① Perceptual motor stage (from birth to 2 years old). At this time, the baby only has "perceptual sports wisdom" and begins to coordinate the activities between sensory perception and action, but there is no representation and thinking; Babies can begin to distinguish themselves from objects and further understand the relationship between actions and effects.
② In the pre-operation stage (2 ~ 7 years old), the signal function of representation and language appears, and children can describe the external world with representation and language, which greatly expands their intellectual activity ability. But children at this stage are not "conservative" and "reversible", and self-centeredness is more prominent.
③ Specific operation stage (7 ~ 12 years old) At this time, children have specific operation ability, and there are "conservation" and "reversibility". "Operation" is the core concept of Piaget's theory of intellectual growth. The so-called operation refers to a special intellectual program that converts information for a certain purpose, and it is reversible. But in this period, children's operation can not be separated from the help of specific things or images.
④ Formal operation stage (12 ~ 15 years old). At this time, teenagers have formal operation ability and their psychological level is close to that of adults. The so-called "formal operation" means "liberating the form from the content", and thinking goes beyond perceiving the specific content of facts or things and develops in the direction of indirectly perceiving or future things. For example, reasoning about various propositions according to assumptions and solving problems.