Cognitive psychology is the study of mental processes such as attention, memory, perception, problem solving, and thinking. Jean Piaget developed the influential theory of cognitive development which proposes that children's thinking progresses through four discrete stages - sensorimotor, preoperational, concrete operational, and formal operational - as their cognitive schemas become more complex through assimilation and accommodation. Piaget's theory focuses on how children actively construct knowledge and understanding from their environment through hands-on learning and exploration.