In 1894, he earned his D.Sc. from the University of Sorbonne. He also edited his journal L' anne Psychologique that year.
Binet, then with his interest in child development peaked, began to study the effect of suggestibility in experiments on children. From this research Binet discovered that age played a major role in the development of children's mental faculties.
In 1890, a man named Theodore Simon applied for doctoral research under Binet's supervision. Simon was a French educator and he had a strong concern for mental abnormality. Around the same time, Binet became interested in studying judgment, attention and reasoning. He was very much interested to study the complex mental processes. He tried greater variety of tests than his predecessors. He also tried to find out just how bright and dull children differ from each other. Regarding the difference of bright and dull, Binet tried all sorts of measures, such as recall of digits, suggestibility, size of cranium, moral judgment, mental addition, graphology and even palmistry.
Binet - Simon Scale
Ultimately, he found that sensory judgment and other simple functions had little relation. Binet's most important work was in intelligence testing. Theodore Simon, his colleague and one of the eminent psychologists of Paris, assisted him to devise a test to measure the mental ability of children. The first scale of intelligence (Binet-Simon scale) appeared in 1905. In this scale, Binet had children do tasks such as follow commands, copy patterns, name objects and put things in order to arrange them properly. He conducted the test on Paris school children and created a standard based on his data. For example, if 70 % of 8-year-old children could pass a particular test, then success on the test represented the 8-year-old level of intelligence.
The Psychology of Reasoning :
In 1886, Binet brought these diverse interests together in his first book, The Psychology of Reasoning. This book was written very clearly and was unambiguously argued. It illustrated the way in which the new scientific psychology made use of three of the most fundamental constructs in its theoretical repertoire, viz., association, and the imaginal basis of thought and unconscious inference.
Theta Wolf, Binet's biographer pointed out that Binet wrote : "The Psychology of Reasoning in part at least to demonstrate that the principles of the association of ideas could explain all psychological phenomena. Binet expressed his views in his book Psychology of Reasoning. The fundamental element of the mind is the image:reasoning is the organization of images, determined by the properties of the images themselves and:images have merely to be brought together for them to become organized:reasoning follows with the inevitable necessity of a reflex."