Aim The author of this article sets out to examine the origins and evolution of the category of idiotism in Philippe Pinel's work. Method This study is based on textual and comparative analysis of Pinel's writings, from the Tableau Général des fous de Bicêtre au nombre d’environ 200 (General table of around 200 cases of insanity treated at Bicêtre), published in 1794, to the last edition of his Nosographie philosophique ou la méthode de l’analyse appliquée à la médecine (Philosophical nosography), published in 1818. Results This analysis has made it possible to retrace the development of the nosographic status of idiotism, to identify its clinical characteristics and highlight Pinel's pessimism with regard to the potential for treatment. The vast majority of cases of idiotism did not support his innovative theory according to which insane subjects retain a small degree of reasoning ability. In fact with many idiots barely any dialogue proved to be possible. Furthermore, very few of them could be cured. In his report on the Wild Child of the Aveyron (1800–1801) and his article on “Mania” (1808), Pinel relates a number of observations of “idiot” children. The founder of psychiatry therefore held that a child could become insane; but this was to be disputed by some of his contemporaries and successors. Discussion Pinel's very negative assessment of idiotism must be qualified; in actual fact his assessment varied depending on its clinical form, whether it was complete or partial, and depending on its cause – physical or moral, inborn or accidental. Conclusions The fact that Pinel considered idiotism as a case apart has no doubt largely contributed to the fact that no study focusing on this category has been published to date. Although his view corresponds to a resurgence of the traditional concept of madness, our study shows that it is more than merely a restatement of this vestige of ancient times.