 The view of models to which economists are most attracted is philosophically problematic, because it is apparently committed to the existence of fictional entities whose properties and causal propensities economists can investigate. In experiments, whether carried out in a laboratory or in the field, experimenters interact causally with flesh and blood experimental subjects, and the outcome may contradict the economist’s predictions. In investigating a model, in contrast, the economist “interacts” with fictional entities, which are arguably nothing other than his or her own thoughts, and the logical implications of the axioms that define the model are never disappointed. This is not to say that the logical investigation of models never results in surprises. Humans are not logically omniscient, and discovering the implications of a set of axioms may be an arduous task. But it is a different task than carrying out an experiment in the laboratory or the field, and ontology of the “worlds” that economists allegedly “create” and then study is deeply puzzling. Although less faithful to economic practice, it is far more intelligible philosophically to regard models as predicates or as definitions of predicates (Hausman 1992). For example, when economists write down a model of a firm with a single output and just two inputs, they are defining a concept that they can use to describe actual firms.