The theory that concentrates on precisely detailing the processes involved in children's thinking is Piagetian theory. Jean Piaget was a Swiss psychologist who proposed a stage theory of cognitive development. According to Piaget, children's thinking progresses through a series of developmental stages, from the sensorimotor stage (birth to 2 years) through the preoperational stage (2 to 7 years), the concrete operational stage (7 to 12 years), and finally the formal operational stage (12 years and up). Piaget's theory emphasizes the active role that children play in constructing their own understanding of the world through their experiences and interactions with their environment.