Summarize the fundamental features of Piaget's theory of cognitive development. According to Piaget, all children pass gradually through the four major stages of cognitive development (sensorimotor, preoperational, concrete operational, and formal operational) and their various substages when they are at an appropriate level of maturation and are exposed to relevant types of experiences. In the Piagetian view, children's understanding grows through assimilation of their experiences into their current way of thinking or through accommodation of their current way of thinking to their experiences.

Piaget's theory of cognitive development comprises four major stages: sensorimotor, preoperational, concrete operational, and formal operational. In the sensorimotor stage (0-2 years old) children understand the world through sensory experience and physical interaction. In the preoperational stage (2-6 years old) children start to develop language and use symbols, but their understanding is still intuitive and not logical. The concrete operational stage (7-11 years old) is characterized by the development of logical thought but still tied to concrete objects and events. Finally, in the formal operational stage (12 years and beyond), children's thinking becomes more abstract and they are capable of hypothetical and deductive reasoning.

According to Piaget, two main processes – assimilation and accommodation – guide cognitive development. Assimilation is the process by which children incorporate new information into their existing cognitive structures. Accommodation is the process by which children adjust their cognitive structures to fit the new information.

Piaget believed that children pass from one stage of development to the next when they reach an appropriate level of physical and cognitive maturation and when they experience relevant types of experiences. So, both the biological maturation and the interaction with the environment are necessary for cognitive development.