Object permanence refers to a set of commonsense beliefs about the nature, properties, and behavior of animate and inanimate objects. The first belief is that objects are permanent entities that exist continuously and independently of one's immediate actions on or perceptions of them. The second and third beliefs stipulate that objects are stable entities whose properties and behavior remain subject to physical laws regardless of one's immediate perception of them. According to Jean Piaget, a Swiss psychologist who formulated a major theory of cognitive development, the understanding that objects exist continuously emerges during stage four of the sensorimotor period (around eight months of age), when infants spontaneously search for and retrieve an object that they see being hidden. For Piaget, however, object permanence is not fully developed until the end of the sensorimotor period (around two years of age), when infants demonstrate through their manual search behavior that they can imagine the behavior and motion of hidden objects.
Baillargeon, Renée. "The Object Concept Revisited: New Directions in the Investigation of Infants' Physical Knowledge." In C. E. Granrud ed., Visual Perception and Cognition in Infancy. Hillsdale, NJ: Lawrence Erlbaum, 1993.
Piaget, Jean. The Construction of Reality in the Child, translated byMargaret Cook. New York: Basic, 1954.