Foucault argues that illegalities became tied up in relations between social classes, and that the resulting conflict between the upper and lower orders led the administrators of society to transform these conflicts of illegality into the concept of a criminal underclass. But because the lower orders were a large, economically necessary group this presented problems. There was a pressing need to control popular revolts and illegality.
Delinquency was the solution. The delinquent was not someone who broke a particular law, but part of a group whose very existence implied illegality and crime. By creating a sub-class if delinquents who were easy to identify and control using the same techniques that operated in the rest of society, these conflicts of power could be resolved. Ultimately, Foucault sees prison and the carceral system as part of a wider system of discipline that developed from the class and economic conflicts of the eighteenth and nineteenth centuries.