JEAN PIAGET

Jean Piaget
(August 9, 1896 – September 16, 1980)
A Swiss philosopher, natural psychologist and developmental psychologist, well known for his work studying children and his theory of cognitive development. According to Ernst von Glasersfeld, Jean Piaget is also "the great pioneer of the constructivist theory of knowing".
The stages of cognitive development
Theory of cognitive development
Piaget served as professor of psychology at the University of Geneva from 1929 to 1975 and is best known for reorganizing cognitive development theory into a series of stages, expanding on earlier work from James Mark Baldwin: four levels of development corresponding roughly to (1) infancy, (2) pre-school, (3) childhood, and (4) adolescence. Each stage is characterized by a general cognitive structure that affects all of the child's thinking (a structuralist view influenced by philosopher Immanuel Kant)[citation needed]. Each stage represents the child's understanding of reality during that period, and each but the last is an inadequate approximation of reality. Development from one stage to the next is thus caused by the accumulation of errors in the child's understanding of the environment; this accumulation eventually causes such a degree of cognitive disequilibrium that thought structures require reorganizing.
The four development stages are described in Piaget's theory as
Sensorimotor stage: from birth to age 2 years (children experience the world through movement and senses and learn object permanence)
Preoperational stage: from ages 2 to 7 (acquisition of motor skills)
Concrete operational stage: from ages 7 to 11 (children begin to think logically about concrete events)
Formal operational stage: after age 11 (development of abstract reasoning).
These chronological periods are approximate, and in light of the fact that studies have demonstrated great variation between children, cannot be seen as rigid norms. Furthermore, these stages occur at different ages, depending upon the domain of knowledge under consideration. The ages normally given for the stages, then, reflect when each stage tends to predominate, even though one might elicit examples of two, three, or even all four stages of thinking at the same time from one individual, depending upon the domain of knowledge and the means used to elicit it.
Despite this, though, the principle holds that within a domain of knowledge, the stages usually occur in the same chronological order. Thus, there is a somewhat subtler reality behind the normal characterization of the stages as described above.
The reason for the invariability of sequence derives from the idea that knowledge is not simply acquired from outside the individual, but it is constructed from within. This idea has been extremely influential in pedagogy, and is usually termed constructivism. Once knowledge is constructed internally, it is then tested against reality the same way a scientist tests the validity of hypotheses. Like a scientist, the individual learner may discard, modify, or reconstruct knowledge based on its utility in the real world. Much of this construction (and later reconstruction) is in fact done subconsciously.
Therefore, Piaget's four stages actually reflect four types of thought structures. The chronological sequence is inevitable, then, because one structure may be necessary in order to construct the next level, which is simpler, more generalizable, and more powerful. It's a little like saying that you need to form metal into parts in order to build machines, and then coordinate machines in order to build a factory