In the wake of developments in France and the United States where early psychiatrists such as Pinel, Esquirol, Belhomme and Seguin advocated "moral treatment" of the insane and classified"idiots" and "imbeciles" as incurable though educable, the Revd C. E. Van Koetsveld initiated his "School for Idiots" in The Hague in 1855. Within two years, he had also opened a boarding facility that accommodated many of his pupils. Legal regulations (the Insanity Act of 1841) demanded a judicial authorization for placing a child in the institution. Thus, the admittance to the first autonomous Dutch institution for the medico-pedagogical treatment of children with mental deficiencies involved a systematic registration of the children's specific characteristics. A local figure of authority, often a physician, was required to fill in a short questionnaire on the condition of the child nominated for placement in the institution by their legal representative such as a parent or guardian. The first aim of this paper is to outline theoretically how physicians and educationists used medico-pedagogical selection to develop a notion of the mental boundaries they thought could be recognized in children's minds. The work of E. Seguin was crucial in this development. He considered these boundaries as hindrances to a "normal" life, for example, for participating in education and at work. Based on his work, other physicians and educationists were stimulated to contribute to new interventions in minding and educating "idiotic" children. The second part of the paper offers the Dutch example. It includes the analysis of 187 files on girls and boys who were admitted to Van Koetsveld's institution during the years 1857 to 1873. The analysis reveals the gender and family characteristics amongst the children, as well as information on medical factors and hereditary properties, which made Van Koetsveld and his staff decide to classify these children as eligible for treatment at the first Dutch "Schoolfor Idiots". (Contains 3 figures and 53 footnotes.)