The book's two parts mirror its double purpose, to explain and to train. The first part answers the theoretical sceptic: the action of spirits on matter, the perispirit as the semi-material envelope through which a spirit moves tables or guides a writing hand, and the classification of manifestations, physical effects, noises and movements, apparitions, and the intelligent communications that matter most to the doctrine. The second part is the working manual.
Kardec catalogues the mediums themselves, of physical effects, hearing, seeing, speaking, and the writing mediums he considered most useful, distinguishing mechanical from intuitive writing; he prescribes the conduct of seances, regular in composition, serious in purpose, free of curiosity-seeking and of any commerce, Spiritism holding mediumship a faculty to be exercised gratuitously; and he devotes his most careful chapters to obsession and fascination, the states in which a deceiving spirit gains ascendancy over a medium, with their remedies in humility and study. The rule the book made famous is its test of messages: a communication signed with an illustrious name has no authority beyond its content, for frivolous spirits borrow great names freely, and everything received must be weighed by reason and by agreement with the teaching received through many mediums, the control of universal concordance. The final chapters print a dictionary of Spiritist terms and dissertations from the spirits themselves.