Perspectives on Science (POS) is a unique research-based post-16 course that addresses the history, philosophy and ethical aspects of science. Our central research question was to what extent is POS successful in promoting high-quality discussion in class and what factors influence this. Through questionnaires, interviews and observations of discussion episodes we were able to identify the most prominent characteristics: the instrumental role of the teacher, the introduction of conflict, and a balance between respect for rational argument and the advocacy of a position with passion. (Contains 1 box.)