Jean Piaget was a Swiss developmental psychologist and philosopher known for his theory of cognitive development. He disagreed with the prevailing view that children are less competent thinkers, arguing instead that incorrect answers from children show their different ways of thinking. Piaget conducted detailed observational studies of children and designed simple tests to study their cognitive development. He identified four stages of development and proposed that children learn through assimilating new experiences into existing mental structures or accommodating their structures based on new information. Piaget's theory shifted the focus of education to understanding children's psychology and cognitive learning processes through active learning like plays and interactions, rather than just product of learning.