The book proposes a sociological model of plural opinion which explains how an actor can uphold two different opinions on the same issue. It is grounded on well known theories to show that in uncertain situation an actor forms an opinion by discussing in interpersonal relations and that these relations, embedded in normatively heterogeneous social circles, push him to agree with various opinions on the same topic. The works of B.Lahire and J.C.Kaufman are presented as well as researches on political behaviour and knowledge about Aids. Recent researches on discussing networks (about health, cancer, aids, or public facilities) bring empirical support to the model and present new method more pertinent to develop a cognitive sociology which stands apart as well of methodological individualism as of holism.