Object permanence is an important intellectual milestone surrounding an infant's ability to mentally "picture" or remember objects and people they have seen before. Without it, the phrase "out of sight, out of mind" comes to life in the truest sense.
The process of achieving object permanence starts around 4 months and begins to show in an infant's behavior around 8 months. It is a concept that is solely devoted to an infant's visual conceptualization of the things in her world; it does not address the other sensory information such as smell, feeling, hearing, or taste.
Object Permanence From an Infant's Point of View
Young infants (newborns to 5 months old) do not yet understand that objects still exist when they are not in sight. They do not have the intellectual skills to remember, imagine, or even think about the things in their world until they are about 4 months old. Even then, those abilities are so small that they are not likely to have any effect upon the baby's behavior.
At or around 8 months, parents may begin to notice a change. If you take that ball and hide it, you will see wonder and curiosity take over. That same child will actively search for the ball, picturing it, and looking for it. He now understands that hidden objects still exist. The intellectual milestone of object permanence has been achieved!