This paper focuses on the education of the deaf pupils at Casa Pia de Lisboa, a Portuguese boarding-school, covering the period from 1820 to 1950. The intention is to show that a historical sedimentation of a scientific discourse about deafness is an effect of a new rationality of government begun by modernity and perfectly fitting a bio-political strategy. The development of medical and psychological sciences and their inscription in the educational landscape allows the appearance of new techniques and instruments in the observation and comprehension of the school population, as well as a rush of experts on social prophylactic. The field of practice offers a single heterogenous surface on which discursive objects are involved in the production of effects of power and types of subjectivities: this article is crossed by the double image normal–abnormal deployed by school practices. The conceptualisation of deafness as an abnormality justified the multiplication of corrective processes directed to an erasing of deafness. The deaf were fabricated as the Other of the educative arena and the hospitality given to them was no more than conditional. The oral lingo was conceived not only as the normal but also as the official language that the deaf had to assimilate.