Madness or passion: the acquittal of a monomaniac (Jules Rousse, on 1855) This case study attempts to analyse the inquiry provoked by the crime of Jules Rousse: a parricide, for which the motives were enigmatic, committed by the son of a well-established family from Bordeaux. The verdict was given after a long judicial inquiry. The analyses of the legal inquiry and the slow process of judicial taxonomy reveals the central role allotted to the questions related to the personality of the accused. This case study adds evidence to the observation of a fundamental transfer from the crime to the criminal operated within judicial practice. The legal inquiry was based on subjective criteria that were supposed to define a subject. These criteria could be summarized within the categories related to the question of passion, character, morality, heredity, suicide, masturbation and cruelty. The expertise given by medical doctors among whom some were very famous like Tardieu, Devergie or Calmeil played a fundamental role. Despite the serious concerns and hesitations formulated by the magistrates, the medical doctors imposed their views upon the members of the jury and Jules Rousse was acquitted.