The reason/no-reason conceptual pair (also declinable in the similar forms of rational/a-rational, logical/a-logical) pervades the history of Western thought from the archaic era up to contemporary times. Perceived in different historical periods and in different cultural forms either as a conflict or as a vital coexistence, the reason/no-reason pair was first theorized and legitimated as a sharp contrast in antiquity with the Pythagorean systoichiai, and at the dawn of the twentieth century it was successfully exemplified by Nietzsche through the opposition between Apollonian and Dionysian principles, which denotes respectively the harmonious, orderly, 'bright' side of the human soul, and the chaotic one, wild, instinctive, passionate, 'dark'. This volume is the outcome of the work of an international research group, which intended to cover some aspects of this dichotomy with the specific end to prove that the two sides of the human 'soul' don't contradict each other - in such a way that one excludes, ontologically and axiologically, the other - but they are rather closely interrelated and interdependent. Scholars with different expertise in the history of thought tackled diachronically some key moments of this story, from different angles and with different approaches, from ancient thought to modern neurosciences. The volume contains contributions of Krzysztof Brzechczyn (Institute of Philosophy, Adam Mickiewicz University), R. Loredana Cardullo (University of Catania), Francesco Coniglione (University of Catania), Santo Di Nuovo (University of Catania), Daniele Iozzia (University of Catania), Syliane Malinowski-Charles (Université du Québec à Trois Rivière), Concetto Martello (University of Catania), Alexandra Michalewski (Paris - Sorbonne, CNRS), Chiara Militello (University of Catania), Sebastian Moro Tornese (United Kingdom), Jean-Marc Narbonne (Laval Université, Canada), Anne Sheppard (Royal Holloway, University of London), Salvatore Vasta (University of Catania), Andrea Vella (University of Catania).