The Swiss developmental psychologist, Jean Piaget, was a founding figure in the experimental science of what he called genetic epistemology. producing a prolific output of writings, some which became seminal texts. At the heart of his work were his constructivist theories of childhood cognitive development. He developed a stage model of cognitive development that has proved very influential right up to the present day. Of course, like so many founding fathers of disciplines, his own theories have now been superseded. But he was undoubtedly a towering figure in the early years of his field, providing important methodology and setting in motion many of the lines of enquiry that researchers still pursue today.
Piaget's driving ambition was to unify biology and logic. He attempted to do this by observing how concepts and language developed in children and how the children interacted with objects. He constructed ingenious interactive experiments to aid this investigation. On a theoretical level he introduced a couple of important concepts to describe how knowledge provides self-regulating symbolic structures. One of these is the idea of assimilation, where the existing knowledge structures modify the new inputs. Accommodation is where the existing knowledge is altered to adapt to the inputs.
Piaget believed that a kind of Hegelian style dialectic of thesis, contradictory antithesis and higher synthesis was at work in the process of knowledge construction. For him children's knowledge developed in a kind of spiral staircase effect. He believed in a dynamic concept of mind in which there was a constant reassessment of knowledge. The processes of assimilation and accommodation allowed a kind of equilibrium to be maintained.
Piaget's most famous claim was that children go through identifiable stages in their learning and development. The first of these is the sensorimotor stage (0-2 years old) where children are considered to be deeply egocentric, their experience driven by movements and senses. Only in the final six months do they start to think symbolically at all. The second stage (2-5 years old) is the preoperational stage and is where motor skills appear although logic is not used and thinking is magical. The third stage (5-11 years old) is the concrete operational stage where children start to think logically but only with practical aids. Finally, the formal operational stage (over 11 years old) is where full abstract thought appears.
The idea of developmental stages is a theme that many subsequent researchers, to this day, have lingered on but without any definitive results. It is difficult to test for, especially in younger children, who might not understand what is expected of them. It may be that some more subtle version of the stage model of cognitive development is correct, or perhaps a recurrent phase model such as the representational redescription model of Karmillof-Smith, for example. This is an empirical matter.
Piaget's legacy though, is as one of the founding fathers of modern, scientific, experimental developmental psychology. His many writings and experiments provided the beginnings of a research program that is ongoing. Although his theories of cognitive development and his thoughts on education practice have been heavily criticised in recent years he was undeniably one of the most significant figures in the history of the field.