This paper is intended to further the discussion on the use and dissemination of 'expert knowledge', and in particular on the relation between the plausibility of scientific discourse and the credibility of the figure of the scientific expert in the domain of policy making and social intervention. An analysis of the concepts of plausibility and credibility cannot be effectively pursued in terms of the traditional discussions on what counts as strict criteria of demarcation between science and pseudo-science (criteria which are indeed notoriously difficult not only to formulate in principle but also, and even more so, to spell out in practice). Such an analysis should instead lead us to rethinking the form as well as ter substance of scient...