As long ago as 1865 I published, under the title, _Unknown Natural Forces_, a little monograph of a hundred and fifty pages which is still occasionally found in the book-shops, but has not been reprinted. I reprint here (pp. xiii-xxiii), what I wrote at that time in this critical study "apropos of the phenomena produced by the Davenport brothers and mediums in general." It was published by Didier & Co., book-sellers to the Academy, who had already issued my first two works, _The Plurality of Inhabited Worlds_ and _Imaginary Worlds and Real Worlds_. "France has just been engaged in an exciting debate, where the sound of voices was drowned in a great uproar, and out of which no conclusion has emerged. A disputation more noisy than intelligent has been raging around a whole group of unexplained facts, and so completely muddled the problem that, in place of illuminating it, the debate has only served to shroud it in deeper darkness. "During the discussion a singular remark was frequently heard, to the effect that those who shouted the loudest in this court of assize were the very ones who were least informed on the subject. It was an amusing spectacle to see these persons in a death-grapple with mere phantoms. Panurge himself would have laughed at it. "The result of the matter is that less is known to-day upon the subject in dispute than at the opening of the debates. "In the mean time, seated upon neighboring heights were certain excellent old fellows who observed the writs of arrest issued against the more violent combatants, but who remained for the most part grave and silent, though they occasionally smiled, and withal did a deal of hard thinking. "I am going to state what weight should be given to the opinions of those of us who do not rashly affirm the impossibility of the facts now put under the ban and who do not add their voices to the dominant note of opposition.