No Object Permanence

In psychology, object permanence is the ability to recognize that an object continues to exist even when you cannot see, hear, or touch it. In Real Life, everyone eventually develops this trait, usually between 8 and 12 months of age.

In fiction, it doesn't always work like that. A good source of comedy is to have a person who should have developed this trait behave as though they never did. An object can be in plain view one second and then hidden from view the next and this person will react as though the object is no longer there, even though it hasn't actually been removed from the room. In a more serious usage, inducing this can be a kind of superpower in fantasy/sci-fi stories.

Use of this trope will often show a severe failure of logic. Characters lacking object permanence will have, at the very least, a normal memory, even though the ability to remember anything depends on object permanence. If you believed that this article only existed while you were reading it, you would find yourself unable to remember it later. This is not a problem in fiction land.

Compare Living Motion Detector for when object permanence depends on the motion of the object. Do not confuse it with No Ontological Inertia, which is when an object really is impermanent.

Literature

 * The Gap Chasm in the Xanth series had a Forgetfulness spell on it that caused something like this: Anyone who went to the Gap Chasm would promptly forget that it existed upon leaving.
 * Piers Anthony's Mute: The hero's mutant power is that once he leaves the view of another person, that person completely forgets him.
 * In Ray Bradbury's short story "No Particular Night or Morning", one of the astronauts develops this due to being in space too long, starts worrying that people, things, even places stop existing once he leaves or otherwise can no longer directly see them, and goes slowly insane as a result.
 * In the Isaac Asimov's Robot City series, two specialised security robots ordered to guard a valuable object have trouble with this concept -- so, rather than lock the object in a safe, they keep it on a table so they can watch it all the time. This doesn't turn out to be a very good strategy.
 * Inverted with the Ravenous Bugblatter Beast of Traal which is such a stupid animal that it thinks if you can't see it, it can't see you.

Live Action TV
"Baby Mike: Hey, you're no longer right in front of me. I can only assume this means I'll never see you again. That upsets me. Waah."
 * In the Wizards of Waverly Place episode "All About You-niverse", Alex attempts to hide herself from her mom after having "borrowed" money from the Sub Station to purchase a vending machine. After Alex hides, Zeke begins talking as if Alex is no longer in the room, even though all she did was cover herself with a sheet.
 * Star Trek Voyager episode "Unforgettable": An alien race called the Ramuran gives off pheromones that interfere with long-term memories. Once a Ramuran leaves another creature's presence, after a few hours the other creature will have completely forgotten about ever meeting the Ramuran.
 * In the Mystery Science Theater 3000 episode Merlins Shop of Mystical Wonders, a poorly-executed magic spell causes Mike to sort-of revert to being a baby.


 * The Silence in in season premier episodes of Series 6 of Doctor Who are a race of aliens that are forgotten as soon as you stop looking at them.

Web Comics
"Father: Peekaboo!
 * In one Saturday Morning Breakfast Cereal strip, the father was playing peekaboo with his child, but the boy replied, "Damn it, Dad, I don't have object permanence."
 * Another strip starts with a baby adrift in a blank void, desperately philosophising about how the universe suddenly ceased to exist. Yes, he's playing peekaboo too.

Baby: Ahahahahaha!"

Western Animation

 * Doofenshmirtz from Phineas and Ferb can't recognize Perry the Platypus without his hat- if Perry takes off his hat in front of him, he assumes Perry has somehow switched places with a regular platypus. Perry uses this to his advantage all the time.

Real Life

 * Truth in Television for very young infants. If you meet someone who isn't a very young infant and has a pattern of acting like this, you should take him to a psychologist.
 * The old myth that ostriches bury their heads in the sand is based on this: the idea being that an ostrich is so stupid that it assumes any threat it can't see isn't real. In truth, ostriches stick their heads in the sand to manage their nests.