Traditionally, power was what was seen, what was shown, and what was manifested. . . . Disciplinary power, on the other hand, is exercised through its invisibility; at the same time it imposes on those whom it subjects a principle of compulsory visibility. In discipline, it is the subjects who have to be seen.

One of Foucault's overarching points in Discipline and Punish is to challenge the idea that how we treat the imprisoned—both the "insane" and the criminal—has become more humane over time. In fact, Foucault argues, our treatment of the incarcerated has become far less humane. Foucault contends that in the Middle Ages and through the Renaissance we concentrated on constraining and controlling the body. This relates, as the above quote indicates, to the idea that power in those times was understood to be manifested through what was visible, what was on display. A king, for example, revealed his power through appearing bodily in public in all his regalia: spectacle was a sign of power. Likewise, the ability to torture—and to publicly torture or kill those who fell out of the confines of the acceptable—was how power was displayed.

Very quickly, however, that all changed, and suddenly, as Foucault notes, all the branded and mutilated bodies disappeared. This reflects a profound shift in the notion of power: from the Enlightenment and onward, it has become more powerful to be unseen or invisible. In the modern institution or prison, it is the prisoner—the powerless—who is constantly under surveillance by invisible eyes. Foucault, for example, spends much time describing the Panopticon, Jeremy Bentham's never-built model prison designed so that the prisoner would ceaselessly be under observation by a warden or guard. We might wonder what Foucault would make of the modern surveillance state, but we can imagine he would be highly displeased.

the soul is the prison of the body

Foucault writes poignantly of how, in the Middle Ages and the Renaissance, prisons and "madhouses" controlled the body but left an individual's soul free. A person might be chained up in a madhouse, but he was left to scream that he was Jesus Christ and to believe it: nobody cared, nobody was trying to "fix" him or "cure" him. He might be in chains, but he was himself. This is because the state was content to manifest its power in the physical display of imprisonment.

All of that changed in the so-called Enlightenment, says Foucault. At this point, society moved from punishment to discipline. In discipline, the state tried to get inside the heads of the imprisoned, such as the "insane," and cure them. Foucault points to supposedly "humane" mental institutions that, rather than chain people up, put them in isolation and had them examine their souls. The attempt was to change the prisoner: to jail the prisoner in the prison of his own mind by making him feel guilty about his former behavior. The idea was that the mentally ill would so internalize the ideas of the powerful—their keepers—that they would self-regulate and not need chains. This, Foucault says, is the true cruelty, the true imprisonment, and the true destruction of the individual.

Is it surprising that prisons resemble factories, schools, barracks, hospitals, which all resemble prisons?

The state wants to control its citizens's minds and souls and mold them to conform to the needs of the state. It does this through a variety of state institutions, from schools to prisons—in fact, through all state institutions. The state wants, every way it can, to exercise power over the body by controlling people's minds so that it can bend the bodies of the population to its own needs and will.