People often wonder why economists analyze models whose assumptions are known to be false, while economists feel that they learn a great deal from such exercises. We suggest that part of the knowledge generated by academic economists is case-based rather than rule-based. That is, instead of offering general rules or theories that should be contrasted with data, economists often analyze models that are "theoretical cases", which help understand economic problems by drawing analogies between the model and the problem. According to this view, economic models, empirical data, experimental results and other sources of knowledge are all on equal footing, that is, they all provide cases to which a given problem can be compared. We offer complexity arguments that explain why case-based reasoning may sometimes be the method of choice; why economists prefer simple examples; and why a paradigm may be useful even if it does not produce theories. * We are thankful to many colleagues and friends with whom we have discussed the issues addressed here over the years. It is practically impossible to recall which ideas were suggested by whom, and we apologize for any inadvertent plagiarism. We thank Hervé Crès, Robin Cubitt, Eddie Dekel, Brian Hill, Doron Ravid, Jack Vromen, and Bernard Walliser for comments on earlier drafts of this paper. We gratefully acknowledges ISF Grant 396/10 (Gilboa and Schmeidler), ERC Grant 269754 (Gilboa), NSF Grants SES-0961540 (Postlewaite) and SES-0850263 (Samuelson).