According to Jean Piaget's theory of cognitive development, intelligence is the basic mechanism of ensuring equilibrium in the relations between the person and the environment. This is achieved through the actions of the developing person on the world. At any moment in development, the environment is assimilated in the schemes of action that are already available and these schemes are transformed or accommodated to the peculiarities of the objects of the environment, if they are not completely appropriate. Thus, the development of intelligence is a continuous process of assimilations and accommodations that lead to increasing expansion of the field of application of schemes, increasing coordination between them, increasing interiorization, and increasing abstraction.
The mechanism underlying this process of increasing abstraction, interiorization, and coordination is reflecting abstraction. That is, reflecting abstraction gradually leads to the rejection of the external action components of sensorimotor operations on objects and to the preservation of the mental, planning or anticipatory, components of operation. These are the mental operations that are gradually coordinated with each other, generating structures of mental operations. These structures of mental operations are applied on representations of objects rather than on the objects themselves. Language, mental images, and numerical notation are examples of representations standing for objects and thus they become the object of mental operations. Moreover, mental operations, with development, become reversible.
For instance, the counting of a series of objects can go both forward and backward with the understanding that the number of objects counted is not affected by the direction of counting because the same number can be retrieved both ways. Piaget described four main periods in the development towards completely reversible equlibrated thought structures. These are the periods described below. As shown below, for Piaget intelligence is not the same at different ages. It changes qualitatively, attaining increasingly broader, more abstract, and more equlibrated structures thereby allowing access to different levels of organization of the world.
Sensorimotor period
The Sensorimotor Stage is the first of the four stages of cognitive development. "In this stage, infants construct an understanding of the world by coordinating sensory experiences (such as seeing and hearing) with physical, motoric actions." "Infants gain knowledge of the world from the physical actions they perform on it." "An infant progresses from reflexive, instinctual action at birth to the beginning of symbolic thought toward the end of the stage." "Piaget divided the sensorimotor stage into six sub-stages". (See Appendix)
"By the end of the sensorimotor period, objects are both separate from the self and permanent." "Object permanence is the understanding that objects continue to exist even when they cannot be seen, heard, or touched." "Acquiring the sense of object permanence is one of the infant's most important accomplishments, according to Piaget."
Preoperational Period
The Preoperative stage is the second of four stages of cognitive development. By observing sequences of play, Piaget was able to demonstrate that towards the end of the second year, a qualitatively new kind of psychological functioning ...