Hervey de Saint-Denys (Marie-Jean-Léon, Marquis d'Hervey de Saint Denys) published this book in 1867, and since then it has become one of the forerunners of the study of lucid dreaming. This is one of the few 19th century works that has lost none of its freshness or usefulness with the passage of time, due to the author's entirely practical foundation. In the second part of this work, devoted mainly to a history of professed views on sleep and dreams from antiquity to modern times, the author expounds his own ideas, based on numerous practical observations, supporting with ample evidence his system of dream analysis and control. Long before there was any talk of "lucid dreams", Hervey de Saint-Denys shows us how we can obtain lucidity and control in our dreams, and how their study will help us to know ourselves.