Boris Johnson has never prepared himself properly, which means his whole life has been a preparation for being unprepared.

But how should the rest of us prepare ourselves for the Johnson prime ministership which now appears almost certain to begin in the middle of next week?

He possesses such a prodigious gift for attracting attention that correspondents from around the globe ask me, because I wrote a life of Johnson, what kind of a Prime Minister he will be.

They wish to know about his ideology, even about the ideological differences within the Johnson family, and have noted with concern the reports that he has no grasp of detail, and no respect for facts, so is unfit for high office.

It occurs to me that a better place to start is with a letter written in Hampstead by John Keats in December 1817, at the age of 22:

“several things dovetailed in my mind, & at once it struck me, what quality went to form a Man of Achievement especially in Literature & which Shakespeare possessed so enormously – I mean Negative Capability, that is when man is capable of being in uncertainties, Mysteries, doubts, without any irritable reaching after fact & reason.”

Johnson cannot be understood by those who demand certainty. The fact-checking mentality, however meritorious it may appear to those who adopt it, is a fatal hindrance in Johnson Studies.

So is the attempt to define him in ideological terms. He has not surrendered his freedom of judgment to an ideology. Like most of us, he is a mixture of instincts, prejudices and flashes of insight, contradictory impulses shaped by an evolving tradition of behaviour and the discovery of what works.

His imagination is always in play. Thoughts, feelings, jokes, images and original turns of phrase crowd in upon his fertile mind. He is a man of Negative Capability.

To the ideologist this sounds recklessly fluid. To the fact-checker it looks like a licence to make things up, i.e. to tell lies.

And to the expert, who yearns to tell us what we can and cannot do, it seems childish.

Like everyone else, I have no way of knowing how Johnson’s prime ministership will turn out. It could be triumph or disaster, or a mixture of the two.

In historical terms, disaster is the more likely outcome. Most Prime Ministers end up being blamed for something.

But there is often an initial period when things go well (cf Blair, Eden, Chamberlain), and Johnson starts with the advantage of low expectations. Fact-checkers, ideologists and experts already find his approach to politics intolerable.

So do commentators as eminent as Matthew Parris, Max Hastings and Bruce Anderson. They regard him (I paraphrase) as an adventurer, a clown, a rogue, an embarrassment.

They wax so vehement they sound blinded by anger, and do him the service of setting the bar so low he may exceed it.

We have just seen, on the far side of the Atlantic, an example of the futility of denunciation. American moralists compete to pass the severest judgment on Donald Trump.

Yet Trump won the election, and has not yet been a complete disaster. The moralists who wrote him off can still expect eventual vindication, but have had meanwhile to endure some painful surprises, including the discovery that millions of Americans have the impudence to laugh at their distress, and to take a wicked delight in doing so.

Johnson is not the same as Trump. Our next Prime Minister is better educated, and less given to sowing enmity. I realise that Remainers who have not forgiven Johnson’s role in the EU Referendum will disagree with the second half of that sentence.

But as Buzzfeed reported a year ago, Johnson is impressed by the President:

 “I am increasingly admiring of Donald Trump. I have become more and more convinced that there is method in his madness.

“Imagine Trump doing Brexit. He’d go in bloody hard… There’d be all sorts of breakdowns, all sorts of chaos. Everyone would think he’d gone mad. But actually you might get somewhere. It’s a very, very good thought.”

Johnson is about to go in bloody hard, and people will find it difficult to tell how serious he is, and whether he really means it.

For much of the time, he may not know himself. Brexit is a negotiation in which you have to advance and retreat according to circumstance, not chain yourself to some fixed position which proves untenable.

There will, he says, be “creative ambiguity” about the £39 billion we are going to pay the EU. He will not deny that we may, in law, be obliged to hand over a large chunk of that money, but he will create uncertainty about when, if ever, Brussels is going to get paid.

At the same time, he will seek to marshal public opinion behind his position. This factor, so important since the 1820s, still tends to be downplayed by members of the Establishment who believe themselves to be in possession of superior knowledge.

If Johnson advances the proposition that we should not pay until we are given a fair deal, the public is likely to side with him, however much the experts insist this withholding of payment simply cannot be done.

Peter Foster, Europe Editor of The Daily Telegraph, reports on Twitter that “TeamBoris by all accounts is in chaos”, with “a vipers’ nest of competing factions all vying” for his ear.

In my view, that is an exaggeration. But Katy Balls of The Spectator recently provided a helpful list of eight different groups which consider themselves entitled to the new Prime Minister’s ear.

Those who feel they are losing that contest will let their dismay be known, which will make it harder to read Johnson’s intentions and work out what his bottom line is, which is as he would like it.

Here, it should be remembered, is a man who is unscrupulous enough to employ experts of his own. He did so at City Hall, but wasted his first six months because with a few exceptions, he had not yet identified the experts he needed.

The Mayor of London serves a fixed term, so could afford to do that. The Prime Minister has an unfixed term. If Johnson wastes his first six months in Downing Street, they will be his last six months.

He is well aware of this fact, and has a better understanding than in 2008 of the capabilities of his colleagues, having served with a number of them in Cabinet.

The situation is fraught with danger, and there in the spotlight will be Johnson. It is a position he has often occupied since the age of 17, and usually though not invariably it suits him.

Trump’s popularity with a section of the American public proceeds partly from his gifts as an entertainer. He is a more practised performer on reality TV, and on Twitter, than the rest of the Republican contenders put together, and uses that ability to set the agenda.

Johnson’s success will depend on his ability to do the same, outflanking the Establishment by mobilising the country. He is not yet a good parliamentarian, something which generally takes far more time than he has devoted to it.

But he is a star performer with the wider public, able to transform the atmosphere when he enters a dull shopping centre on a quiet Wednesday afternoon.

His critics will regard his use of this talent as vulgar and unparliamentary, but his prime ministership will be immeasurably strengthened if he can carry the public with him, persuade them to back his version of Brexit, and put the Conservative Party in a position to win a general election.

We live in a free country. That remark used often to be made when someone had made a remark in questionable taste. One suspects that during the Johnson prime ministership, we shall be hearing it pretty often.