Once education was made mandatory for all French children from ages six to fourteen, it became necessary to determine which children might be considered retarded. To do so the question arose concerning what test should be given to children that might possibly have learning disabilities to determine whethery they should be placed in a special class. Binet attempted to find a way to establish the differences between normal and abnormal intelligence and to measure such differences. Experimenting with his own daughters, Binet gave them problems to solve but also demanded an explanation for the solution they provided for his questions. These experiments resulted in new ideas concerning human reasoning and its relationship to images. Digital facsimile from Google Books at this link.

Binet–Simon intelligence tests. As early as 1895 Binet had published a plan for studying intelligence. English translation, 1912.