In Vienna, in 1825, a weak and listless teenager is talked about a lot. In broad daylight he falls into a deep sleep and changes his personality. While sleeping, he reads, writes, plays cards, challenges the doctors with amusement, performs the most surprising exercises with his eyes closed. A new subject has appeared, or a second "I" has now supplanted the first. Cavalletti meticulously records the disturbing appearances of this second "I" in the psychology and in the literature of the last two centuries. In a scenario dominated by amnesia and sleepwalking, hallucinations and daydreams, the bourgeois subject, whose identity seemed so solid, is revealed to be inhabited by masks that defy any mastery, in the grip of a doubling that can in no way be reassembled.In the cases that Henri Bergson will study carefully, in the visions gained by Théophile Gautier with the help of hashish, fixed by Poe in the projections of the nightmare or reversed by Döblin in comic parodies, the personalities multiply and fight each other, and even life and death exchanges their parts. And, in the end, the identity of the Western subject turns out to be a shadowy and constitutively double figure, who lives only in his faintings and his forgetfulness, in his losses and his distractions. Immemorial and, precisely for this reason, unforgettable.