Sensorimotor and Preoperational Stages
Jean Piaget (1896–1980) was a Swiss developmental psychologist who proposed that children's thinking develops through four qualitatively distinct stages — not just accumulating more knowledge, but developing fundamentally different ways of understanding the world. This framework remains the most influential model in developmental psychology, though contemporary researchers have refined some age ranges and recognized that development is more continuous than Piaget's stage model implies. The Sensorimotor Stage (birth to approximately 2 years) is characterized by the infant learning through sensory experiences and physical actions — they understand the world through what they can see, touch, hear, and manipulate. The defining cognitive achievement of this stage is object permanence: the understanding that objects continue to exist even when they are out of sight. Before approximately 8–12 months, an infant who watches a toy hidden under a blanket will not search for it — out of sight is literally out of mind. After object permanence develops, the infant will actively search for the hidden toy. This milestone is why peek-a-boo is both delightful and educational — it practices the understanding that something hidden still exists. By the end of the sensorimotor stage (around 18–24 months), toddlers begin to represent objects and events mentally — the beginning of symbolic thinking. The Preoperational Stage (approximately 2–7 years) marks the explosion of language, imagination, and symbolic play. Children in this stage use words, images, and symbols to represent objects and experiences. They engage in pretend play — using a block as a car, or acting out scenarios with dolls. Piaget described this stage by what children cannot yet do: they lack conservation (the understanding that quantity does not change with appearance change), they are egocentric in the Piagetian sense (difficulty seeing things from another's perspective), and they engage in animistic thinking (attributing life and intentions to inanimate objects). Piaget's conservation task is striking: pour water from a short, wide glass into a tall, narrow glass — the preoperational child believes there is now more water, because the level is higher. They focus on one dimension (height) while ignoring another (width), a limitation called centration.