Pierre Janet, answered from the texts and cited to the page.
In the hysteric the unity of the mind is broken. A part of the field of consciousness has split away — a sensation, a memory, an idea — and continues to exist and to act on its own, beneath the awareness and outside the will of the patient, who no longer knows it is there. These dissociated elements are most often fixed ideas: intense ideas, born very often of an emotional shock, a trauma, that the mind could not assimilate and could not discharge, and which, cut off from the rest, persist unchanged and unaging in the subconscious, expressing themselves in the symptom.1
The paralysis, the anesthesia, the amnesia, the somnambulism, the attack — these are not caprice or simulation; they are the acts of a subconscious fixed idea.23 And because the symptom is the expression of a hidden idea, to cure it we must find that idea, bring it back to the light, and dissolve or transform it. The subconscious is not a metaphysical depth; it is the sum of the psychological facts that have fallen out of the personal synthesis and go on working in the dark.45
This disposition is common with everyone. Now, a normal individual may, by an effort of attention, get out of the group of remembrances, awakened mechanically by the situation wherein he finds himself, but an hysterical cannot do so without assistance. If we consider our somnambulists with fixed ideas, we remember that with them the fixed ideas were already subconscious, isolated, outside of personal perception.The Mental State of Hystericals, p. 440
Finally, this subconscious character of the fixed ideas with hystericals plays a great part in the therapeutics of these affections. We formerly showed that it was necessary to look up, so to say, these subconscious phenomena in order to attack them, and that one could not treat the hysterical accident before having reached those deep layers of thought within which the fixed idea was concealed.The Mental State of Hystericals, pp. 412–413
It brings about two things: a blank in the general consciousness, which is represented by an amnesia, and an exaggerated and independent development of the emancipated idea. Now we find nothing of the kind here; we do not find one distinct idea, one precise system that has emancipated itself from consciousness a great many different ideas seem to characterize the somnambulism.The Major Symptoms of Hysteria, p. 65
In a word, all the psychological phenomena that are produced in the brain are not brought together in one and the same personal perception; a portion remains independent under the form of sensations or elementary images, or else is grouped more or less completely and tends to form a new system, a personality independent of the first.The Mental State of Hystericals, pp. 493–494
But the most important work that has come to confirm our earlier studies is, without contradiction, the article of MM. Breuer and Freud, which recently appeared in the Neurologisches Cejitralblatt. We are very glad that these authors, in their independent researches, have been able to verify ours with so much precision, and we thank them for their kindly reference to us.The Mental State of Hystericals, p. 496
Dissociation and the traumatic subconscious fixed idea are Janet's foundational and now widely-vindicated discoveries; he is recognized as the founder of the systematic study of dissociation and psychological trauma. For much of the twentieth century the Freudian unconscious (organized by repression and sexuality) eclipsed Janet's dissociation model, but the late-century revival of trauma studies and the dissociative disorders (PTSD, dissociative identity disorder) explicitly reclaimed his account of trauma, dissociation, and the pathogenic fixed idea as prescient and clinically sound. The qualifications are that specific fin-de-siècle hysteria presentations were shaped by their clinical culture and are read skeptically today, and that the mechanisms of dissociation are now understood in more differentiated terms; but the core — that trauma can split the mind and that split-off material drives symptoms — is a living foundation.