This paper have been presented at the second congress “Psychiatry and violence” in Lausanne, September 2008. It proposes a critical reflection on epistemological and technical basis from an empirical observation of plethysmography used in the assessment of sexual deviance. The consideration of ethnographical and theoretical data leads to the interrogation of the dichotomous division between deviant and non deviant fantasy. This division will be put compared to the other conceptions of the fantasy revealing this content and this reach. Finally, the rhetoric of the neutral stimulus – keyword of the device – is examined closely to reconsider his validity.