* ABSTRACT In his early writings, the late Joseph Ben-David put forward a model accounting for certain kinds of cases of the emergence of new disciplines. We argue that Ben-David's explanations fall squarely within the sociology of knowledge - and, in fact, have many points in common with explanations in the tradition of the 'strong programme' in that discipline. Specifically, Ben-David's central theoretical concept - that of 'role hybridization' - overlaps the notion of 'multifunctionality' of theories. Ben-David's approach is superior to that of the 'strong programme' in as much as, being hermeneutical and not causal, it avoids the dangers usually besetting the sociology of knowledge - notably relativism. In the second part of the paper we analyze the emergence of Ludwik Fleck's ideas and - using Ben-David's model - we relate them to the social context within which Fleck was working. We show that social conditions induced the outsider bacteriologist Fleck to adopt a strategy of role-hybridization. This strategy gave rise to 'idea-hybridizations' which led to his original, multifunctional theory of scientific growth. Ludwik Fleck's Roles in Society: A Case Study Using Joseph Ben-David's Paradigm for a Sociology of Knowledge Gad Freudenthal and Ilana Lowy To Joseph Ben-David: in memoriam The purpose of this paper is to present in outline a paradigm for sociology of knowledge and to illustrate its fruitfulness by applyin to explain sociologically the genesis and development of Ludwik Fle philosophy and sociology of science. The paradigm we describe is - we suggest - implicit in the work of the late Joseph Ben-David: his (largely justified) reputation as an un- compromising opponent of the sociology of knowledge notwithstanding, several of Ben-David's studies can in fact be construed as sociological accounts of the emergence and the contents of ideas - usually, although not always, scientific ideas - which fall squarely within the sociology Social Studies of Science (SAGE, London, Newbury Park and New Delhi), Vol. 18 (1988), 625-51