By far the greater part of the theories of the soul which have been put forward during the last two thousand years or more are based on introspective inquiry--that is, on "self-observation," and on the conclusions which we draw from the association and criticism of these subjective experiences. Introspection is the only possible method of inquiry for an important section of psychology, especially for the study of consciousness. Hence this cerebral function occupies a special position, and has been a more prolific source of philosophic error than any of the others (cf. chap. x.). It is, however, most unsatisfactory, and it leads to entirely false or incomplete notions, to take this self-observation of the mind to be the chief, or, especially, to be the only source of mental science, as has happened in the case of many and distinguished philosophers. A great number of the principal psychic phenomena, particularly the activity of the senses and speech, can only be studied in the same way as every other vital function of the organism--that is, firstly, by a thorough anatomical study of their organs, and, secondly, by an exact physiological analysis of the functions which depend on them. In order, however, to complete this external study of the mental life, and to supplement the results of _internal_ observation, one needs a thorough knowledge of human anatomy, histology, ontogeny, and physiology. Most of our so-called "psychologists" have little or no knowledge of these indispensable foundations of anthropology; they are, therefore, incompetent to pronounce on the character even of their own "soul." It must be remembered, too, that the distinguished personality of one of these psychologists usually offers a specimen of an educated mind of the highest civilized races; it is the last link of a long ancestral chain, and the innumerable older and inferior links are indispensable for its proper understanding. Hence it is that most of the psychological literature of the day is so much waste paper. The introspective method is certainly extremely valuable and indispensable; still it needs the constant co-operation and assistance of the other methods.