After an exhaustive study of many nervous patients afflicted with phobias and obsessions a tentative explanation of these symptoms urged itself upon me. This helped me afterwards happily to divine the origin of such morbid ideas in new and other cases, and I therefore believe it worthy of reporting and further examination. Simultaneously with this “psychological theory of phobias and obsessions,” the examination of these patients resulted in a contribution to the theory of hysteria, or rather in an alteration of the same, which seems to imply an important and common character to hysteria as well as the mentioned neuroses. Furthermore, I had the opportunity to look into the psychological mechanism of a form of indubitable psychic disease and found that my attempted observation shows an intelligible connection between these psychoses and the two neuroses mentioned. At the conclusion of this theme I will describe the supporting hypothesis which I have used in all three cases. I. I am beginning with that alteration which seems to be necessary for the theory of the hysterical neuroses. That the symptom-complex of hysteria as far as it can be understood, justifies the assumption of a splitting of consciousness with the formation of separate psychic groups, has attained general recognition since P. Janet, J. Breuer, and others have given out their interesting work. Less understood are the opinions concerning the origin of this splitting of consciousness and concerning the rôle played by this character in the structure of the hysterical neuroses. According to Janet’s[36] theory, the splitting of consciousness is a primary feature of the hysterical alteration. It is due to a congenital weakness of the capacity for psychic synthesis, and to a narrowing of the “field of consciousness” (champ du conscience) which as a psychic stigma proves the degeneration of hysterical individuals. In contradistinction to Janet’s views, which in my opinion admit the most manifold objections, are those advocated by J. Breuer in our joint communication. According to Breuer, the “basis and determination” of hysteria is the occurrence of peculiar dream-like conscious states with a narrowed association capacity, for which he proposes the name “hypnoid states.” The splitting of consciousness is secondary and acquired, and originates because the ideas emerging in the hypnoid states are isolated from associative communication with the rest of consciousness.