The Daily Telegraph today reports that Theresa May “will threaten to take Britain out of the single market unless the UK is given full control of its borders”. She will do so later this month in what is described as “a significant Brexit speech designed to counter claims she has no plan for leaving the EU”.

We shall have to wait for the full text of her speech to see how much of a plan it contains, or whether it can more accurately be described as a statement of objectives. The word “vision” is used, which suggests the latter.

The aim seems to be to convince everyone that the British Government is serious about regaining control of immigration, and is prepared to leave the single market if this is demand is not met.

Since many people in Brussels, including some British officials, seem not to have realised this is the case, that is an entirely sensible point to make, and is consistent with May’s general determination to show she will not be pushed around.

She needs to indicate the destination at which she intends to arrive, or the damaging assumption will continue to circulate that she can be fobbed off with minor concessions.

But although we need to know the destination, it would be foolish, at this stage, to expect full disclosure of the route by which she intends to get there.

Sir Keir Starmer is quite right, on behalf of the Opposition, to demand that she explain herself. But she would be madly imprudent to yield to his entreaties. Part of not being pushed around is not yielding to Sir Keir’s fashionable but misplaced belief in “transparency”.

Since 1846, when Sir Robert Peel broke the Tory Party, and consigned it to the wilderness for a generation, by betraying his backbenchers and repealing the Corn Laws, successive leaders have known it was their duty, “at all costs and at almost any sacrifice, to avoid repeating Peel’s action” (as Robert Blake puts it in The Unknown Prime Minister, his masterly life of Bonar Law).

The party has to be kept together. Otherwise it has nothing to offer the nation. This elementary point is regarded, by those who choose to regard politics as an exercise in moral idealism untainted by practical considerations, as disreputable.

David Cameron’s great achievement was to keep the party together. He did so by subcontracting to the British people the decision on whether or not to leave the European Union.

That too is regarded by purists as a disreputable manoeuvre. In strict constitutional theory, I agree with them: in a parliamentary system, such a decision ought to be taken by MPs.

But as a matter of practical politics, Cameron knew that to go on trying to keep the decision inside Parliament was just not feasible. His own MPs were too divided on the issue, and he could not afford to go on losing MPs, activists and voters to UKIP.

Like Harold Wilson, he admitted his weakness and decided to call a referendum. The British people proceeded, in their wisdom, to give the Tories a slim overall majority at the general election of 2015, only to reject, the following year, Cameron’s energetic but unpersuasive advice about how to vote in the referendum.

So Cameron was out, but has left his successor in a commanding position. He killed the Lib Dems with kindness, shot UKIP’s fox, and by defeating Ed Miliband, precipitated the rise of the unelectable Jeremy Corbyn.

And yet May knows there are still deep divisions within the Tory Party, and indeed within her Cabinet, about what Brexit means. The last thing she wants to do, or indeed ought to do, is destroy the only party strong enough to implement Brexit, by becoming needlessly and precipitately specific about her “plan” for doing so.

She has to preserve, for as long as possible, a degree of ambiguity, so that people can believe, or hope for, what they will. This may not be a glorious form of politics, but it is the only practical one. A route map that satisfied Dominic Grieve would not satisfy Jacob Rees-Mogg.

If Corbyn commanded the loyalty of more than about ten Labour MPs, he could mock the divergences on the Tory benches. But he knows that if he tried that, he would be drowned in a great roar of laughter at his own expense.

So the task of trying to understand May, and hold her to account, falls to other figures on the Left. They are, however, impeded not just by her inscrutable manner, but by their own refusal to treat Conservative politicians, and indeed conservative-minded voters, with any kind of fair-mindedness.

The hatred and contempt with which the Left greeted the rise of Margaret Thatcher are today directed at her successors. Hence the tendency to underestimate the persuasiveness, with the wider public, of figures as diverse as David Cameron, Boris Johnson, Michael Gove, Nigel Farage and Donald Trump.

Mark Lilla, an American essayist and historian of ideas, makes this point with subtlety and lucidity in his latest book, The Shipwrecked Mind. He begins by observing that university libraries contain vast numbers of books about revolutions, and almost none about reaction:

“We have theories about why revolution happens, what makes it succeed, and why, eventually, it consumes its young. We have no such theories about reaction, just the self-satisfied conviction that it is rooted in ignorance and intransigence, if not darker motives. This is bewildering. The revolutionary spirit that inspired political movements across the world for two centuries may have died out, but the spirit of reaction that rose to meet it has survived and is proving just as potent a historical force, from the Middle East to Middle America. This irony should pique our curiosity. Instead it arouses a kind of smug outrage that then gives way to despair. The reactionary is the last remaining ‘other’, consigned to the margins of respectable intellectual inquiry. We do not know him.”

What Lilla does not say (and one does not blame him for not saying it, for his book is a delightfully slight affair of only 144 pages) is that many on the Left suppose ‘conservative’ and ‘reactionary’ are synonymous terms.

The reactionary is homesick for a supposedly idyllic past, and yearns to return to it. The conservative knows this is not possible. “A state without the means of some change is without the means of its conservation,” as Edmund Burke pointed out in his Reflections on the Revolution in France.

A more recent writer, Michael Oakeshott, remarks in his masterpiece, Rationalism in Politics, that tradition “is pre-eminently fluid”, though the rationalist attributes to it “the rigidity and fixity of character which in fact belongs to ideological politics”.

Some Conservatives make this error too: they suppose their tradition is somehow hidebound and inflexible, when actually it is far more adaptable and resourceful than socialism or communism have proved to be.

Oakeshott warns against trying to simplify a tradition of political behaviour and turn it into a mere guidebook or crib, which will supposedly enable anyone, no matter how inexperienced, to acquire “a political training in default of a political education”.

The yearning for a “plan” to carry out Brexit is a rationalist illusion. It rests on the assumption that politics is a static business, where everything can be worked out in advance. The better analogy is with a sea voyage, where nothing is predictable, experience is more valuable than theory, and judgment will be needed to make the winds and the waves work in one’s favour, rather than drive one aground or onto the rocks.

In her forthcoming speech, May will quite rightly offer a better idea of which port she intends to make for during the Brexit negotiations. If, however, she were to tell her crew exactly how she proposes to arrive there, she would not just be guilty of preposterous technocratic hubris, but would most likely provoke a mutiny.