The main purpose of our existence on earth-aside from the sacred and paramount duty of securing our salvation-is undoubtedly to make ourselves masters of the tangible world around us, as it stands revealed to our senses, and as it was expressly made subject to our will by the Creator. We are, however, at the same time, not left without information about the existence of certain laws and the occurrence of certain phenomena, which belong to a world not accessible to us by means of our ordinary senses, and which yet affect seriously our intercourse with Nature and our personal welfare. This knowledge we obtain sometimes, by special favor, as direct revelation, and at other times, for reasons as yet unknown, at the expense of our health and much suffering. By whatever means it may reach us, it cannot be rejected; to treat it with ridicule or to decline examining it, would be as unwise as unprofitable. The least that we can do is to ascertain the precise nature of these laws, and, after stripping these phenomena of all that can be proved to be merely incidental or delusive, to compare them with each other, and to arrange them carefully according to some standard of classification.