Jean Piaget’s idea that children of about eight or nine months of age develop awareness/the idea that objects continue to exist even when one cannot see them. Before this stage, as far as the infant is concerned, items that are not within eyesight range do not exist.
Piaget developed this theory by conducting very simple tests with infants involving blankets and toys. If the child was old enough to know about object permenance than when he covered the toy with a blanket, they would still try to reach for it. If the child was not at that stage, they would move on.
Example: Young babies who have not yet developed a sense of object permanence often seem particularly delighted by “peek-a-boo” or other games involving a “vanishing person” or object.
This website has a wonderful section about ages in month to month segments that showcase different levels of awareness: http://www.associatedcontent.com/article/1928862/piagets_theory_of_object_permanence.html
Fournier, G. (2016). Object Permanence. Psych Central. Retrieved on August 12, 2017, from https://psychcentral.com/encyclopedia/object-permanence/