Prison time is a very severe punishment. John Stuart Mill likened it to being consigned to a living tomb. Any society that employs it should do so with care and restraint. Yet we do not.

Partly because we think that prison is a humane punishment, it is drastically over-used in many countries, to the point of cruelty. Aside from failing in humanity, prison does not even perform well at the specific functions generally asked of a criminal justice system - namely, deterrence, retribution, security and rehabilitation.

We need to reconsider our overreliance on prison, and whether other types of punishment - even capital and corporal punishment - may sometimes be more effective and humane.

The fundamental problem with prison time, as Mill noted, is that its severity is hard to imagine. After all, many of us frequently find that what with one thing and another we have spent the whole day indoors, and we don't find that we have really suffered for it. It is hard to imagine quite how it must be to be confined to a small space and narrow routine for periods of years, or even until death.

There is no great drama to focus on. No particularly terrible things happen. Just more of the nothing. Attempting to multiply our feelings about spending one day indoors does not really get us there.

A punishment that is hard to imagine will not work very well. First, people contemplating breaking the law will not be especially deterred by dread of the punishment. In particular, though the concept of prison as an institution may be somewhat daunting, it is hard to contemplate the difference in severity of spending different lengths of time in one. Duration is a rather abstract dimension, and the difference between one year and five years - especially the cumulative difference - is hard to imagine.

This rather undermines the logic of the influential "law and economics" approach, particularly popular in America, which supposes that people commit crimes based on a rational calculation of the risk and severity of punishment set against their likely gains. If you are dissatisfied with the amount of crime in your society, the solution is to ramp up prosecutions and prison sentences (up to the point where the social costs of deterring crime exceeds the costs of further crime). This approach has the virtue of treating criminals as rational individuals like ourselves, rather than as members of a strange and dangerous sub-species. But it assumes that we can accurately foresee and compare the benefits and costs of crime.

Second, a punishment that is hard to imagine will also fail to satisfy the moral outrage of those who have been wronged. If a child is run down by a drunk driver, not only the parents but the society as a whole demands a severe punishment. Though a criminal justice system cannot be run on populist grounds in particular cases (that would just be mob rule), in order for justice to be seen to be done it does need to respond to popular demands and perceptions. Thus, even though the professionals staffing the justice system may understand the severity of prison time as a punishment, their judgement may well be superseded by the pressures of popular opinion. This is most evident where populist politics is integrated directly into the justice system, such as by the election of judges and prosecutors in America.

Where prison is the only severe punishment available, and length of time the only measure of severity, one will naturally find that very long sentences will be handed out. On an impartial view of the matter, the severity of the punishment often seems quite disproportionate. Yet even so the victims and those who sympathize with them remain dissatisfied. After all, aren't some prisons like hotels, with televisions and private bathrooms no less! To many people even ten years confinement to such a place hardly seems a just punishment for driving over an innocent child.

This dissatisfaction lies behind the dismaying popularity of inhumane prison conditions, seen most clearly in the pervasiveness of sly jokes and official winking about prison violence and rape. One can understand this phenomenon as a reaction to the imaginative shortcomings of simple prison time as a punishment. If prisons are understood as places of physical and sexual violence, then a prison sentence takes on a much more dramatic character that is easier to imagine for both potential criminals (deterrence) and victims of crime (retribution).

But this is a very dissatisfactory fix. In effect the punishment of prison time comes in two parts. The term of imprisonment that society's justice institutions decide is right and proper. And an additional corporal punishment component outsourced to the most vicious and violent thugs in the relevant prison community to determine and administer. That corporal punishment regime is out of society's control, but remains our responsibility. It falls most heavily upon the weakest and most vulnerable prisoners, not the most wicked, and makes society into torturers by proxy.

Two criminal justice functions unrelated to punishment are also relevant to thinking about prison: rehabilitation and security. The rehabilitation argument is a humanitarian one. Dysfunctional people commit crimes, including terrible crimes, but many of them can be made better. Prison time appears to offer a way to impose rehabilitation on criminals, since they are a captive audience. This was an important argument by nineteenth century liberals for proposing prison as the best form of punishment and thus the only legitimate one. (Hence the rather optimistic terminology of "correctional facilities" and "penitentiaries.")

Yet rehabilitation as an aim fits poorly with the punishment emphasis, as is clear from the generally high re-offending rates of ex-convicts in many countries (particularly the ones that use prison the most).

It is possible for those who genuinely want to make a change in their lives and are willing to work to change their character and tackle problems like drug addiction to use their time in prison productively if they are provided with adequate support and counselling, during and after their sentence. But most prison systems do not provide that support because they are dominated by a righteousness ethic of punishing bad guys rather than a social-work ethic of helping to mend broken people.

In addition, the very circumstances of prison - long term isolation from the rest of society, from positive relationships such as family and friends, and from positive responsibilities like regular work - do not favour rehabilitation. Convicts' lives are likely to be more dominated by the prison community they are living in than the values of the normal society to which they are supposed to be re-oriented. Thus it is not surprising that many people who spend significant time in prison seem to become further acculturated in criminal values and attitudes and actually emerge less willing and able to function normally in society. Others are traumatized by the experience of merely surviving in such a community.

Can prison at least be justified by the security it provides? It is true that criminals can't prey on society so long as they are locked up. So in this limited sense, prison works. But that is not enough to justify the way we use it. Prison has costs that must be weighed against this benefit. There is the obvious opportunity cost to society - the nursing care for the elderly, the subsidised child-care for the poor and so on, that we must give up to pay the bills for locking all those people into dark little boxes for years and years. There are the prison officers whose working lives would otherwise have been spent contributing to society in another, perhaps more satisfying way. And there is the dreary colourless psychological suffering of the prisoners themselves and the years and relationships lost to them. Security is important, but if we can achieve it by better means than imprisonment then we have a moral duty to do so.

Even if someone has committed a serious crime and there is general agreement that they deserve to be punished severely, that does not necessarily mean that they present a danger society needs to be protected from. Corporate fraudsters for example can be made safe relatively easily by removing their rights to manage companies. Likewise even those who commit very serious violent crimes may not be particularly dangerous - for example, women who kill abusive husbands do not go around killing other people.

Quite often, people are sentenced to prison for the worst thing they have ever done, and not for being dangerous kinds of people. Thus, little to no security benefits for society are achieved from their stay in prison. Of course, there are some people whose violent character really does present an unacceptable risk to society. But these are a small minority of those who are now sent to prison. The way we use prison assumes that all convicts are criminal characters, which is not only false, but a very inefficient way of trying to achieve security.

Don't be misled by the cold-bloodedness of the term. An inefficient criminal justice system is one that inflicts unnecessary and hence unjustified suffering. This is the very definition of cruelty.

The need for reform

We should recognize the failures of our moral imagination that lead us to overuse prison time as a punishment. Because prison is such a severe punishment, excessive use of it is an offence to justice that should arouse our moral indignation. Millions of people are serving unjustifiably long sentences in living tombs as a result of our inability to take prison seriously.

Our criminal justice systems should be much more restrained in their use of prison as punishment, and much more insulated from popular demands for excessive sentences. Society at large has a responsibility to reconsider how and why we punish, and its centrality in the way our institutions are organised. A more generous and rigorous approach to rehabilitation incorporating at least some elements of restorative justice seems particularly important for a society that wants to call itself civilized.

Restorative justice sees crime as a problem for society to solve together rather than a person to be punished by the state. Crime hurts, justice should heal. Most criminals are people who need mending, not further marginalisation. Most victims - and a great deal of crime has no victim but the state - would be more satisfied in the long run to be a part of a process of restitution than to be observers of a government run retribution programme. It seems eminently more fitting and more constructive for the fraudster who embezzled old people's savings to be sentenced to work two days a week in a nursing home rather than to increase the amount of suffering in the world and redirect taxes from the care of the elderly by sending him to prison.

Moreover, if we are interested in the moral rehabilitation of law-breakers then we need to encourage them to reflect on the wrongness of what they have done and the harms they have inflicted on others, as restorative justice sets out to do. In contrast, the punishment approach attempts to teach respect for others through the strange device of inflicting suffering on the criminal. The prisoner is thereby directed to reflect on his imprudence not his moral failure, to regret his crime only because it has brought such harms upon himself. Prison thus functions in the same amoral way that hell does in some religions.

I recognise that a fully restorative justice system is a political non-starter. The drive for retribution has become so entrenched in the moral psychology of modern societies that we are unlikely to give it up any time soon. So let me return to challenging the justification for prison as punishment in its own terms and raise the stakes. Even if you believe that the judicious infliction of suffering is required by justice and is an effective means of deterring crime, you should still reject prison because there are better options available.

Prison time is so awful a punishment that even "barbaric" alternatives like flogging and execution may often be more effective and more humane. Many people would automatically deny this. But the very reason for this reaction - that such punishments are extremely unpleasant to contemplate - is actually their advantage over prison.

Flogging is brutal and ugly. Yet that in itself does not mean it is cruel or inhumane or otherwise unfit as a punishment. Punishments, by definition, are supposed to be very unpleasant. Prison time is a form of psychological torture. Considered less emotively, flogging has advantages that measure up well against prison, especially longer sentences (of more than a few months). Its drama makes it much easier to imagine, indeed to over-imagine, and so it should work better than prison as a deterrent. For the same reason, flogging also seems better able to satisfy demands for retribution by those who have been wronged. Seeing someone strapped to a frame and having their skin ripped from their body seems to me to convincingly satisfy the requirement that justice be seen to be done in a way that prison cannot.

Yet, unlike prison, achieving these effects doesn't require that large chunks of a person's life be thrown away, together with their relationships and mental well-being. Thus, exactly because of the revulsion it inspires, we can expect flogging to be a more efficient punishment because the total suffering it inflicts is less. That makes it less cruel; more humane.

Execution is an even more dramatic punishment that may be justified by very severe crimes, such as sadistic murder. I do not suppose the threat of judicial execution is a particularly persuasive deterrent to the people who commit such crimes (indeed, it is very hard to understand how such people look at the world). Nonetheless it seems a fitting and proportionate punishment. Such crimes are almost the definition of evil and on the retributive view undoubtedly merit a very severe punishment. Furthermore, in such cases society has a legitimate fear of the perpetrators ever again being allowed to live freely amongst them, and a legitimate distrust of claims about successful rehabilitation.

Yet I do not think that even sadistic murderers deserve any punishment more severe than execution. I do not believe that the perpetrators deserve to be locked away forever in a living tomb, often in solitary confinement for their own protection. While long-term prisoners do have a considerably shorter life-expectancy than the rest of us, that still allows for several decades of monotonous hopelessness before a miserable, unmourned death. I follow John Stuart Mill here in considering that death is not the worst thing that can happen to a person, and that merely allowing someone's continued existence is not mercy.

A truly humane approach to punishment must consider the severity of the punishment from the convict's perspective, the undramatic but unrelenting mental suffering that a life sentence means. If we are too cowardly to execute them ourselves, we should at least allow such prisoners to end their lives and their suffering, as the euthanasia rights activist Philip Nitschke has recently argued.

Humane punishment?

Let me conclude by re-examining what we mean by a "humane" punishment. It seems to me that prison time is a humane punishment in one particular respect. It is humane to those who impose it.

Because prison time appears to consist of removing pleasures rather than inflicting suffering, societies find it all too easy to choose as a punishment for their troublemakers. Furthermore, because it is difficult to imagine how awful it must be to be deprived for years on end of all the things that make life desirable or valuable, societies do not feel any great moral compunction about employing it liberally. It is therefore the perfect punishment for a society too cowardly to face up to the moral responsibilities that come with inflicting punishment.

Punishment takes place within a moral relationship: punishers should be alive to their moral responsibility for deliberately inflicting pain and suffering on another human being. That is why true punishment has a tragic element - it is always carried out with some regret, even when it is just. Indeed, it is because we recognise that punishment is an awful thing to do to someone that we are pushed to carefully consider in every case whether it is morally justified and proportional.

The risk posed by "humane" punishments like prison is that they can be imposed with little regret. And when the act of punishment becomes so easy, the decision to punish is no longer treated with the moral seriousness it deserves. The result is that the way society punishes wrongdoers and troublemakers will no longer reflect a considered judgement about what justice requires.

Consider the debate about judicial execution in America. Because execution is generally acknowledged as a severe punishment, its proper application is considered a momentous responsibility. Not only are the few hundred death penalty cases much more carefully prosecuted by the state (on average), but a number of NGOs concern themselves entirely with investigating and scrutinizing whether justice has been done. Indeed, the conscience of a civilized society deserves no less.

Yet it is striking that the more than 100,000 people sentenced to decades of suffering in United States prisons do not receive such careful attention to the justice of their cases. Sending large numbers of people to prison for a very long time does not seem to stir the public's conscience in the same way as a "real punishment" like execution. It should.

Thomas Wells is a philosopher based in Rotterdam. He blogs at The Philosopher's Beard.