The relationship between power and knowledge is central to Foucault's work. Discipline and Punish essentially charts the reorganization of the power to punish, and the development of various bodies of knowledge (the human sciences) that reinforce and interact with that power. The modern power to punish is based on the supervision and organization of bodies in time and space, according to strict technical methods: the modern knowledge that Foucault describes is the knowledge that relates to human nature and behavior, which is measured against a norm. Foucault's point is that one cannot exist without the other. The power and techniques of punishment depend on knowledge that creates and classifies individuals, and that knowledge derives its authority from certain relationships of power and domination.
The body as an object to be acted upon, but also as the subject of "political technology" is present throughout the work. Beginning with public execution, where the body is horrifically displayed, Foucault charts the transition to a situation where the body is no longer immediately affected. The body will always be affected by punishment—because we cannot imagine a non-corporal punishment—but in the modern system, Foucault says, the body is arranged, regulated and supervised rather than tortured. At the same time, the overall aim of the penal process becomes the reform of the soul, rather than the punishment of the body. Eventually, the concepts of the individual and the delinquent replace the reality of the body as the focus of attention, but the body of the criminal still plays a role. If anything can be seen as constant in this work, it is the idea that the body and punishment are closely linked.
Foucault's project in Discipline and Punish is to account for the modern penal system, but he also presents a genealogical account of the modern soul. This is not only due to the fact that the soul gradually replaces the body as the focus of punishment and reform. It is also due to the fact that modern processes of discipline have essentially created that soul. Without the human sciences and the various mechanisms of observation and examination, the normal soul or mind would not exist. Ideas such as the psyche, conscience, and good behavior are effects created by a particular regime of power and knowledge. For Foucault, examining that regime is a way of looking deep into our souls. His history of the soul is also a powerful critique, because it makes us confront what we have become by excluding and marginalizing certain elements of our society.
The relationship between the prison and the wider society cannot be stressed enough. For Foucault, the prison is not a marginal building on the edge of a city, but is closely integrated into the city. The same "strategies" of power and knowledge operate in both locations, and the mechanisms of discipline that control the delinquent also control the citizen. Indeed, the methods of observation and control that Foucault describes originated in monasteries, in hospitals and in the army. Related to this point is Foucault's argument that we cannot abolish the prison, because our ways of thinking about and carrying out punishment will not allow it. The prison is part of a "carceral network" that spreads throughout society, infiltrating and penetrating everywhere.
This is perhaps more a stylistic point than a theme, but Discipline and Punish reveals Foucault's addiction to contrast and contradiction. From the shocking image at the beginning of the work to the many numbered lists and pairs of terms, the work is structured around a series of conceits and paradoxes. The idea that prison contains its own failure is a good example. Critics have criticized Foucault for what they see as his chronic obscurity, but at least part of the problem comes from his attitude to language and discourse. Discourses are complex structures in which people can become "trapped": perhaps the experience of being trapped inside some of Foucault's more difficult sentences is meant to echo this. Or perhaps, like many French philosophers, he was just fond of linguistic jokes.