In the invisible (No invisível - Inglês):

Allan Kardec recommended prudence and knowledge as necessary preparation for the practice of mediumship, as they are the means for avoiding conscious and unconscious errors. In the Invisible is an indispensable book for students of mediumistic experimentation. Léon Denis wrote this treatise on experimental Spiritism, which, besides studying the laws that govern the communications between the invisible and physical worlds, presents a large number of spirit-related cases researched by scholars and the conclusions they reached. The author concludes that, “The constant and in-depth study of the invisible world, which is also the world of causes, will be the great resource and the inexhaustible reservoir on which thought and life must be nourished. Mediumship is the key.”