They are varied the theories on the cognitive development that different authors have posited along the years (Shaie, Vygotsky…) But definitely the work of the Swiss psychologist Jean Piaget (1896-1980) is one of the most widespread and a reference for the rest of theories of the psychology of the development or evolutionary. 

Genetic epistemology.

The theory piagetiana split of a philosophical system on a biological base of the knowledge. The philosophical problem of the that split Piaget is an epistemological question, the what is and how produces  the knowledge, tackled of empirical form on the biological suppositions and the investigation psicólogica. Therefore, to this epistemological question add him  the question ontogénica, the how develops  this knowledge purchased. What finishes  knowing like genetic epistemology. 

For Piaget there is a fundamental truth: the functional continuity that exists between the life and the thought. Therefore, it deduces that it does not exist discontinuity between the thought of the boy, the one of the adult and the scientist, but this thought goes  generating ones from others from the previous biological base. The novelty is what characterises mainly to the knowledge.  The increase of the knowledge and as they elaborate  the new structures of the same from the precedents is the problem to the that tries to give answer the genetic epistemology that proposes Piaget.  

The knowledge comes from of the interactions between the subject and the objects, which involve interdependent activities between them according to a continuous process and in spiral that the subject will have to go learning to coordinate to implement new actions from the previous. 

Processes of change. 

These processes of change produce  from two complementary processes and inseparable: the adaptation and the organisation. 

• Adaptation (external plane), produces  when in the interaction of the individual with the half, the organism experiences useful beneficial changes for the survival. Inside the adptación exist two basic processes fundamental from the biological and intellectual point of view: 
  • Assimilation: process by which the previous psychological structures impose  to the new elements modifying them to integrate them in the knowledge. The organism answers to a stimulus because it already possesses a diagram or psychological structure to be able to interpret it. 
  • Acomodación: Process by which modify  the psychological structures interns in function of the variations of the external conditions. They adjust  the previous diagrams to do them consistent with the new experiences. 
• Organisation (internal plane), process whereby go  integrating these changes by means of internal restructurings of the system in his group.

The psychological structures, according to Piaget. 

The internal organisation part of the concepts action, diagram or psychological structure. 

Action: half whereby the subject goes in in contact with the objects to know them and modify them and be source of knowledge through the adjust of the generic underlying diagram to assimilate the object. 

Diagram: that that there is repetible and generalizable in an action. Therefore, it is a pattern of behaviour organised, considered like the basic unit of the cognitive operation. 

The stadiums of the development, according to Piaget. 

Having in base an organisation sincrónica or horizontal of the actions and diagrams or psychological structures the cognitive development conceives  like a process by means of which the diagrams of action go  differentiating and diversifying from the most elementary forms until the most complex. Besides, this development forms part of a process of combination and coordination of all the structures of the group that allow like this speak of different levels or evolutionary stadiums: 

On these foundations, evolve the famous four stadiums of the development according to Piaget, also called periods or levels: 

1. Period of the development sensoriomotor: it produces  between 0-2 years
   roughly. The evolution of the intelligence bases  in feelings and in going exploring repetitivamente the objects that surround to the boy or girl.
2. Period preoperatorio or representative stage: until the 7-8 years. The boy or girl to begins to express what want to, incorporating something that differentiates him of other living beings: the language. It uses the imitation or the games like road of communication.
3. Period of the concrete operations: of 7-11 years. The boy or girl is
   able to order, classify and relate the things applying the logic.
4. Period of the abstract or formal operations: until the 15 years. The boy or girl is able to relate abstract operations: part of
   hypothesis, values them and studies and treats to solve them. It incorporates the
   thought adult.

Three are the basic criteria that use Piaget to identify these forms of structural organisation:

1. Wholes structured, each one of them has to can explain the diversity of the behaviours observed in the corresponding stadium.
2. General sequence of the stadiums, has to remain clear how situates  each stage inside the sequence. 
3. Constant order of succession in the distinct structures and stadiums.

These are some of the basic postulates of the theory of the cognitive development of Piaget, that is much more extensive and has served of base for many other theories. However, it has been criticised in several appearances. Perhaps the criticism of greater repercussion is the scarce paper that attributes him  to the language and the social factors inside the development which is paradoxical since his theory bases  in verbal reports of the subjects.