Piaget was a Swiss psychologist who developed one of the most influential theories of cognitive development. He believed that children construct an understanding of the world through active learning rather than passive absorption. According to Piaget, children progress through four distinct stages of cognitive development - sensorimotor, preoperational, concrete operational, and formal operational - as their biological and mental capacities develop. While influential, Piaget's stage theory has been criticized for being too rigid in its age boundaries and the assumption that all children develop at the same pace.