Jean Piaget was a Swiss psychologist known for his influential theory of cognitive development, which posits that children actively construct their understanding of the world through a process of learning that involves balancing assimilation and accommodation. He identified four distinct stages of cognitive development: sensorimotor, preoperational, concrete operational, and formal operational, with varying abilities manifesting at each stage. Piaget argued that development occurs naturally through interaction with the environment, emphasizing that educational approaches should respect these developmental stages rather than attempt to accelerate them.