Well at first babies don’t have object constancy. If they can’t see something, it’s gone. Then at around 4-6 months if something is taken out of sight, they’ll look for it. They’ve begun to hold onto the idea of it even when they can’t actually see it.

It’s kind of cool to watch. The idea is more complex as it starts to apply to attachment. How do you know your person is going to come back?

If parents talk to their toddlers about it gently and create familiar rituals around separating and coming back together (and actually come back like they said they would), children learn that their world is predictable and their adults are reliable. So they learn to hold onto the idea of the person who isn’t there and it’s comforting because they know from experience that their person will be back soon.

If things are chaotic and/or the child isn’t helped with that developmentally appropriate anxiety, they might not trust that their person will come back or might doubt that they exist at all when they’re out of sight.