This book turns upside-down the world of rationality. Rationality has been philosophers’ concern for hundreds of years, if not back to ancient times. Now, in trying to determine just what rationality consists in, we find it even more complex than supposed. That supposition has not been thoroughly enough investigated. This work aspires to bring out and bring together the facets of this peculiar phenomenon, rationality. It is both more complex than presumed, yet more accessible than may have been feared. This book’s systematic approach should make this mystery much less obscure. While for argument conceding that common assumption that those interested in rationality need only rely on the intuitions about this phenomenon, even moderate research reveals the concept’s profound fuzziness. By the end, though, rationality, as is appropriate, and its criticisms provide much more luminous light than has long been missing.