It is commonly assumed that psychologists studying human reasoning may safely ignore the normative issue of rationality, and that philosophers may analyze rationality without reference to empirical research on actual human reasoning. Presenting five arguments against such a mutually exclusive dichotomy, this paper concludes that although empirical and normative study can and must be differentiated, they should not be conducted independently of each other. At the very least, psychologists must consider normative evaluation of rationality in order to meaningfully investigate human reasoning and its development, and philosophers must consider empirical research on human reasoning if their conclusions about rationality are to have any relevance to specifically human rationality. Even better, it may be useful to conceive the study of reasoning and rationality as a single, transdisciplinary endeavor in which empirical and normative considerations, though conceptually distinct, are inextricably intertwined. (RH)