Jean Piaget's cognitive theory of development explains how children's thought progresses through four distinct stages, each characterised by different ways of understanding and interacting with the world. The first stage, the sensorimotor stage (birth to about 2 years), involves learning through senses and motor actions. The preoperational stage (about 2 to 7 years) is marked by the development of symbolic thought and language but also by egocentrism and a lack of logical reasoning. The concrete operational stage (about 7 to 11 years) sees the development of logical thinking about concrete events and objects, including the understanding of conservation. Finally, the formal operational stage (from about 12 years and into adulthood) involves the ability to think abstractly and hypothetically. Piaget's theory emphasises that cognitive development is a continuous process of adaptation through assimilation (incorporating new information into existing schemas) and accommodation (modifying existing schemas to fit new information).