In Psychology, object permanence is the understanding that objects would continue to exist even when they cannot be seen, heard or touched.

Think of this experiment that Piaget carried out. Infants over 2 years old could understand that their toys behind the card are still there, and would vie to grab hold of them; while baby infants would not cry when their mothers leave the room.

Think of stars. When the wind blows and the stars, high above in the sky, blink as if they are going to disappear; you would never have to worry, cause you know the stars will still be there. When a dark big patch of cumulonimbus covers the stars, you know they will always be there for ages.

Now think of me. When I am gone, will you still think of me? When I am out of your sight, will you be like a toddler over 2 years old, frantically searching for me? Or would you regard I am no longer here, out of sight, out of mind?