object permanence
knowledge of the continued existence of objects even when they are not directly perceived. According to Jean Piaget, object permanence develops gradually in infants during the sensorimotor stage of cognitive development. Milestones that indicate the acquisition of object permanence include reaching for and retrieving a covered object (about 8 months), retrieving an object at Location B even though it was previously hidden several times at Location A (the A-not-B task; about 12 months), and removing a series of covers to retrieve an object, even though the infant only witnessed the object being hidden under the outermost cover (invisible displacement; about 18 months). Recent research using nonreaching tasks suggests that infants display some knowledge of object permanence at an earlier age than that suggested by Piaget.