In a lesser degree, amnesia only affects limited periods of life. Epileptics and hysterics often present the phenomenon of _ecmnesia_, a term chosen by the eminent professor of clinical medicine at the university of Bordeaux, M. Pitres, who was the first to point out this phenomenon with hysterical subjects. The patient forgets a part of his life, believes he is ten, fifteen, thirty years younger than he really is, and behaves as though he were at the age he thinks he is. The souvenirs of his ulterior life cease to be accessible to his conscious personality, which finds itself brought back exclusively to the elements which constituted it, at the time the ecmnesia carries him to. Every idea, foreign to that diminished personality, remains unintelligible to him. In order to make him understand, we must speak to him only of what he knew at the epoch to which he has been brought back. Besides these disappearances or _amoindrissements de la personnalité_ of the personal consciousness, which may be permanent or transitory, we also observe _qualitative_ without _quantitative_ alterations of the personal consciousness. These are changes or variations of personality, which have been well studied in hysterical subjects, but which also exist in other invalids, notably epileptics and victims of certain poisons.[9] [9] Interested readers will find a complete analysis of these facts in Azam’s celebrated work, _Hypnotisme et double conscience_, Alcan; in Pitres’ book, _Leçons sur l’hystérie_, Alcan; and in Janet’s _L’automatisme psychologique_, Alcan. It is essential to know at least these three books, if we wish to observe, profitably, the delicate phenomena I am discussing in this chapter. To sum up, the personal consciousness is susceptible of total or partial disappearance, or of being replaced by another consciousness which can be absolutely foreign to the normal personal consciousness, or preserve more or less close relationship with it, _e.g._ the patient who undergoes a change of personality may retain all the souvenirs of the normal personality A and those of the new personality B. But in an almost absolute manner the normal personality A is ignorant of all which concerns B. This is the type of periodical amnesia. The clinical study of diseases of personality permits observation of the above facts. I ought to say that, in practice, they do not present the simplicity of the _schéma_ which I have just given. Curious problems arise from the nature itself of amnesia, its degree, its mechanism, problems impossible to treat here.