By Michael J. Formica, MA, Ed.M, NCC, LPC in Enlightened Living:
Sigmund Freud had a nephew named Ernst with whom he would play peek-a-boo. Freud would hold a teddy bear at the edge of Ernst's crib and then drop it out of sight. Ever the scientist, Freud noticed that, at a young age, Ernst would immediately lose interest when he could not see the bear. When the bear came back, so did Ernst, so to speak. As Ernst got older Freud noticed that, when the bear was out of view, Ernst would reach over the edge of the crib to find the bear.
From this experience Freud developed a theory of cognitive development that would later come to be called ‘object constancy'. Basically, object constancy suggests that, at some point in our early development, humans express the capacity to understand that ‘out of sight' doesn't mean ‘gone'. This is a very important idea, as it is one of the core elements of interpersonal relationship and informs everything from romantic love to jealousy to Borderline Personality Disorder.