This paper discusses the bases of a secondary physics teacher's contradictory conceptions of the nature of science. The data were obtained from a series of four interviews and three months of classroom observation. One of the interviews, a group interview (having as interviewees the subject of this study and four other experienced physics' teachers), has been shown to be a very powerful way of challenging the teacher's views, of making him reflect about his ideas, and at the same time of reducing the risk of the interviewer suggesting particular ideas. An analogy to Piaget's (1929) early methodology supplied a framework with which to categorise the interviewee's replies. This categorisation has provided information about the bases that underpin the teacher's different and conflicting views on the nature of science.