Jean Piaget
Piaget served as professor of psychology at the University of Geneva from 1929 to 1975. As he delved deeper into the thought-process, he became interested in the nature of thought itself, especially in the development of thinking. Finding relatively little work done in the area, he had the opportunity to give it a label. He called it genetic epistemology, meaning the study of the development of knowledge. And so he developed the idea of stages of cognitive development. These constitute a lasting contribution to psychology. This idea has been extremely influential in pedagogy, and is usually termed constructivism.
(See "Constructivism (learning theory)")