This book lays no claim to be a comprehensive treatise upon the psychology of insanity. The number of independent schools of thought existing at the present day, and the fundamental divergence in their methods of investigation, make it obviously impossible to compress such a treatise into the limits of a small volume. All that has been attempted here is the presentation of certain recent developments in abnormal psychology which have already yielded results of fundamental importance, and which seem to offer an exceptionally promising field for further investigation.

An endeavour has been made to develop the subject-matter of the book in a systematic manner, so that the general principles which it is sought to establish may appear in as clear a light as possible. The adoption of the systematic method in a work of this size, however, renders a somewhat dogmatic mode of presentation inevitable. It is necessary, therefore, to warn the reader that many of the theories to which he will be introduced have not as yet been firmly established, and that they are to be regarded rather as suggestive hypotheses which will ultimately require considerable alteration and improvement.

It must be clearly understood, moreover, that no attempt has been made to cover the whole field of insanity. On the contrary, certain sections of that field have been more or less arbitrarily selected, mainly on the ground that they yield fruitful results to psychological methods of investigation.