"In the course of the long and laborious studies to which I have consecrated many a night, as a relief or by-play in more important work, I have always observed in these phenomena the action of a force the properties of which are to us unknown. Sometimes it has seemed to me analogous to that which puts to sleep the magnetized subject under the will of the hypnotizer (a reality this, also slighted even by men of science). Again, in other circumstances, it has seemed to me analogous to the curious freaks of the lightning. Still, I believe I can affirm it to be a force distinct from all that we know, and which more than any other resembles intelligence. "A certain savant with whom I am acquainted, M. Fremy, of the Institute, has recently presented to the Academy of Science, apropos of spontaneous generation, substances which he has called _semi-organic_. I believe I am not perpetrating a neologism bolder than this when I say that the force of which I am speaking has seemed to me to belong to the _semi-intellectual_ plane. "Some years ago I gave these forces the name _psychic_. That name can be justified. "But words are nothing. They often resemble cuirasses, hiding the real impression that ideas should produce in us. That is the reason why it is perhaps better not to name a thing that we are not yet able to define. If we did, we should find ourselves so shackled afterwards as not to have perfect freedom in our conclusions. It has often been seen in history that a premature hypothesis has arrested the progress of science, says Grove: 'When natural phenomena are observed for the first time, a tendency immediately arises to relate them to something already known. The new phenomenon may be quite remote from the ideas with which one would compare it. It may belong to a different order of analogies. But this distinction cannot be perceived, since the necessary data or co-ordinates are lacking.' Now the theory originally announced is soon accepted by the public; and when it happens that subsequent facts, different from the preceding, fail to fit the mould, it is difficult to enlarge this without breaking it, and people often prefer to abandon a theory now proved erroneous, and silently ignore the intractable facts. As to the special phenomena in question in this little volume, I find them implicitly embodied in three words uttered nearly twenty centuries ago,--MENS AGITAT MOLEM (mind acting on matter gives it life and motion); and I leave the phenomena embedded in these words, like fire in the flint. I will not strike with the steel, for the spark is still dangerous. '_Periculosum est credere et non credere_' ('It is dangerous to believe and not to believe'), says the ancient fabulist Phaedrus. To deny facts _a priori_ is mere conceit and idiocy. To accept them without investigation is weakness and folly. Why seek to press on so eagerly and prematurely into regions to which our poor powers cannot yet attain? The way is full of snares and