This is the first monograph to consider the significance of madness and irrationality in both Spanish and Spanish American literature. It considers various definitions of madness and explores the often contrasting responses, both positive (figural madness as stimulus for literary creativity) and negative (clinical madness representing spiritual confinement and sterility). The concept of national madness is explored with particular reference to Argentina, where the country’s vast expanses have been seen as conducive to madness, while the urban population of Buenos Aires is especially dependent on psychoanalytic therapy. The discussion considers both the work of lesser-known writers such as Nuria Amat, whose personal life is inflected by madness, and that of larger literary figures such as José Lezama Lima, whose poetic concepts are suffused with the irrational. The conclusion draws attention to the other side of reason as a source of possible originality in a world dominated by the tenets of logic and conventionalised thinking.