THERE is never a good time to write about this. But we are about to begin a new legal year. The nation’s judges are gathering in cathedrals around the land to pray for guidance at the start of the legal term. So, this is a good a time to think about whether it makes sense to lock people in prison for as long as we do.

Ordinarily, the subject of prison sentences enters public discourse only in the wake of some terrible crime. Then, in outrage at one particular atrocity, we tend to fulminate on the inadequacy of the sentence handed out by the courts. To the victims of the family, most especially in the case of murder, the punishment never seems sufficiently to fit the crime.

Yet it is a longstanding legal adage that hard cases make bad law. Moral indignation is a poor basis for the establishment of general legal principle. This was brought home to me during the past two years as I served on the Independent Commission into the Experience of Victims and Long-term Prisoners (News, 24 June), chaired by the Rt Revd James Jones, a former Bishop of Prisons who also chaired the independent panel into the Hillsborough disaster. We took evidence from prison staff, reformers, academics, and, most importantly, prisoners and the families who had suffered at their hands.

It is commonly assumed by politicians, popular press, and the public that increasing the length of prison sentences offers society better and better protection. The evidence we uncovered shows that this is not the case.

During the past 20 years, the number of people sentenced to ten years or more has doubled. In the past decade, the number of people imprisoned for 20 years or more has quadrupled. Yet this constant increase in sentence length, at ever greater cost to the taxpayer, fails to achieve the results which legislators desire. Listening to both victims and to prisoners has brought us to the understanding that the existing system serves neither well.

The purpose of prison, the law says, is to punish, protect the public, reduce crime, and to reform and rehabilitate the offender. In practice, balance within those desiderata is rarely achieved.

This is because the content of the sentence is as important as its length. Yet prison fails adequately to address issues such as mental health, drug use, or education for life skills. So, however long the sentence, many offenders come out of prison as dangerous as when they went in — or more so. Without reform and rehabilitation, there is little reduction in reoffending, and society is no safer.

Our report, Making Sense of Sentencing, calls for a review by the Law Commission of both the content and the length of sentences. We recommend a strengthening of the part played by the Sentencing Council to promote better public understanding of sentencing and greater public confidence in the way prison works. Above all, we ask for a structured consideration by politicians across all parties — prompted and stimulated by the Justice and Home Affairs Select Committees — to debate the purpose, length, and content of prison sentences.

The urge for retribution is understandable. But something more thoughtful and analytical is required. Otherwise, our prisons stand as testaments to vengeance rather than justice.

Making Sense of Sentencing is available to read here