The subject treated in the following pages has made great progress in the course of forty years. Now what we are concerned with in psychical studies is always unknown forces, and these forces must belong to the natural order, for nature embraces the entire universe, and everything is therefore under the sway of her scepter.
I do not conceal from myself, however, that the present work will excite discussion and bring forth legitimate objections, and will only satisfy independent and unbiased investigators.
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