Rome made everyone a citizen. Then it learned what citizenship was for.

This week the country is arguing about who belongs. Who gets a council house, who gets to work, what a person owes before they can draw on what the rest of us built. It sounds like a modern quarrel about migration and welfare. It is one of the oldest questions a state can ask, and Rome gave its answer in the year 212 - an answer that should stop us cold.

For most of Rome's history, being a Roman citizen was a prize. It carried rights that other people in the empire did not have, and it was handed out sparingly: to Italians, to soldiers who had served their time, to favoured cities. People wanted it precisely because not everyone had it. A thing that everyone holds stops being a status and becomes just the weather.

Then the emperor Caracalla issued one of the most sweeping laws in history. In a single edict he made almost every free person in the empire a Roman citizen. Vast numbers who had been outsiders were, on one morning, inside.

It sounds generous. It was not meant to be. The historian who lived through it, Cassius Dio, set down the real reason: the emperor needed money, and citizens paid taxes that non-citizens escaped. He did not widen the circle to honour anyone; he widened it to tax them. His "real purpose", Dio wrote, "was to increase his revenues by this means".

Here is the part that matters for us. It worked as a way to raise tax and failed as everything else. When citizenship was rare, it bound people to Rome: they had climbed to reach it, and they had something to lose. When it was universal and free, it bound no one to anything. You cannot feel lucky to hold what was handed to everyone at once for the treasury's convenience. Sever membership from contribution - make it a thing you are simply owed and never a thing you owe - and you end up where Caracalla did: a proud name that means a little less each year, propping up a loyalty that quietly drains away.

The generous reading has a case, and it is worth stating plainly: they say a confident nation shows its strength by sharing what it has, freely and without small print.

But sharing is not the same as giving away the thing that made the sharing worth having. A country is not a pile of benefits waiting to be handed round. It is a bargain between the people who put in and the people who take out, and it holds only while both believe the bargain is fair. The instinct that you should put in before you draw out is the oldest glue a country has, and calling it cruelty does not dissolve it.

So when this week's row is drawn as cruelty against openness, look again: underneath it is the very thing Rome got wrong. A country can choose to be open - that is a real choice, and a defensible one - but it cannot decree that belonging costs nothing and still expect belonging to be worth having.

Rome found out the hard way. You can hand citizenship to everyone, or you can keep it worth having. You cannot do both. We are learning it again now, one council house at a time.

An opinion of the house.