In another twist in the argument, Foucault reveals that the modern prison is not a prison at all, but a penitentiary. The penitentiary combines the different functions of a workshop, in which prisoners engage with the world of production and a hospital where medical observation operates. These additions are made partly for economic reasons and partly to increase the efficiency of power in the prison. This change is matched by the redefinition of the subject locked in the prison: the prisoner becomes the delinquent. This category is explored in the next section.

The penitentiary is also a place in which knowledge is important. The observation and classification of delinquents depend on, but also help to create, a new kind of science. This is human science, particularly criminology. Criminals in the pre-modern period were written about in popular literature, but were not known in the sense Foucault means. Knowledge is an organized collection of facts about a subject or individual, obtained by specific technical means. When the prisoner becomes an individual or delinquent, he can become the subject of knowledge. Power justifies the claims that the human sciences make about knowledge, and so the delinquent is caught up in a complicated relationship between disciplinary power and the knowledge that it creates.