As for whether rationality alone is “sufficient”—meaning that by itself, it alone can deliver important predictions—Arrow argued convincingly that rationality alone does not get you very much. To derive useful results, theorists have to add auxiliary assumptions, such as assuming that everyone has the same utility function, meaning the same tastes. This assumption is not only demonstrably false, but it immediately leads to all kinds of predictions that are inconsistent with the facts. We are not Econs and we are certainly not identical Econs.
