The most dramatic moment of James Graham’s play This House shows the Labour whips discussing whether they can summon an MP called Alfred Broughton back for a crucial vote, even though he is dying of heart failure.

“Ann, it’ll kill him,” the deputy whip tells his colleague Ann Taylor. But she knows that James Callaghan’s government will fall without Broughton’s vote, so she replies: “He’ll die happy.”

In the end, Callaghan over-ruled the whips, Broughton stayed at home – and Labour lost the no-confidence motion by a single vote. The year was 1979. Eighteen years of Tory rule followed.

This House recently finished a nationwide tour after several runs in the West End. Its success is not down to audiences’ unquenchable lust for arcane parliamentary procedures, but because it reveals how people react to extreme pressure. The play also shows how a functioning democracy relies on trust, which is both powerful and fragile.

Broughton’s physical presence was needed in Westminster that night because the “pairing” system had broken down. That meant absent MPs were no longer matched with an opposing politician who also agreed to skip the vote. In a hung parliament, this had terrible consequences. Because the opposing whips lost trust in each other, sick and dying MPs had to be wheeled through the division lobbies whenever there was a possibility of a government defeat.

Any liberal democracy rests on the assumption that there are some weapons that its politicians agree not to use

Now, the pairing system is once again under threat: in a Brexit-related vote last week, the Lib Dem deputy leader – and new mother – Jo Swinson discovered that her pair was not honoured. Theresa May has insisted this was an “honest mistake”, but the Conservatives’ story has now changed so many times it feels like a bad pitch meeting for a soap opera. The current line is that the whips were thinking about breaking other pairs, but didn’t, and were not thinking of breaking this one, but accidentally did. By the time you read this, however, the chief whip Julian Smith’s story might well be that: a) bigger boys told him to do it and ran away; b) it wasn’t him but his evil twin, José Smith; or c) it was all a dream.

There is real anger about how Swinson was treated, with Tory MP Heidi Allen bravely criticising her own whips’ office. “I refuse to be tarnished by this behaviour so will not stand by and say nothing,” she tweeted. “Integrity and honesty are fundamental to our democracy.”

She is right. The reason that pairing matters is, again, nothing to do with partisan point-scoring or an obsession with musty traditions. It matters because systems need rules. Democracy is no exception.

If pairing breaks down, parliamentary votes become about the survival of the fittest – literally. In 1978, Labour’s Joe Harper repeatedly postponed emergency surgery so that he could vote. He died in office.

Today, a breakdown in pairing would cause needless stress and ruin lives. It would also give the Conservatives an advantage in knife-edge votes because more opposition MPs are pregnant or new mothers. The Tories would, in effect, be rewarded for having just 67 female MPs compared with Labour’s 119. (Why not allow proxy or electronic voting? you ask. A group of MPs is pushing the idea, but progress is slow.)

The consequences of a pairing breakdown would be grisly. And so would the message sent out by the government: we will play dirty to win these votes. Once a convention like that is broken, it’s hard to restore.

Yet any liberal democracy rests on the assumption that there are some weapons that its politicians agree not to use. They don’t lock up their political opponents on spurious corruption charges, jail dissidents or shut down newspapers that print things they don’t like. They don’t suggest that the other side disagrees with them because they are unpatriotic, or traitors, or in hock to “globalists” or “Rothschilds”, or whatever the euphemism is this week.

All of these things are happening somewhere, right now – and as with the pairing system, the prisoner’s dilemma applies. It is better for all of us if democratic norms are respected. But there is an immediate, short-term reward for any politician who dares to break them. So we have to be defend them as strongly as we can.

After all, what happens when trust begins to evaporate from a political system? Just look across the Atlantic. No one believes a word Donald Trump says. He can stand next to Vladimir Putin one day and casually exonerate the Russian leader from interfering with the US election, then retract those remarks 24 hours later. His own party, his own cabinet, even his own White House officials don’t trust him. The result is paralysis and instability.

In his bestselling book Sapiens, Yuval Noah Harari asks how modern humans manage to live in such big, complex societies compared with our ancestors. The trick, he argues, is that we create myths that allow us to co-operate on a grand scale. Once, these myths were about the god who brought rain or protected our cattle. Now, they are about the existence of nations, currency systems and private companies. “A disaster might kill every single one of Peugeot’s employees and go on to destroy all of its assembly lines and executive offices,” writes Harari. Nonetheless, the company could be rebuilt, because “Peugeot is a figment of our collective imagination”.

These myths require trust. Money works because have all agreed to treat a scrap of paper as it if has value. Pensions work because we are confident enough to pay into the pot, assuming there will be something in there for us one day. Democracy works because we believe it’s better than arming our children with cricket bats and trying to annex our neighbour’s patio.

If civilisation means anything, it is this gossamer-thin web of rules, courtesies, myths and collective beliefs, underpinned by an agreement not to screw people over just because we can. So, yes, the process of pairing might be dull, but the principle is vital.

Helen Lewis is associate editor of the New Statesman