A man stands in front of a doorway facing two haystacks. Suddenly he is attacked by an ape (actually a second man in a King Kong costume). The assailant retreats and, in full view of his victim, hides under the haystack on the right. The first man dashes off to fetch a weapon but, while he is absent, King Kong changes his hiding place. Man number one returns with a stick. Where will he go to exact retribution?

This isn’t a trick question or a political allegory. The scene has been staged as part of an experiment by psychologists to test “theory of mind”. Our man will, of course, target the haystack on the right. We, the audience, know that King Kong is no longer there. But we can also surmise that stick-wielding man doesn’t know what we know, because he left the scene at the crucial moment. We are capable of constructing a mental image of the world as it is perceived by someone else and distinguishing it from our own view.

Most people acquire this skill at the age of around five. Younger children presume the world exists only as they experience it. They impose their knowledge on the man in the video, and expect that he too will know that the haystack on the right is empty.

According to research published in the latest edition of the journal Science, some ape species may be better at theory of mind than previously thought. Researchers monitored the reactions of chimps, bonobos and orangutans to the haystack video and observed that their responses were closer in sophistication to what might be expected from adult humans.

But what highly evolved primates can do under laboratory conditions turns out to be challenging at the level of national politics, where debate is regressing into a state of solipsistic toddlerism. The ability to separate a subjective impression of the world and an exterior reality populated by independent actors with interior realms of their own is breaking down.

Britain’s demand to leave the European Union cannot be understood without recognition that the club contains 27 other member states, each with its own domestic political dynamics. None can afford to bring much generosity to a Brexit negotiation.

Theresa May and her ministers depict the Brexit transaction wholly in terms of British needs to be met by foreigners

The European parliament, containing another multitude of competing interests, will also have a say. Even presuming skilful British negotiation, the best deal must, for the sake of EU integrity, contain costs that continental governments can cite as the penalty for surrendering full membership. Theresa May might think that a price worth paying, but it still has to be paid.

Yet May and her ministers depict the transaction wholly in terms of British needs to be met by foreigners. There is no theoretical reckoning with external interests, just an infantile projection of “Europe” as the mechanism that dispenses concessions on demand. David Davis tells parliament there will be “no downside to Brexit” (a point on which the Treasury, leaking bleak forecasts of shrunken tax revenues, evidently disagrees). Liam Fox offers a picture of tariff-free utopia so lacking in diplomatic perspective and practical detail it might as well be drawn under the cabinet table in crayon.

There is something childlike too in the way libertarian Brexiters complain about the emphasis being placed on migration. The Conservative MEP Daniel Hannan laments that the goal of making Britain a deregulated trade entrepot has somehow been interpreted as an instruction to close the borders. That wasn’t what Brexit was supposed to be about at all, fret the free market zealots. What counts is the non-xenophobic leave campaign they ran inside their heads, not the one that erected posters in the street warning that the Turks were coming.

The remain side has its own obtuse streaks, the least attractive of which is the habit of treating leave voters as victims of stunted enlightenment – dismissing their choice as economic self-harm driven by stupidity or racial spite. This isn’t a failure to apprehend the existence of autonomous minds in others, but it comes close, by denigrating the critical capability of those minds to assess a situation and respond rationally. It is hardly empathic.

Reluctance to credit opponents with an operative conscience is now endemic in political argument. On the left, there is hair-trigger readiness to see every proposal to manage migration as a Nuremberg law in the making. On the right, it has become normal to treat warnings about the economic consequences of Brexit as rhetorical sabotage, as if there is no difference between the observation of a threat and the treasonous wish that it be realised.

The writer Christopher Hitchens once identified this as the argumentative strategy of tinpot ideologues, whereby “if an opponent thought he had identified your lowest possible motive, he was quite certain that he had isolated the only real one”. This is not a new style, but it feels to me – and it would be only fair to concede that I might be wrong here – that it is spreading, encroaching on the available space for compromise. It is a rejection of the idea that politics is a shareable space at all, because there is no reason to respect the argument of an opponent whose motive is beneath contempt. Malice requires no rebuttal.

In such a climate, the ability of politicians to see things from alternative points of view, which should be a debating strength, risks becoming a handicap. The candidate who finds traces of validity in a rival proposition risks moral contamination in the eyes of core supporters. Moderation and caveat sound murky. The crowd-pleasing method is to project total certainty, from which flows emotional clarity (but no workable solutions).

No one is immune to bias and prejudice. People change their minds quite rarely. But every adult – and, it transpires, even the bonobo monkey – is capable of theorising the existence of a mind different to their own. To study or shut out that view is a choice we can make. It is easier sometimes to take the child’s-eye view, to wrap ourselves in politics that fit snugly around our instinctive preferences, and to howl when the blanket slips.

But if Britain goes any further down that path, it won’t be having a debate about Brexit or much else. It will be having a political tantrum.