“With rationality we must imperatively go as far as it can take us. Since it does not lead us to the ultimate goal, that is, to know what is real, we must then rely on less certain guides.” A fundamental text by Bernard d’Espagnat, physicist, philosopher and director of the Institute of Theoretical Physics at the University of Paris-Orsay. In an authentic and direct way, d’Espagnat investigates and verifies the very foundations of physics and the relationship between it and the philosophy of knowledge.