Clarke's argument is, as we already know, twofold: i) that we cannot afford to keep detaining more and more prisoners, and ii) that the current justice system is broken, particularly when it comes to rehabilitation, and needs mending from the ground up. He concentrated on the second of these – the custodial "revolving door" – on the Today programme earlier, and was persuasive in parts. No-one sifting through the reoffending statistics would argue that the British justice system is working as well as it should. The government has some encouraging ideas, particularly in the area of payment-by-results schemes, for fixing this.
For many people, though, fixing the prisons system need not mean shrinking it. Couldn't population figures be kept up at the same time? After all, the evidence of the last decade is that prison does a great deal to cut crime rates; not to mention appeal to the sense that misdeeds must not go unpunished. Here, the government will point to the public finances, and shrug that needs must. They will also point to plans to make community sentences tougher and more punitive. But the argument will only really be won if crime falls, prison costs go down and rehabilitation improves. Until that hopeful day, this remains a live issue.