The word "psychosis" is the one which prevailed as the scholarly term for insanity. The perspective adopted to define the issues is the history of scholarly sociability. The time frame (1947-77) was chosen to reflect the career of French psychiatrist Henri Ey, the central figure in the post-war Evolution Psychiatrique group. Referring to the Traité de Psychiatrie (in the Encyclopédie Médico-Chirurgicale, or EMC) which Ey edited until his death, I study the perpetuation of a tradition of thought whereby psychiatrists classified mental illnesses in two categories, "acute psychoses" and "chronic psychoses." The thesis retraces the genesis of the word "psychosis" and analyzes the reactivation of the Evolution Psychiatrique group leading to the publication of their collective work (1955). Ey's theory of "organodynamism," inspired by the work of British neurologist John Hughlings Jackson, became the touchstone for establishing links between issues related to the concept of psychosis, the writings of EMC contributors, and the longer-term history of mental illnesses. My interpretation of the texts is oriented toward a series of four principal themes: acute or chronic delusional states, the neo-jacksonian theory of epilepsy, and the psychoanalytic theory of schizophrenia. Lastly, in the form of an epilogue, I consider the revised and updated edition of the EMC (1956-77), in which it is possible to observe some tension between classification, of which the concept of psychosis is an integral part, and a series of medical, intellectual, and cultural innovations, as well as disputes (anti- psychiatry). At the end of the period studied, I question whether the large categories of mental illnesses, classified in the past under the word "psychoses," are still pertinent in relation to these upheavals. The fact that some of Ey's associates, like Henri Ellenberger and Georges Lantéri-Laura, chose a historiographic or methodological approach is a radical departure from the outline of a general theory of psychopathology.