The eighteenth- and nineteenth-century movement to rationalize and humanize legal systems that had accumulated haphazardly over centuries — to replace a mass of precedent, local custom and savage penalty with codified law, proportionate punishment, and procedures a citizen could actually know in advance.
Beccaria's On Crimes and Punishments (1764) opened the argument in Europe, attacking torture and capital punishment on the ground that deterrence depends on the certainty of penalty rather than its severity, and that a state which tortures has already conceded the argument about its own legitimacy. Bentham was the most systematic reformer of the era, drafting codes for several countries and attacking the common law as a fiction that let judges make law retrospectively while pretending to find it. Samuel Romilly worked the same program through Parliament, chipping away at the Bloody Code's two hundred capital offenses; John Howard's prison surveys made conditions a public question; Peel's reforms in the 1820s consolidated much of it into statute.
Most of what they proposed is now simply what law is: codification, proportionality, the presumption of innocence, professional policing, prisons intended to do something other than hold people until trial or execution.
The movement's legacy is genuinely double-edged, and the tradition's own most famous artifact makes the point. Bentham's panopticon — a prison designed so that inmates could always be watched and never know when — was proposed as humane reform, replacing the lash with architecture. Foucault later read it as the emblem of a new kind of power that disciplines by making people watch themselves, which is not what its designer had in mind and is difficult to dismiss.