I will give the name of ‘medium’ to any person capable of producing any of the phenomena previously mentioned. I adopt the word ‘medium,’ because it is consecrated by custom and has received the precise signification I mention. Some philosophers criticise this definition. Their criticisms are, I think, misplaced. In metaphysics it is easy to give definitions which, though elegant, are founded upon nothing. In physics—I use this word in its etymological and primitive sense—a being can only be defined by its properties. Definitions of this kind state a fact, which is all we can require of them; they serve one purpose, which is to avoid a long periphrase. Any other definition would lead to the supposition, that the veritable knowledge of the cause of the phenomena observed or of the properties recorded, was known; now, it seems to me impossible to affirm the real cause of the facts I have observed. I confine myself to stating them without forming any hypotheses. A medium is, therefore, a person in presence of whom ‘psychical’ phenomena can be observed. I use this word ‘psychical’ with regret, because it implies a hypothesis. As a rule it is necessary to experiment with mediums in order to discover them. Their gifts are often latent, and only reveal themselves if conditions favourable to their manifestation are supplied. This is not always the case, and there is generally a chance of coming across a medium when experimenting with persons in whose presence certain irregular abnormal noises are heard, certain movements of furniture are spontaneously produced. Such things are far from being as uncommon as one would think. This assertion may seem paradoxical, but such is not the case. I have met with good mediums who were ignorant of the existence of their faculties; yet, when I questioned them, I discovered that they frequently heard little ‘raps’ upon the wood of their bed or upon their night-table, without attaching any importance to it. Others have often noticed the displacement of ordinary articles. Sometimes, but more rarely, the facts observed are so intense that the house appears to be haunted. We are often tempted to attribute to fraud the phenomena of haunting. I believe accounts of this nature are not all false, and I shall perhaps try and show this in a future work. We must not reason like one of my friends, a man of vast erudition and superior intelligence, who one day said to me: ‘A little girl from thirteen to sixteen years old is always to be found in haunted houses—as soon as the little girl is taken away the phenomena cease!’ Granted! Things generally happen thus; only the little girl may not be the voluntary cause of the phenomena: she may be the involuntary cause of them, a medium in activity, producing supernormal phenomena of the nature of those observed at spiritistic seances.