A lot of nonsense has been expressed on both sides in the debate on grammar schools. But it can still be summed up like this: one side claims to have all the evidence while the other flounders when asked to provide any.

Given this imbalance, the interesting question is not who is right. Pretty much everyone who has considered if academic selection at the age of 11 enhances social mobility has come up with the same answer: it is good for a few and bad for the many. Even the Department for Education’s consultation on bringing back selection says ‘there may be an association with poorer educational consequences for those pupils not attending selective schools in areas where selection is allowed.’ They just think selection in the future could be different from selection in the past.

There is no more point arguing against the weight of evidence on this than on other issues: it is for people with too much time in their lives, too little history in their heads and a Twitter app in their hands. (I’m looking at you Arron Banks.)

The interesting question lies elsewhere: why does the discussion about grammar schools continue to flourish in such unpropitious circumstances? It is like the weeds outside my home. They grow back every time I uproot them, blast them with a flamethrower or squirt weed killer at them.

There is, I think, one good reason why the argument will not die which has gone unrecognised in the debate: some politicians emphasise the needs of society more than the needs of individuals and some do the opposite.

Even the fiercest opponents of grammar schools admit a minority of children do better academically in them. But they focus more on the majority, who do not. On the other side of the debate, those who support grammar schools aim for the best fit between individual people and their education. So they worry less about the collective impact. They say it is imperative that those who could benefit from selective education have the chance to experience it and, if that provides new challenges for the rest of the education sector, then you should fix those other challenges rather than ending selection. (For example, you make sure the alternatives to grammar schools are better than they were in the bad old days.)

In other words, many of those wanting to see academic selection become more common again at school age do not refute the current evidence; they just think it is not the killer blow that the opponents of grammar schools assume. They also see selection flourishing elsewhere in national life, including in other parts of the education system, such as in entry to university. It is because the UK university system is so selective and hierarchicial that it includes what the Times Higher Education claims to be the best university in the world.

The difference between those who place the interests of society above the interests of individuals and those who put individuals above society goes to the heart of politics. It is part of the difference between parties of the left and parties on the right, and why parties of the left favour the term ‘collective’ while those on the right dislike it. It even divides single parties: who would deny that Thatcherism focuses more on the needs of individuals than One Nation Conservatism?

In reality, each person – not just each political party – is a mix of the individualistic and the communitarian. For example, a good manager simultaneously cares about the health of their organisation and its individual members of staff. The same goes for politicians: MPs oscillate between acting in the interests of society as a whole and acting in the interests of individual citizens. It is the difference between fixing the problems raised by someone at a constituency surgery and being a Minister, frontbench spokesperson or Select Committee member. These two roles clash to comic effect in the film In the Loop.

(In the real world, the tension between the needs of society and the needs of individuals can lead to behaviour that seems hypocritical – such as when a politician helps an individual migrant earn the right to remain in the UK while calling for tough restrictions on migration overall. Or when they oppose school selection for the country while benefiting from it for their own children. On Tuesday, Emily Thornberry, the Shadow Secretary of State for Defence, told Channel 4 News: ‘I’m a mother and I’m a politician and as a mother I will do the best for my kids and as a politician I will do the best I can for all kids.’)

Most big political questions can only be answered by balancing the needs of individuals against society. For example, should we reduce spending on welfare because of the way it can disincentivise good behaviour or should we ensure no individual lacks the basics? Should we turn back refugees because of the pressure they put on public services or should we welcome them as individual people with something to offer? Should we put more focus on preventative health campaigns or spend as much as we can on each individual patient?

This may all seem obvious but wise people can differ in their answers to such questions and the debate over who has the best evidence on grammar schools has lost sight of this fundamental truth. Surely a healthy debate on education policy – just as with on welfare, refugees or healthcare – needs both individualistic and more communitarian voices to be heard loudly and clearly? After all, a defining feature of any successful educational institution is that is helps individuals flower within a collective environment. Isn’t that precisely the goal of democratic government too?

After all, weeds once thought to be annoying have sometimes turned out to serve a useful purpose.