Jean Piaget believes that for the development of cognitive skills the most important thing is how the child thinks. Therefore, his theory used observations taken from children of different ages by a method he called „clinical". Based on these clinical observations, which led to theoretical conclusions, Piaget described the development of intelligence as a sequence of stages (sensory-dynamic, pre-operational, concrete operations and formal operations), divided into sub-stages. J.Piaget believes that child development occurs in these criteria-determined stages. Stages occur in a fixed order and are irreversible: once a student enters a new stage, he thinks in ways that characterize the stage regardless of the level of task and the context the task is presented. New forms of more complex structure and operation become available in every stage. In the last stage thinking is complete, with openings made as meaning to the probability of higher scientific knowledge. Piaget's stage theory remains a landmark and a solid, invaluable starting point for education.