Hide a toy under a blanket, and a young baby acts like it stopped existing

Jean Piaget found that infants younger than about eight months won't search for an object the moment it's hidden from view, as if out of sight truly means out of mind. The capacity to mentally hold onto things you can't currently see — object permanence — builds up gradually over an infant's first two years.

— Jean Piaget, The Construction of Reality in the Child — 1954 English translation
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