Foucault shows that the modern prison system has never succeeded in reducing crime; moreover, that was never even the purpose of the prison system. In a brilliant reframing of the issue, Foucault writes,
But perhaps one should reverse the problem and ask oneself what is served by the failure of the prison; what is the use of these different phenomena that are continually being criticized; the maintenance of delinquency, the encouragement of recidivism, the transformation of the occasional offender into a habitual delinquent, the organization of a closed milieu of delinquency. Perhaps one should look for what is hidden beneath the apparent cynicism of the penal institution, which, after purging the convicts by means of their sentence, continues to follow them by a whole series of ‘brandings’ (a surveillance that was once de jure and which is today de facto; the police record that has taken the place of the convict’s passport) and which thus pursues as a ‘delinquent’ someone who has acquitted himself of his punishment as an offender? Can we not see here a consequence rather than a contradiction? If so, one would be forced to suppose that the prison, and no doubt punishment in general, is not intended to eliminate offences, but rather to distinguish them, to distribute them, to use them; that it is not so much that they render docile those who are liable to transgress the law, but that they tend to assimilate the transgression of the laws in a general tactics of subjection. Penality would then appear to be a way of handling illegalities, of laying down the limits of tolerance, of giving free rein to some, of putting pressure on others, of excluding a particular section, of making another useful, of neutralizing certain individuals and of profiting from others. In short, penality does not simply ‘check’ illegalities; it ‘differentiates’ them, it provides them with a general ‘economy’. And, if one can speak of justice, it is not only because the law itself or the way of applying it serves the interests of a class, it is also because the differential administration of illegalities through the mediation of penality forms part of those mechanisms of domination. Legal punishments are to be resituated in an overall strategy of illegalities. The ‘failure’ of the prison may be understood on this basis.Discipline and Punish, p. 172
We can only see the prison system as a failure if we (mistakenly) assume that its purpose is to reduce crime, rehabilitate offenders, protect the public, etc. Foucault is arguing that the prison system differentiates illegalities. In other words, the system establishes a sense of order. It takes the nameless mass of offenses and divides it up into something that makes more sense. It’s like the way 18th-century-naturalists classified plants and animals; this allowed the naturalists to study the various plants and animals. Likewise, differentiating crimes allow criminologists to study crime and criminals–or the create knowledge.
In systems-theoretical terms, the penal system (which encompasses prisons, probation, etc), builds up its own complexity as it filters the unmanageable noise of its environment. It places crimes and criminals on a grid. It takes the meaningless mass of offenses and organizes it or rationalizes it.
This is similar to how modern warfare took the pre-modern mass of warriors, which functioned as great object designed to mow down or crush the opposing army, and (thanks to the rifle) differentiated it into ranks and files. The mass of warriors–with the king fighting alongside everyone else–became became individual soldiers. A hierarchy was established, and the generals and their assistants stood away from the field of battle and directed the movements of the men in the field. This is a kind of rationalization as Luhmann uses the term.
So in the context of the penal system, offenses are placed on a grid; they are hierarchized and placed on continua–e.g., manslaughter, second-degree murder, first-degree murder, capital murder.
The penal system and the legal system are two different, operationally autonomous systems. As far as the law is concerned, prison simply takes away a person’s liberty–and the death penalty takes away their right to life. The law functions negatively, taking something of value away from the offender. The law has no interest in rehabilitation or reducing the crime rate. Once a person is convicted and sentenced, the legal system loses interest. It doesn’t want to dirty its hands with active punishment or get involved in dubious efforts to reform or rehabilitate offenders. And it cares nothing about rising or falling crimes rates.