Object permanence is the understanding that objects continue to exist when they are no longer visible. Piaget considered it a major achievement of the sensorimotor stage, developing gradually over the first two years. Before acquiring object permanence, infants act as if hidden objects cease to exist — "out of sight, out of mind." The development of object permanence reflects the infant's construction of a stable, enduring representation of the physical world.
Piaget's Substages
Piaget described six substages of sensorimotor development, with object permanence gradually emerging. Before 8 months, infants do not search for hidden objects. From 8-12 months, they search for objects but make the "A-not-B error" — searching in the original hiding location (A) even after seeing the object moved to a new location (B). Full object permanence, including understanding of invisible displacements, develops by 18-24 months.
Renee Baillargeon's violation-of-expectation studies (1987) suggested that infants as young as 3.5 months expect hidden objects to persist — they look longer when a hidden object seems to have vanished. This is much earlier than Piaget claimed and suggests that object permanence as a concept may develop before the motor ability to search for hidden objects. However, whether longer looking truly indicates conceptual understanding or simpler perceptual expectations remains debated.