Professor Thury thinks that this fluid, which he calls psychode, is a substance which forms a link between the soul and the body; but there may also exist certain wills external to ourselves, and of unknown nature, working side by side with us.
The chemist Crookes attributes the phenomena to psychic force, this being the agent by which the phenomena are produced; but he adds that this force may well be, in certain cases, seized upon and directed by some other intelligence. "The difference between the partisans of psychic force and those of Spiritualism," he writes, "consists in this: we maintain that it is not yet proved that there exists a directing agent other than the intelligence of the medium and that presence and actions of the spirits of the dead are felt in the phenomena, while, on the contrary, the Spiritualists accept as an article of faith, without demanding more proofs thereof, that these spirits are the sole agents in the production of the observed facts."
Albert de Rochas defines these phenomena as "an externalization of motivity," and considers them to be produced by the fluidic double, "the astral body" of the medium, a nerve-fluid able to act and perceive at a distance.
Lombroso declares that the explanation must be sought simply in the nervous system of the medium, and that we have in the phenomena transformation of forces.
Dr. Ochorowicz affirms that he has not found proofs in favor of the Spiritualistic hypothesis, any more than he has in favor of the intervention of external intelligences, and that the cause of the phenomena is a fluidic double detaching itself from the organism of the medium.
The astronomer Porro is inclined to admit the possible action of unknown spirits, of living forms different from our own, not necessarily the souls of the dead, but psychical entities to be studied. In a recent letter he wrote me that the theosophic doctrine appeared to him to approach the nearest to a solution.[82]
Prof. Charles Richet thinks that the Spiritualistic hypothesis is far from being demonstrated, that the observed facts relate to an entirely different order of causes, as yet very difficult to disentangle and that in the present state of our knowledge no final conclusion can be agreed on.
The naturalist Wallace, Professor Morgan, and the electrician Varley declare, on the other hand, that sufficient proof has been given them to warrant them in accepting without reserve the Spiritualistic doctrine of disembodied souls.
Prof. James H. Hyslop, of the University of Columbia, who has made a special study of these phenomena, in the Proceedings of the London Society for Psychical Research, and in his works Science and a Future Life and Enigmas of Psychical Research, thinks that there are not yet enough severely critical verifications to warrant any theory.
Dr. Grasset, a disciple of Pierre Janet, does not admit displacement of objects, or levitation, or the greater part of the facts described in this book as proved, and thinks what is called Spiritualism is a question of medical biology, of "the physiopathology of the nervous centres," in which a celebrated cerebral polygon with a musical conductor named O, plays an automatic rôle of a very curious description.