Rationality has been philosophers' concern stretching back to ancient times. But just what is rationality? In trying to answer this question, rationality is found to be more complex than supposed. This book investigates this supposition and thereby aspires to bring together the facets of the peculiar phenomenon that is rationality. Rationality is shown to be both more complex than presumed and yet more accessible than many may have feared. One argument concedes the common assumption that those interested in rationality need only rely on intuitions about this phenomenon. Yet, even moderate research reveals the concept's profound fuzziness. This book aims to set forth a theory, explanation, and unification of the kindred and disparate understandings of this elusive concept, rationality.