Piaget was a Swiss psychologist whose main contribution to psychology was the belief that intelligence was the product of a natural and inevitable sequence of developmental stages. He did not suggest that intelligence was simply innate, but rather developed as a result of the constantly changing interaction of the child and environment. Piaget believed that the child went through distinct stages of development. Each stage is characterised by qualitatively different ways of thinking about the world.

During development, the child develops mental structures called schemas, which enables him or her to solve problems in the environment. Adaptation is brought about by the processes of assimilation (solving new problems using existing schema) and accommodation (changing existing schemata in order to solve new experiences.) The importance of this viewpoint is that the child is seen as an active participant in its own development, rather than a passive recipient of either biological influences (maturation) or environmental stimulation.Having outlined how schemas change and develop, in general, Piaget then described changes that took place during the four main stages of development:Sensori-Motor Stage (birth to two years.) The baby's intelligence is essentially practical, i.e.

its interactions with the environment consist of overt actions, either sensory (seeing, hearing etc) or motor (grasping, puling etc.) Lack of object permanence characterises this stage.Pre-Operational Stage (two to seven years.) The child tends to be influenced by how things look, rather than by logical principles or operations.

A predominant features of the pre-operational child is a profound sense of egocentrism.Concrete Operational Stage (seven to eleven years.) In this stage children develop the structures to perform actions mentally (in the presence of actual objects.) This is best seen in the ability to conserve.Formal Operational Stage (eleven to fifteen years.) The thinker can manipulate ideas or propositions and can reason solely on the basis of verbal statements.

This ability to think hypothetically extends to the de-centration of the adolescent.Piaget had a tremendous influence on psychology for many years, and his ideas have been instrumental in much of the educational thinking of the later part of the 20th century. His critics claimed that Piaget underestimated the abilities of young children. Like-wise, Piaget's theory is often described as 'anti-educational', in that it describes a sequence of events not readily alterable by education or training.Vygotsky believed that cognitive development was founded on social interaction.

According to Vygotsky, much of what children acquire in their understanding of the world is the product of collaboration with others. He constructed the idea of a zone of proximal development, which are those tasks which are too difficult for a child to solve alone, but which s/he can accomplish with the help of adults or more skilled peers. As a result of shared dialogues with these helpers, who provide hints and instructions as well as encouragement, the child is able to internalise the 'how to do it' part of the task as part of their inner (private) speech. This can then be used by the child on later occasions when they tackle a similar task on their own.Vygotsky introduced two explanatory concepts on how this could be accomplished: Inter-subjectivity is were two people (i.

e. the child and the helper) begin a risk with different levels of skill and understanding. As each adjusts to the perspective of the other, the helper has to translate their own insights in a way that is within the grasp of the child, and the child develops a more complete understanding of the task.Scaffolding refers to the way in which the support offered by a helper gradually decreases as the child assumes more of the strategic planning; and eventually gains the competencies to master similar problems without the aid of an adult or more knowledgeable peer.

Vygotsky's theory has profound implications for classroom learning. Teachers guide, support and encourage children, yet also help them to develop problem-solving strategies that can be generalised to other situations. The role of expert peers is stressed in the Vygotskian approach. Co-operative learning symbolises the whole spirit of Vygotsky's approach to development, children learn best not when they are isolated, but when they interact with others, particularly with competent others who can provide the guidance and encouragement to master new challenges.

The concept of developmental stages is often taken to mean that development is discontinuous. However, for Piaget, development is a gradual and continuous process of change, although later stages build on earlier ones (which is why the sequence is invariant.) (Krebs and Blackman, 1988.)Children can reach later stages without having gone through earlier ones (Horn, 1976), as when some children walk without having crawled.

Whether the concept of stages is valid or not, is hotly debated by both supporters (e.g. Flavell, 1970) and opponents (e.g. Sternberg, 1990.

) As Flavell (1982) remarked, "The theory may, in varying degrees, be unclear, incorrect and incomplete."Although some researchers have accepted the validity of stages, they have suggested that there are cross-cultural differences in the rates of development in the various cognitive domains. Development of the cerebral hemispheres, apparently, overlaps with the timing of Piaget's stages. (Thatcher et al, 1978.

) Conversely, others (e.g. Riegel, 1973, and Labouvie-Vief, 1980) have argued that some people reach stages beyond the formal operational stage.The main difference in explaining human cognitive development in terms of discontinuous and continuous change is the idea of the "child as the scientist" (Piaget) and "the child as the apprentice" (Vygotsky.) (Rogoff, 1990.

)While Piaget's theory has, arguably, had the biggest influences on education, methodological concerns have lead to the growth of support for Vygotsky. However, both may serve to obscure, rather than to uncover, children's understanding.