This article shows the importance of the early formation of British multiculturalism as a crucial backdrop for understanding the Hart-Devlin debate of the 1960s. Hart criticized Devlin’s “disintegration thesis,” showing that it fails sociologically. However, Devlin’s position does not focus on the abstract dangers of social disintegration, but is concerned, rather, with specific aspects of the British legal tradition. Contrary to other European cultures, liberty, and tradition are not opposed in common law culture. British liberalism relies on certain legal standards, such as the “ordinary Englishman,” which seemed challenged in the face of mass immigration during the late 1950s. Rather than a confrontation between liberal and conservative camps, the Hart-Devlin debate reveals two conflicting notions of liberty, and a profound crisis in post-war British legal culture.