Jean Piaget was a renowned 20th century Swiss psychologist who pioneered the study of child development. He contributed significantly to the study of cognitive development in children and is best known for proposing that children progress through four distinct stages of cognitive development - sensory-motor, preoperational, concrete operational, and formal operational. According to Piaget's theory, children actively learn about the world through experiences and experiments, progressing from simple reflexes as infants to acquiring the ability to think logically and understand abstract concepts as adults.