According to him, there is an orderly sequence of stages in cognitive development. Thus progression from one stage to another is always fixed. The child can not adopt the strategies of a later stage at an early stage of development. It is because he must have to acquire and exercise the strategies and schemas of earlier stages. For example, in order to understand words, the child must know letters first or we can not teach Electronic Physics to a child of Ausbel commenting upon the development stages of Piaget writes “Piaget’s stages are identifiable, sequential phases in an orderly progression of development that are qualitatively discriminable from adjacent phases and generally characteristic of most members of a broadly defined age range”.

The stages of cognitive development are related in the sense that they represent forms of adaptation, but these forms are qualitatively different, that is the adaptive functions are transformed as the child moves from one stage to the next. This theory of development is quite different from the theory of associationists who emphasize the gradual accumulation of responses.

Important stages of Piaget’s theory of mental development are given below.

  1. Period of sensorimotor adaptation (from 0-2 years): The infant starts this stage by his reflex activities and then he reaches to the stage of sensorimotor schemes by practice and accommodation. The intellectual development at this stage is marked by the following characteristics.

(a) Object concept formation : Objects exist in the mind of an adult whether they are present before the person or not. But in the psychological world of a child, only those objects exist which have a psychological presence for the child, ie, he can look and touch it. As soon as they are removed from the eyes of the child, they cease to exist for the child. When the child experiences the same object repeatedly, he develops the concept of permanence of objects. This concept is developed in the first year of life. When this type of structure is formed in the mind, practice of concepts is started by the child.

(b) Coordinated space : Spatial world at first is totally uncoordinated for the child. Each sensory modality has its own space. By the end of two years, the child develops the concepts which are marked by the coordination among different objects and between objects and body of the child

(c) Objectified causalty : The concept of cause and effect relationship is established when action of the child brings about an effect. Then this effect is taken as the cause of that event.

(d) Objectification of time : By the end of two years the child is also able to objectify time.

  1. Period of symbolic and preconceptual thought (2-4 years): Ideas begin to develop in the mind of the child at the third year. By imitation and observation, he demonstrates that he is capable of extending his physical world to metaphysics. He begins to use syrnbols in the 4th year. Now he can represent the environment in the absence of perceptual cues only on the basis of ideas.

III. Period of intuitive thought (4-8 years): At this stage the child can use various concepts in different situations, i.e., he is able to generalize his past and present experiences, His thoughts are not based on logical reasoning. It is rather governed by intution. The child can talk about this and that momentary static condition in a separate manner but he can not properly integrate various sets of conditions into an integrated whole due to lack of logical weakness, i.e. logical coherence is not found here. For example, a child can write fine sentences representing various topics but he can not write a compact essay.

  1. The period of concrete operation (8-12years): Here the child is able to direct his attention away from the static condition, ie, he can accommodate with all the successive changes taking place in the system. It means that he can reorganize his future on the basis of his past experience as well as trends at present. Reasoning power of the child is well developed during this period. Some examples of what a child can do are handling and manipulating the numbers in various ways by various operations, arrangement of objects and materials into various groups and sub groups, ordering of objects according to one or more attributes and so on. Piaget has described a term “grouping” for this stage of cognitive development. This period also shows some logical inconsistency in the thinking of the child. Piaget has coined a term “syncretism” for this defect. Despite this, beginning of abstract operation is seen during this stage.
  2. Period of formal (abstract) operation (12-18 years): During this period, the thought process of the child becomes systematic, consistent and reasonably well integrated. Reality guides the contemplation process of the child. The formal operation of thinking starts with the formulation of hypothesis, (deductive reasoning) and reaches at its peak when hypothesis is tested by experiments, ie, inductive reasoning. At this stage, the boy can manipulate any abstract idea in his own way and form new idea entirely different from the old one. His wisdom lies in the expertise administration of the unforeseen. Whenever an adolescent faces any problem with the help of formal operation of thought he identifies various relevant variables which may be helpful in the solution of that problem and then all these variables are manipulated by him. Thus, development of formal operation ‘capacity enables the adolescent to transfer understanding from one situation to another and to evaluate the results on the basis of its pros and cons. Physical (concrete) objects are never required for thinking (operation) during this period. The thought process is completely abstract here