Fundamental research has the scientific research aim to improve scientific theories for improved understanding or prediction of natural or other phenomena. Piaget developed his theory of cognitive development on the basis of his Fundamental research on the nature and development of human intelligence. The theory explains how a child constructs a mental model of the world. Piaget disagreed with the idea that intelligence was a fixed trait, and regarded cognitive development as a process which occurs due to biological maturation and interaction with the environment. Piaget proposed four stages of cognitive development: the sensorimotor, preoperational, concrete operational and formal operational period. Albert Einstein called Piaget's discovery "so simple only a genius could have thought of it."