Over the years, a person has been interested in the origin of dreams. In his book, M. Jouvet tells how dreams are born, what relationship exists between sleep and wakefulness and how it led him to the discovery of "paradoxical sleep" - a state in which the brain is extremely active. It is at this moment that our consciousness works very clearly. To understand him, the author turns to the experience of philosophers - among them Locke, Hegel, Kant, Hume, Husserl, and speaking of the subconscious, recalls Freud. In conclusion, M. Jouvet describes new studies aimed at studying the capabilities of the brain - the main mystery of man, which no one has yet managed to unravel.