“No phenomenon of the life of the soul is so wonderful and so variously interpreted as consciousness. The most contradictory views are current to-day, as they were 2,000 years ago, not only with regard to the nature of this psychic function and its relation to the body, but even as to its diffusion in the organic world and its origin and development. It is more responsible than any other psychic faculty for the erroneous idea of an “immaterial soul” and the belief in “personal immortality”; many of the gravest errors that still dominate even our modern civilisation may be traced to it. Hence it is that I have entitled consciousness “the central mystery of psychology”: it is the strong citadel of all mystic and dualistic errors, before whose ramparts the best equipped efforts of reason threaten to miscarry. This fact would suffice of itself to induce us to make a special critical study of consciousness from our monistic point of view. We shall see that consciousness Is simply a natural phenomenon like any other psychic quality, and that It is subject to the law of substance like all other natural phenomena.
The only source of our knowledge of consciousness is that faculty itself; that is the chief cause of the extraordinary difficulty of subjecting it to scientific research. Subject and object are one and the same in it: the perceptive subject mirrors itself in its own inner nature, which is to be the object of our inquiry.”