The problem of hysteria, although for some years past it has engaged the special attention of the best neurologists and psychologists, is yet very far from being solved. The chief object which authors have had in view at recent congresses has been to obtain a satisfactory definition of this affection. A definition, however, as Claparéde justly observes, being only a citation of the characters which constitute the nature of a thing, should be the crown and not the foundation of the study of any malady. Hysteria cannot, then, be defined without first establishing the nature of this affection, without studying the mechanism of its morbid phenomena. Now, an essentially pathogenic study of this disorder has not yet been made. It has been supposed to be the consequence of a narrowing of the field of consciousness (Janet), the suppression in consciousness of painful impressions of sexual origin (Freud), of suggestibility (Babinski), of an exaggeration of the plastic power of the imagination (Hartemberg), of torpor of the organic cerebral mechanisms (Sollier), of a tendency to reversion, to atavism (Claparéde), of disorder of the cortical or sub-cortical reflexes (Raymond), of an exaggerated emotional reflex (Bernheim), of perversion of the highest cerebral functions (Bastian); but it has not been explained by what mechanism all these conditions are produced, and provoke the morbid phenomena. It is just here that the chief difficulty of all these theories lies; it is the point by which we may judge of their respective value. Even if we assume, for instance, that hysteria is due to a limitation of the field of consciousness, what, we may ask, is the primary cause which determines this modification of the personality? By what mechanism does a suggested idea or a painful representation translate itself, or become converted, into somatic phenomena, or into an exaggerated emotional reflex? It does not suffice to tell us that this affection is characterised by a flexile suggestibility or by an exaggeration of a plastic imagination; we require also an explanation as to what determines the plastic properties in the case of these patients. What is the cause which induces the torpor of the organic functions of the brain as in Sollier's theory, the disorder of cortical or sub-cortical reflexes as in that of Raymond?