Aim of this paper is to analyse, from a philosophical point of view, the theatrical dimension of “mental illness” between XIX and XX century, with special attention to the complexity of the psy-chiatric problems posed by simulations in cases of hysteria and possession. Problems of simulation was effectively unsolvable in nineteenth-century neurology – because the technique of hypnotism was not really able to distinguish simulation from illness (as well as conscious and un- or semicon-scious from simulation), the boundary separating authenticity from masquerade was necessarily murky. Focused on the works of Pierre Janet, Charles Richet, Traugott K. Oesterreich, the critical analysis of theories of “dédoublement de la personnalité” and duplicity of consciousness highlights on the other hand the theoretical possibility to resolve this aporia.