Britain has never solved a full prison. It has only moved the overflow.

This week the country argued about who gets let out of prison early. Nobody asked the older question underneath it: why is there a back door at all?

Because there has always been one. For two and a half centuries Britain has promised more punishment than it has built room for, and every single time the gap has been closed the same way - with something temporary that turned out to be permanent.

For more than a century the country did not really have a prison problem, because it had an ocean. People punished by being sent away were put on ships to America. Then the American war shut the route. In 1776 transportation to America was suspended, and the suspension was described as temporary.

The overflow had to go somewhere. It went onto old ships rotting at their moorings in the Thames - hulks, still afloat but no longer able to sail. Parliament authorised them for two years. They were still in use when the act finally expired in 1857: eighty years of a two-year measure, renewed and renewed, and along the way widened.

The ocean had already reopened in another direction. From 1787 the ships sailed to Australia instead, and kept sailing until 1868, carrying about 162,000 men and women. Then that door shut too, and for the first time the country had to hold its own prisoners at home, in buildings it had to pay for.

The honest reformer looks at all this and says we have the problem upside down. On their telling, prison mostly makes people worse, short sentences achieve nothing but cost, and a country that locks up more of its people than its neighbours has a sentencing problem rather than a building problem. They would say the back door is not a failure at all - it is the system quietly correcting a promise it should never have made.

That is the strongest version of the case, and part of it is true. But it answers a charge nobody made. Set aside how many people ought to be locked up: you can argue that either way, and put it to a vote. The charge is narrower and harder to escape. The sentence the public is told about and the sentence actually served are two different things, and the difference is managed quietly, by officials, under the word temporary. If we really are punishing too harshly, then say so in daylight and shorten the sentence in law. What is not allowed is to keep the loud sentence and serve half of it in the dark.

And this is not history. In 1997 the government moored a ship at Portland and put prisoners on it, as a temporary measure to ease prison overcrowding. It was used as a prison ship between 1997 and 2006. More than two centuries after the first hulk, the same country reached for the same answer and used the same word.

A state may punish hard or punish lightly. Both are defensible, and people can vote on either. The one thing it may not do is announce one sentence, serve another, and call the difference an emergency.

For two and a half centuries the emergency has been the plan.

An opinion of the house.