I think it’s very important that as we set out in this journey we are positive about the outcome for the very good reason the outcome will be fantastic for this country,” Boris said, probably imagining himself to be a bit like Cicero.

The problem, in other words, is not the government’s lack of a plan; the problem is its critics’ lack of faith. In a familiar phrase, the Telegraph headlined its report: “Boris Johnson criticises John Major for talking down UK’s post-Brexit prospects”.

The left is no better. In any discussion of the failings of Corbyn’s leadership of the Labour party, it won’t be long before someone blames the polls, or the by-election results, on either the lack of support from the parliamentary Labour party, or the hostility of a media that never liked him in the first place. “Of course he’s struggling,” the implication runs. “Your lack of belief is a self-fulfilling prophecy.” Dead fairies, everywhere you turn.

It’s easy to see why the Tinkerbell strategy would be such an attractive line of argument for those who deploy it: it places responsibility for their own f*ck-ups squarely on their critics, thus rendering them impervious to attack. Corbyn’s failure becomes the fault of the Blairites. A bad Brexit becomes the fault of Remoaners, and not those who were dim enough to believe it would easy to begin with. Best of all, the more right your critics turn out to be, the more you have to blame them for.

But being impervious to criticism is not the same as being right, and to think this strategy is a recipe for good government is to mistake a closed loop of true believers for objective reality. Jeremy Corbyn is unlikely to start winning elections, no matter how hard the faithful believe. However much you talk up Scotland, that oil is still going to run out

And whatever the right-wing press do to convince themselves that Boris Johnson is right, and John Major is wrong, it is unlikely to affect the negotiating position of the 27 other states in the slightest. At the end of the day, our faith matters a lot less than the facts on the ground. There is no such things as fairies.