's theory of cognitive development, children progress through four stages of development: sensorimotor, preoperational, concrete operational, and formal operational.

1. Sensorimotor Stage (Birth to 2 years): During this stage, infants explore the world through their senses and actions. They develop object permanence, the understanding that objects continue to exist even when they are out of sight. They also begin to develop basic mental representations and engage in pretend play.

2. Preoperational Stage (2 to 7 years): In this stage, children start to use language and symbols to represent objects and events. They engage in egocentric thinking, where they struggle to see things from others' perspectives. They also exhibit animistic thinking, attributing human-like qualities to inanimate objects. However, their thinking is still limited by concrete, tangible experiences.

3. Concrete Operational Stage (7 to 11 years): During this stage, children become more capable of logical thinking and understanding conservation, which is the understanding that quantity remains the same despite changes in appearance. They develop the ability to engage in concrete operations, such as classifying objects and understanding cause and effect relationships.

4. Formal Operational Stage (11 years and older): In this final stage, adolescents and adults gain the ability to think abstractly and hypothetically. They can engage in deductive reasoning and understand complex concepts. They can also think about multiple perspectives and engage in hypothetical-deductive reasoning.

Piaget believed that children actively construct their understanding of the world through interactions with their environment. He also emphasized the importance of cognitive development being driven by both maturation and experience.