Eren: Salt?! L-liar!! I mean, salt?! That would be a treasure trove! Merchants would've already exhausted the supply!!
Armin: No, that's the thing! The "sea" is so huge, it can't be depleted!
Eren: That's just silly.
The world has suffered some kind of massive calamity. Humanity has survived, but at a great cost. In addition to the population decrease, much of humanity's knowledge about the world and how it works has been lost.
How can an author demonstrate the sheer scale of cultural, historical, and intellectual loss? By having the characters rediscover some scrap of information that is laughably common knowledge in the real world and react to this information with shock, disbelief, and amazement. Expect at least one character to become fascinated and obsessed with the rediscovered knowledge, vowing to travel off in search for whatever-it-is (if it's a location, animal, etc.) or to try and build it (if it's a form of technology). Expect at least one other character to scoff at the implausibility of these "facts" and question whether the long-dead writer of the text was just making stuff up.
A variant is where the characters (usually children) will hear the lost knowledge told to them by an elderly relative who remembers the days when this knowledge was common. Again, their reactions will range from rapt wide-eyed wonder to "Granny must be going senile."
Sometimes the common knowledge will be lost because that thing no longer exists in the future world (e.g.: a child asking her grandmother to tell her about that strange extinct creature called the "lion"), thus further demonstrating how far the world has regressed.
Compare Future Imperfect, where the knowledge is still there but has been misinterpreted over the years. Compare to Aliens Never Invented the Wheel, which is where aliens don't have something common with humans. Not to be confused with "Common Knowledge".
- The page quote is from Attack on Titan which takes place in a world where the last remnants of humanity are holed up in a walled off territory about the size of Texas, hiding in fear of the people-eating giants that have overrun the world. Information about the outside world is not only scarce — it's forbidden by the government, and Eren mentions that the book Armin is reading from is illegal to own.
- In Gargantia on the Verdurous Planet, an ice age caused some humans to fly off into space, while others stay on Earth. The protagonist, Ledo, is from the space-faring society, but rediscovers Earth That Was purely by accident. Since the people in space have few resources and are fighting a never-ending war against another species, he's Conditioned to Accept Horror and has to learn about a society where people are viewed in more than utilitarian terms. Also, he is completely Squicked by the fact that people on Earth eat animal carcasses.
- Invoked and Played for Laughs in Asterix and the Chieftain's Shield. At the time the volume was first published, in 1967, the exact location of Vercingetorix's capital and his last stand against Caesar, Alesia, was not known. In the comic, Asterix and Obelix travel through Arvernia, the general region where Alesia was in, and the local Arverni, still sore for the defeat, angrily (but obviously falsely) claim to not know where Alesia is and refuse to discuss it any further. A note from the authors then claims that this is the reason why the location of Alesia was eventually forgotten.
- Legends of the Dead Earth: In Wonder Woman Annual #5, the Unremembered have regressed thousands of years since their Generation Ship was struck with disaster. They have forgotten the meaning of words such as "sky," "night" and "stars" and know nothing of planets, space vessels, the universe or anything else beyond the worldship. Most notably, most male members of the Unremembered believe that women make babies "out of some contrary nature unique to them." There is a growing number of men, such as CatXon, who believe that men are just as much to blame for babies being conceived but it is widely seen as a "preposterous belief."
- In WALLE, when Captain McCrea (whose ancestors spent hundreds of years lounging around on the luxury space-ship ''Axiom'' after fleeing Earth) accesses the computer database, most of the facts he's getting all excited over are commonplace knowledge in the real world, like the definition of "dancing" and "farms".
- Played for laughs in Idiocracy. By the time of the dumbed-down future, people have no concept how to care for plants, and think that the sports drink Brawndo is "what plants crave." The main character tries to explain the (basic) science behind watering them, but the knowledge flies over their heads, so he settles for telling people that he can talk with plants, and that they've told him what they want is water.
- The Book of Eli:
- All copies of the Bible have been destroyed, to the point where a big driving force of the plot is various characters' attempts to protect or steal a rare surviving copy. Explained as after a terrible third world war nearly drove humanity to extinction, most of humanity decided that religion was to blame for all the world's ills and had all religious texts burnt. The ending reveals that Alcatraz has been converted into a printing factory, where some individuals protect and feverishly work to reprint any salvaged copies of the Bible, Qu'ran, Talmud etc.
- When Carnegie holds a gun on Solara outside the wreckage of the old couple's house, Eli reveals that he hid the Bible behind their TV. When Carnegie orders one of his young henchmen (born after the nuclear war) to check the TV, he replies with a blank look and a puzzled, "The what?" Carnegie rages, "You're fuckin' SHITTIN' me!" before telling Redridge (who is old enough to know what a TV is) to check it.
- Played for laughs in Sleeper. Miles Monroe wakes up in the year 2173 after spending 200 years as a Human Popsicle. The people of that time have lost most of their historical knowledge due to a nuclear war, so they ask him questions about 20th century artifacts (e.g. "chattering teeth") and people. Miles makes up a bunch of lies about them for his own amusement ("Bela Lugosi...was the mayor of New York for a while" and "Charles de Gaulle...was a very famous French chef.").
- In the world of Mad Max: Fury Road this is demonstrated when Nux, one of Immortal Joe's War Boys whose entire short lives are dedicated to fighting and dying gloriously for their master, doesn't know what a tree is and refers to it as "that thing" until someone else provides the word. In context it's quite sad.
- A joke from Communist Eastern Europe: Somewhen in The Future, a child reads in an old book about butter. The child asks everyone what this means, but nobody can remember. Finally, they find a definition in a dictionary: "A spread for bread, not used anymore by progressive people for its connotation with capitalism."
- Another joke is about Culture Clash between people from different times:
1: Sorry, I'm late. Long line for sausages.
2: What's a "line"?
3: What's a "sausage"?
- Some version then follow up with a third person asking "What's "sorry"?"
- The Books of Ember:
- In The City of Ember, the simple fact that the surface world exists becomes this, as does the existence of animals, candles, matches, the sun, the sky, and many other things.
- In The People of Sparks, the Emberites don't know that the Earth is round, and don't know about things like animals, trees, trucks, rain, seasons, and gods.
- In fact, this is the whole point of The Giver. The title character's actual occupation is "Receiver of Memory," and his role in the community is to be the only one with access to Lost Common Knowledge (such as the existence of death, color, and animals), in case it comes in handy to the regime. For example, the Giver tells Jonas that he was consulted when the powers that be were considering increasing the number of children born, and he said no because he knew why they'd started controlling population numbers, to avoid starvation. Of course he and Jonas come to feel that everyone else is being deprived of the knowledge of good things and that they're unfairly burdened being the only ones to know about things like starvation.
- Taken to an extreme in Ayn Rand's novella Anthem: the protagonist rediscovers the very concept of individuality and the word "I".
- In the dystopian world of the Delirium Series, where love is considered a disease, the protagonist Lena does not know what poetry is. She considers it to be incredibly beautiful when she hears it, however.
- Civilization has (sort of) recovered by the time of The Wheel of Time series' protagonists, but periodically they come across ideas or information that strike them as absurd. One villain is trying to distract one of the heroes, for instance, and mentions off-handedly that in her time, people were able to travel to other worlds, "even worlds in the sky". Only the last bit fazes Nynaeve, who's pretty familiar with other dimensions — but worlds in the sky? That's just ridiculous. Bonus points to the Aes Sedai who have spent thousands of years trying to figure out how ancient people could fly. They're certain it must have been a magical effect, but no-one has ever managed it. It's on the verge of becoming discredited as a myth, possibly a mistranslated text. Perhaps the ancient Aes Sedai could fly that way, but only the readers (and a few Aiel, who don't become Aes Sedai) know they had airplanes.
- An Instinct for War features a future Forever War in which American soldiers sees a page from Clausewitz's On War and dismisses it as being enemy propaganda as they can't read it.
- The Dark Tower is set in a world that has "moved on", having suffered some never-specified apocalypse and slowly decaying. During a brief confrontation between Roland and the Man in Black early in the series (near the end of The Gunslinger), the Man in Black makes mention of the great deeds done by their progenitors; among other things, they cured cancer and walked on the Moon. Roland flatly refuses to believe the latter.
- In Sheri S. Tepper's The Visitor, the inhabitants of Bastion have been trying to rediscover the Lost Arts. However, they assume these were magic arts. Thus they explain science in magical terms. Actual science is known as Scienceism and considered heresy.
- In The Lost Fleet Captain Geary is periodically stunned when confronted with traditions, regulations, or beliefs that are not only contrary to his era but also just plain wrong. The biggest example, and what makes Geary so deadly, is the art of coordinating fleet-wide maneuvers at light-fractional velocities. Most officers learned this from more experienced officers before the war, but the massive casualties suffered by the fleet eventually wiped out anyone who knew.
- In A Song of Ice and Fire, the Valyrian people knew how to do a lot of things (Valyrian steel, roads, construction...) but everything was lost when Valyria got destroyed, and the few survivors (the Targaryens) did not know those secrets.
- Eternity Road. According to legend the October Patrol used a submarine to salvage human literature and remove it to a place called Haven before society collapsed completely from The Plague, thus preserving human knowledge and culture for future generations. The plot involves an expedition to find this Haven, however their scholars scoff at the idea of a ship that can travel underwater, regarding it as a fanciful embellishment by later storytellers. Because of this they fail to realise that two of the doors at Haven form an airlock to a submarine docking chamber that has now flooded.
- An In-Universe case occurs in the original Dragonlance trilogy. On the edge of death, Raistlin frantically goes through the library of Astinus, trying to find a method in the books to save his life. He seems to have everything he needs except "The Key" which book after book mentions and the ancient mages apparently all knew, but none of the books actually explains what the Key is. When Raistlin realizes that none of the books will do so and the knowledge is apparently lost to time, he pretty much throws a fit.
- "The Quiz Broadcast" on That Mitchell and Webb Look is a post-apocalyptic game show that plays this for laughs. For example: "Pre-Event sources talk about "hope". What was hope?" Note that it is made clear that the game show takes place not that long after the Event. That's right: the Event was so horrible that people lost common knowledge of things they'd personally experienced for the bulk of their lives.
- In Star Trek: The Original Series, the episode "For The World Is Hollow And I Have Touched The Sky", the Fabrini built a generational starship into an asteroid to escape their dying homeworld. 10,000 years later, their descendants are completely oblivious to the fact they are living in space at all. Of course, it helps that the ones in charge make sure no-one finds out.
- Star Trek in general sometimes does this, with knowledge having been lost after the world war that devastated the earth and the cultural transformation brought on by replicators, the Federation, and alien influence. Unfortunately the writers don't always remember what things have been lost, so that the phrase "room service", which has been used several times in the shows without any hint that it's some archaic phrase, in one episode leaves the crew standing around in confusion until Data tentatively suggests that it's about cleaning the room.
- In the Doctor Who episode "The Space Pirates", Zoe, being from the futuristic twenty-first century, doesn't know what a candle is. (Though she'd had no trouble recognizing them in previous episodes).
- An episode of Blake's 7 had the protagonists arguing over whether all humans came from Earth originally. Given that the series used both Human Aliens and Future Imperfect tropes, either argument is plausible.
- In the The Twilight Zone (1959) episode "Elegy," a trio of astronauts lands on a planet that resembles Earth in the late 20th century, despite it being the year 2186 and them being in another solar system. Their landing site is a farm, complete with trees, a barn, hay, and a machine that Captain Webber and Kirby don't recognize. The third among them, Meyers, explains that it's called a tractor, a device that was in use on Earth before World War III.
- In the RPG Paranoia, most inhabitants of Alpha Complex display this attitude about anything relating to the semi-mythical Outdoors.
- Warhammer 40,000 is all over this trope. Since nearly every world in the Imperium is insanely specialized, consisting of one climate, or both, what's commonplace in one world may be totally unheard-of in another. Someone who grew up in an Underhive, miles deep beneath the towers of a continent-spanning city, may be awestruck at the thought of a "sky"; meanwhile someone who grew up in a farm-dedicated agri-world would boggle at the thought of a Forge World that is almost wholly dedicated to manufacturing. This doesn't even get into the Mechanicus having such a strangle-hold on all things science after so much was lost over time, nobody has any idea HOW all of the technology works only THAT it works, and this includes many in the Mechancium itself!
- In BIONICLE, the Matoran on Mata Nui used to live in the city of Metru Nui, but they lost all their memories of their old lives when the Makuta trapped them all wiped their memories so he could control them when they awaken. The Toa Metru awaken the Matoran by giving up their Toa powers, and guide them in tribal living as Turaga.
- In Time Traveller's segment of The Cave there's a (rather empty) museum with several examples, overlapping with Future Imperfect:
- A few examples of lost (or misinterpreted) knowledge crop up over the course of Civilization: Beyond Earth. In most cases it's less that the knowledge is plainly lost, as that very few people bother to learn it. The Purity affinity naturally has a building (the aptly-named Terra Vault) and wonder (Deep Memory) that are dedicated to the preservation and exhibition of what does remain in hopes of averting this trope.
Samatar Jama Barre: [on Deep Memory] You may say: "I remember," but we must also remember what our parents remember, and what our parents' parents remember as well.
- Dragon Age: Inquisition: Almost everything about ancient elven society has been forgotten by modern elves, including the fact that vallaslin, face tattoos worn by all Dalish, were once slave markings. This is also a huge motivation for Solas's plan to restore the pre-Veil world, since he feels modern elves are shadows of what they once were, and is dismayed that most modern Thedosians (of all races) don't know anything of what had been common knowledge in his day.
- Horizon Zero Dawn : Humanity is in a primitive tribal state without much knowledge of the old world, even though you have these obviously futuristic machine animals stomping around. Figuring exactly out what happened in the past is a major plot point in the game.
- Star Wars: The Old Republic: An absolutely tragic case where a Republic PC searches through apocalyptic logs to learn what happened to a colony that disappeared shortly after The Empire bombed Taris into a toxic waste planet. After generations of devaluing education to survive endless famine, disease, failing crops, low fertility, still-births, rackghoul plagues, and more, the last leader of the last generation (only ten people) can only remember:
"Promised Land" Leader: They say there used to be a city here. I know what a city is — lots of houses... We've stopped giving birth. The old records say it was... [slowly] "tak-sick radie-ashtion."
- In The Last of Us, while some modern technology still exists, mostly in the quarantine zones, Ellie has a hard time believing that ice cream trucks used to be a thing.
- Stand Still, Stay Silent is set in a world where Iceland, Norway, Sweden, Denmark and Finland are the only surviving nations as far as the characters are concerned. At some point, one of the mages ends up in a spirit world location that is set up just as it was Just Before the End, and the location's resident naturally offers him a brown-colored beverage that has to be either tea or coffee. Upon drinking it, the mage calls it "soup" and says it tastes gross.
- In Peace on Earth, two squirrel pups ask their grandfather what "men" are, since humans have driven themselves to extinction.
- Often played for laughs in Futurama:
- "The Series Has Landed": Although the moon has long since been colonized, the site of the Apollo 11 landing has been forgotten. Fry and Leela only find it by sheer luck. A plaque inside suggests that it was restored some time between now and the 31st Century, then abandoned. In a bit of Fridge Brilliance, when Fry and Leela find the original Apollo 11 site, they take refuge in the ascent stage of the Lunar Module. A plaque on one wall states "Placed here by the nitpickers' society."
- "Mother's Day": After all machinery has rebelled, Fry has to reintroduce the concept of the wheel to the others. Of course, Fry being Fry, he gets the shape wrong.
- "A Big Piece of Garbage": To stop a giant ball of garbage from hitting Earth, they need to hit it with another ball of garbage. Unfortunately, since all matter is recycled, garbage doesn't exist as such, so it's up to Fry to teach the world how to make it.
- "A Clone of My Own" has Farnsworth demonstrating to Cubert his failed attempt at a universal translator, which only translates into "an incomprehensible dead language":
- The Simpsons, "Lemon of Troy". Bart has an Imagine Spot where people from the future bring him back to life using "technology" (namely, shooting a ray gun at a piece of graffiti he's made). He wows them by taking out a yo-yo.
Man 1: What's normal to him amazes us.
Man 2: He will be our new god.
- As many historians and archaeologists can confirm, there's a lot we don't know about everyday life and technologies of the past. Usually we have something better, but obsolete crafts still interest scientists. This is what experimental archaeology is about.
- Possibly the quintessential example is Roman concrete, the use of which can be seen at any surviving Roman road or non-urban structure (dams, bridges, etc.). ~2000 years later and it survives with minimal wear, structures still serving their purpose, yet the complete, working recipe for it remained lost to the ages. In 2017, scientists at the University of Utah rediscovered the process.
- More like, we had the recipe, but when scientists tried to follow it, the results were different than the Roman examples. Apparently, nobody bothered to write down that you need to use sea water.
- Up until the 18th century, occasionally unearthed stone tools were explained either as a natural phenomenon (pottery shards grow underground like potato or beet) or magic (flint tools are created by gnomes or by a lightning strike).
- Drilling holes in stone tools without metal, using just sand and sticks or bones was demonstrated only in the late 19th century.
- The purpose of bâtons de commandement ("rods of command") or pierced rods. Those are palaeolithic artefacts (12-23 thousand years old) made out of a 15-20 cm Y-shaped piece of a deer horn with 1 or more holes drilled. Originally though to be a purely ceremonial sign of an experienced warrior. Current hypotheses include: a tool to straighten arrows or spears (by passing them through the hole), a spear thrower (tested, proven to work), a symbol of fertility, a dress clasp, a calendar (most rods are decorated with patterns), and a leatherworker tool.
- How moai were moved. A lot of theories were proposed, including Ancient Astronauts, but experiments have revealed that these huge statues can be "walked" using ropes and the same leverage techniques commonly used to to move a heavy barrel. Rapa Nui oral history always said the statues walked but this was thought to be figurative until the technique was rediscovered.
- The suspension system of pre-Roman British chariots that allowed enough precision for archers to shoot from a moving chariot on a bad road. Researchers who studied the Wetwang chariot burial around 2010 concluded that it was a wooden frame hanging from wooden arches on rawhide strips.
- "Ganosis", a coating to protect polished marble from the elements. Well known in Ancient Greece, involved wax and heat, but only a few sculptors knew the exact process. Lost at the time of Roman conquest, rediscovered only in the 20th century.
- Another Ancient Greek formula allowed ivory to be turned into a soft and malleable wax-like substance and also reversed the transformation. Also rediscovered only in the 20th century.
- "Greek Fire" (actually used by the Byzantines, who were Roman) is a fairly well-known incendiary substance that was used as a weapon. Its properties were recorded by contemporary writers, but not its formula. A number of possibilities have been put forward, and weapons with a similar effect are easy to make today, but the actual composition has been lost to history.
- Damascus Steel was created in ancient times to make swords and was legendary for its strength, flexibility, and sharpness, not to mention its beauty. Modern metallurgy can produce slightly better steel but the original method is lost to the ages. Today's scientists have even discovered carbon nanotubes in these ancient blades.
- Similarly, few historians bothered to record mundane "obvious" information like what life was like on a farm in ancient China, or the workings of a marketplace in the Songhai Empire; leaving modern day historians of Rome learn about the gossip, vulgar humor, "night life" and general street culture from studying things like ancient Bathroom Stall Graffiti, the Sigil Spam of ancient mercantile groups and manufacturers, and lewd carvings on the walls of ancient alleyways.
- A similar rarely discussed bit of information about ancient Egypt is that the workers carving the hieroglyphics were a bunch of Deadpan Servile Snarkers and aspiring political cartoonists. There are more than a few hilarious, crude, and hilariously crude Easter Eggs, Creator In Jokes and MSTs hidden in corners of many tombs and monuments. These often give insight into their actual opinions about those whose grand histories they were recording, and expand our understanding about how much is false and what can actually be considered somewhat reliable.
- It's well known that the Roman army was able to rotate their troops through the front line, but nobody is sure of the details of how it was done in the face of the enemy. The people who wrote histories either didn't know, or considered it as obvious and non-noteworthy as which end of the sword you hold in weapon drills.
- We have evidence that the story of the pied piper of Hamelin is Based on a True Story. We don't know how accurate it is, however, because the oldest surviving mention of it simply says that "it is 100 years since our children left". The author no doubt expected any reader to know exactly what had happened.
- Throughout the Western world, salt and pepper shakers are standard dining-table equipment. However, older sets — from as recently as the 1850's — include a third shaker. Nobody alive knows what was put in it: sources from the period when the three-shaker set was common just say things like, "The usual condiments," or "salt and so forth." Bill Bryson's book At Home discusses the matter in some detail and lays out some of the theories as to what the mystery substance might have been.