There are books in which the author says so clearly and in such precise terms what he has to say that any commentary weakens their import; and a preface becomes superfluous, sometimes even prejudicial. Dr. Maxwell’s work belongs to this category. The author, who has long given himself up to psychology, has had the opportunity of seeing many interesting things. He has observed everything with minute care; and having well thought out the method of observation, the consequences, and the nature itself of the phenomena, he lays bare his facts and deducts therefrom a few simple ideas, fearlessly, honestly, _sine ira nec studio_, before a public which he hopes to find impartial. To this same public I address the short introduction, with which my friend Dr. Maxwell kindly asked me to head this excellent work. My advice to the reader may be summed up in a few words. He must take up this book without prejudice. He must fear neither that which is new, nor that which is unexpected. In other words, while preserving the most scrupulous respect for the science of to-day, he must be thoroughly convinced that this science, whatever measure of truth it may contain, is nevertheless terribly incomplete. Those imprudent people who busy themselves with ‘occult’ sciences are accused of overthrowing Science, of destroying that bulwark which thousands of toilers, at the cost of an immense universal effort, have been occupied in constructing during the last three or four centuries. This reproach seems to me rather unjust. No one is able to destroy a scientific _fact_. An electric current decomposes water into one volume of oxygen and two of hydrogen. This is a fact which will be true in the eternal future, just as it has been true in the eternal past. Ideas may perhaps change on what it is expedient to call electric current, oxygen, hydrogen, etc. It may be discovered that hydrogen is composed of fifty different bodies, that oxygen is transformed into hydrogen, that the electric current is a ponderable force or a luminous emission. No matter what is going to be discovered, we shall never, in any case, prevent what we call to-day an electric current from transforming, under certain conditions of combined pressure and temperature, what we call water into two gases, each having different properties, gases which are emitted in volumetrical proportions of 2 to 1. Therefore, there need be no fear, that the invasion of a new science into the old will upset acquired data, and contradict what has been established by savants. Consequently psychical phenomena, however complicated, unforeseen, or appalling we may now and then imagine them to be, will not subvert any of those facts which form part of to-day’s classical sciences. Astronomy and physiology, physics and mathematics, chemistry and zoology, need not be afraid. They are intangible, and nothing will injure the imposing assemblage of incontestable facts which constitute them.