Jean Piaget developed a theory of intellectual development consisting of 4 stages:
(1) Sensorimotor stage (birth to 2 years) where infants develop object permanence and causality.
(2) Preoperational stage (2 to 7 years) where symbolic thought emerges but reasoning is egocentric.
(3) Concrete operational stage (7 to 11 years) where logical thought allows for conservation and classification.
(4) Formal operational stage (12 to 15 years) where abstract reasoning, hypothesis testing, and transferring knowledge to new problems occurs. Piaget believed cognitive development arises from interactions between maturing biological factors and environmental experiences.