Abstract
English It is widely accepted that a person's existing knowledge plays a critical role in learning science. Two different aspects of this role are illuminated by the research reported in this paper. A graduate tutor in freshman physics was interviewed on three separate occasions. The first interview was used to determine what metaphysical commitments he held in relation to special relativity theory and showed the significant role his commitments played in his acceptance of counter‐intuitive aspects of the theory. The second interview was used to show the stability of his commitments four months later and to present the metaphysics underlying the theory in comparison and contrast with his own. The third interview, ten days later, repeated parts of the first interview and showed that significant changes in his metaphysical commitments had occurred. This case study demonstrates firstly the importance of the learner's metaphysical commitments as components of existing knowledge and secondly the effectiveness in facilitating conceptual change of an instructional strategy which explicitly addresses the learner's existing knowledge. An explanation of the effectiveness of instruction is given by an analysis of the interviews in terms of a previously developed model of learning as conceptual change.