Throughout history, many people have made amazing contributions to the school of psychology.One of these was Jean Piaget and his theories on the cognitive development stages. Jean Piaget was born in Neuchatel, Switzerland in 1896. Here he studied at the university and received a doctorate in biology at the age of 22. Following his schooling he became increasingly interested in psychology and began much research and studying of the subject. From this research Piaget created a broad theoretical system for the development of cognitive abilities. His work, in this way, was much like that of Sigmund Freud, but Piaget emphasized the ways that children think and acquire knowledge. Piaget referred to his theory as genetic epistemology. This is defined as the study of the acquisition, modification, and growth of abstract ideas and the abilities as on the basis of an inherited or biological substrate, an intelligent functioning that makes the growth of abstract thought possible. Piaget derived his theories from directly observing children and by questioning them about their thinking. Piaget viewed intelligence as an extension of biological adaptation that has a logical structure. One of the central points of his theories was that of epigenesis. This is that growth and development occur in a series of stages, each of which is built on the successful mastery of the previous stage.Piaget described four major stages leading to the capacity for adult thought. Each stage is a prerequisite for the following stage, but the rate at which different children move through different stages varies with their heredity and environment. Piaget's four stages are the sensorimotor stage, the stage of preoperational thought, the stage of concrete operations and the stage of formal operations. The first stage that Piaget felt all children go through was the sensorimotor stage. This stage occurs between birth and two years of age.