The intolerance of certain savants is equalled by that of certain dogmas. To take an example, Catholicism considers psychical phenomena as the work of the devil! Is it worth while at this hour to discuss so obsolete a theory? I think not. However, superior ecclesiastical authorities, with the tact and sentiment of opportunism which they often show, permit many Catholics to undertake the experimental study of psychical facts. I cannot blame them for recommending prudent abstention to the mass of the faithful; spiritism appears to me to be an adversary with which they will have to reckon very seriously some day. The simplicity of its doctrines ensures it the _clientèle_ of simple souls enamoured of justice, that is to say, of the immense majority of mankind. But this question is foreign to psychical facts themselves. As far as my experience permits me to judge of them, these phenomena contain nothing but what is natural. The devil does not show his hoof here, timorous souls may feel reassured; if the tables claim to be Satan himself, they need not be believed; summoned to prove his power, this grandiloquent Satan will be a sorry thaumaturgist. Religious prejudice, which proscribes these experiments as being supernatural, is just as little justified as scientific prejudice, which sees therein nothing but fraud and imposture. Here, again, the old adage of Aristotle finds its application: Justice lies midway. May my book determine a few experimenters of goodwill to try to observe in their turn. May it help to dispel from the mind of gifted mediums their fears of being ranked with insane and disordered intelligences, or looked upon as being in partnership with the devil. May it especially contribute to make metapsychic phenomena come to be considered as natural facts, worthy of being usefully observed, and capable of enabling us to penetrate more deeply than any other phenomena into a real knowledge of the laws which govern Nature. APPENDIX A _An Appreciation on Certain Documents published on the subject of Fraud._ The question of fraud is so important that I feel I should not only give the results of my own observations, but also my appreciation of some of the principal documents published on the subject. With the exception of Richet and a few others, representatives of science in France are very ill informed on this question, as I have endeavoured to show. They overlook the immense work which has been done in the United States and in England; consequently it is very difficult to discuss the question with these savants, they are either ignorant or feign to be ignorant of what others have done. I have shown that their experiments are defective and their methods open to criticism.