The construction of reality
In Geneva, the psychologist Jean Piaget spent decades observing children to understand how they acquire knowledge. He moved to the city in 1921 to become the director of studies at the Jean-Jacques Rousseau Institute. Here, through simple but carefully designed experiments, he first systematically studied how infants develop an understanding of reality. His work showed that one of the fundamental concepts—that an object continues to exist even when it cannot be seen—is not innate. This concept is called object permanence.
Piaget’s primary test subjects were his own three children, Jacqueline, Lucienne, and Laurent, born between 1925 and 1930. Through games involving hiding a toy under a blanket, he documented the stages of this mental construction. An infant who has not yet developed object permanence acts as if a hidden object has vanished completely. Piaget's work, particularly his 1937 book The Construction of Reality in the Child, detailed this progression as part of the "sensorimotor stage" of cognitive development, which spans from birth to about two years of age.
The stages of knowing
Piaget outlined six substages in the development of object permanence. From birth to around 4 months, infants live in a world of transient images; what is out of sight is literally out of mind. Between 4 and 8 months, they begin to show progress, such as reaching for a partially hidden object. A significant shift occurs between 8 and 12 months. Infants will now actively search for a completely hidden object.
This leads to a famous and repeatable phenomenon known as the "A-not-B error." An infant who repeatedly finds a toy hidden in location 'A' will continue to search at 'A' even after watching someone move it to location 'B'. This error shows their understanding is still incomplete; their memory of the object is tied to their own action of finding it in the first spot. Full object permanence, the ability to form a mental representation of an object and follow its movement even when unseen, does not solidify until around 18 to 24 months. While later research using different methods, like measuring infant gaze, suggests a basic awareness might appear as early as 3.5 months, Piaget's observational framework established the field.