The motivation for the work reported in this paper is the belief that not only is it beneficial to reuse knowledge but it is essential if we wish to build knowledge-based systems (KBS) that meet the needs of users. The focus of most KBS research is on complex modelling at the knowledge level which requires a knowledge engineer to act as the intermediary between the expert and the system. The type of reuse primarily considered is the reuse of ontologies or problem-solving methods so that improvements can be made in system quality and development time. However, there is little focus on the needs of users to access the knowledge in a variety of ways according to the individual's decision style or situation. The system described in this paper seeks to support the user in a number of different activities including knowledge acquisition, inferencing, maintenance, tutoring, critiquing, 'what-if' analysis, explanation and modelling. The ability to ask different types of questions and to explore the knowledge in alternative ways is a different type of knowledge reuse. The knowledge acquisition and representation technique used as the foundation is known as ripple-down rules (RDR). To support the exploration activities, RDR have been combined with formal concept analysis which automatically generates an abstraction hierarchy from the low-level RDR assertions. The paper suggests that rapid and incremental KA together with retrospective modelling can be used to provide the user with a system that they can own, build and explore without the difficulties associated with capturing and validating the conceptual models of experts via the mediation of a knowledge engineer.