This paper contributes to the reappraisal of sociological theories of modernity inspired by the sociology of scientific knowledge (SSK). As much as these theories rely on received ideas about the nature of science that SSK has called into doubt, so do they rely on ideas about the public understanding of science. Public understanding of science has been assumed to conform to the monolithic logic and perception of science associated with rationalization, leading to an impoverished view of the cognitive outlook of the modern individual. Rationalization has become the basis for the construction of theoretical critique of science divorced from any clear reference to public understanding, with the result that theory has encountered considerable problems in accounting for public scepticism towards science. However, rather than question rationalization, the more typical strategy has been to propose radical changes in the modernization process, such as postmodernism and the risk society. Against this, an alternative view of public understanding is advanced drawn from SSK and rhetorical psychology. The existence of the sociological critique of science, and SSK in particular, suggests that the meaning of science in modernity is not monolithic but multiple, arising out of a central dilemma over the universal form of knowledge-claims and their necessarily particular, human and social grounding. This dilemma plays out not only in intellectual discourses about science, but also in the public's understanding of science. This argument is used to call for further sociological research into public understanding and to encourage sociologists to recognize the central importance of the topic to a proper understanding of modernity.