The origin of cases is a central issue in cogni- tive models of case-based reasoning. Some recent work proposes the use of weak methods for gener- ating solutions when a relevant case is not avail- able, and chunking the solutions into cases for potential reuse. Our theory of case-based spsi- tial planning and navigation suggests a different approach in which mental models of the world provide a way for solving new problems and ac- quiring cases. These mental models also pro- vide a scheme for organizing the case mmeory, adapting old cases, and verifying new plans. The use of multiple methods, such as the case-based and model-based methods, raises another impor- tant issue in reasoning, namely, how to oppor- tunistically select and dynamically integrate the methods. Our theory sujjgests the use of simple meta^reasoning to recursively select an appropri- ate method as the problem is decomposed into subproblems. This leads to the dynamic integra-tion of different methods where one method is used for one subproblem and a different method for another subproblem.