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At the beginning of his career, Pierre Janet had the opportunity to observe a patient, Achille, who seemed to be possessed by a demon. After acquainting himself with old studies on this theme, Janet observed that his patient indeed presented all the classic signs of possession. He concluded therefore that Achille was possessed. But this diagnosis was immediately criticized by theology specialists who asserted that Achille was in fact only pseudo-possessed. Thirty years later, without ever having reexamined this patient, Janet admitted that he had been mistaken: Achille was only pseudo-possessed. Based on this change in diagnosis, I will first of all emphasize the importance of institutions and their agents in establishing pertinent and legitimate diagnoses. I will then try to grasp the interest that two sub-systems of meaning in competition with one another (here the Church and medicine) might have in together qualifying an object as fake or pseudo. It is possible that the acceptance of this designation as fake or pseudo might correspond to a particular moment in the relations between two sub-systems of meaning in competition over the definition and institution of reality. Between phases of confrontation to capture reality and eliminate competing sub-universes, phases appear where reality is negotiated. These phases are probably marked by the use of notions such as pseudo, fake, and imitation.