It will therefore be well at the outset to indicate exactly what it is proposed to include in the present investigation into "Spiritualism." The alleged phenomena of Spiritualism may be roughly divided into two classes--physical and mental. Those which belong entirely to the latter class are outside the scope of this book. It is proposed to examine those phenomena of the former class, the reality of which may fairly be assumed to be proved by scientific evidence. The scope of the work is thus reduced to reasonable proportions. There are several groups of phenomena which appear to violate, or at least to extend in a striking manner, laws recognised by Physical Science. The evidence to be relied on will be that of scientific men of high standing, and of other persons of unquestioned literary and social position. There is, however, an important respect, in regard to which this inquiry is placed in an entirely different position to any ordinary scientific investigation, and one which adds greatly to the difficulties of the student. Ordinary experiments conducted in a physical laboratory can be repeated again and again under similar conditions, and similar results will follow. If attempts are made to reproduce the phenomena of Spiritualism, under what appear to be precisely similar conditions, by means which have previously been successful, failure to obtain the wished-for results may very probably follow. It is no use to rebel and to feel inclined to abandon the pursuit as useless! That would be most unscientific! The inquirer finds himself in the presence of a subtle elusive influence, which he seems unable to control, and which refuses to submit to the laws which govern physical experiments. On the other hand, perseverance may be richly rewarded. An unexplored field of scientific research of unlimited extent may open itself to view. Something of that joy may be experienced which the search into the unknown alone can give. Mr. Arthur James Balfour, in an address on the occasion of the annual dinner of the Royal Literary Fund, in 1893, said:-- "My friend, Lord Kelvin, has often talked to me of the future of science, and he has said words to me about the future of science which are parallel with the words I have quoted to you about the future of art, and with the hope which I have expressed to you with respect to literature. He has told me that to the men of science of to-day it appears as if we were trembling on the brink of some great scientific discovery which should give to us a new view of the great forces of Nature, among which and in the midst of which we move. If this prophecy be right, and if the other forecasts to which I have alluded be right, then indeed it is true that we live in an interesting age; then indeed it is true that we may look forward to a time full of fruit for the human race--to an age which cannot be sterilised or rendered barren even by politics."