In the papers appearing for the first time in this edition (Chapters XIV. and XV.), Dr. Jung develops his ideas of introversion and extroversion, a contribution of the first importance to psychology. He agrees with Freud in regarding the neuroses to be the result of repression, but differs in his view as to the origin of repression. He finds this to lie not in sexuality _per se_, but rather in man's natural tendency to adapt to the demands of life one-sidedly, according to his type of mentality. The born extrovert adapts by means of feeling, thought being under repression and relatively infantile. The introvert's natural adaptation is by means of thought; feeling being more or less repressed remains undeveloped. In either type the neglected co-function is behind the adapted function. This inequality operating in the unconscious, brings about a conflict, which in certain subjects amounts to a neurosis, and in others produces a limitation of individual development. This view shifts the interpretation of repression on to a much more comprehensive basis than that of sexuality, although there can scarcely be a repression that does not include this instinct on account of its deep and far-reaching importance in man. There is no doubt that some even scientific persons have a certain fear of whither the study of the unconscious may lead. These fearful persons should be reminded that they possess an unconscious in spite of themselves, and that they share it in common with every human being. It is an extension of the individual. To study it is to deepen the self. All new discoveries have at one stage been called dangerous, and all new philosophies have been deemed heresies. It is as though we would once more consign radium to its dust-heaps, lest some day the new radiancy should over-power mankind. Indeed this particular thing has proved at once most dangerous and most precious. Man must learn to use his treasure, and in using it to _submit to its own laws_, which can only become known when it is handled and investigated.