Equivalent Exchange



""Humankind cannot gain anything without first giving something in return. To obtain, something of equal value must be lost. That is alchemy's first law of Equivalent Exchange. In those days, we really believed that to be the world's one, and only truth." "

- Alphonse Elric, in the Opening Narration to Fullmetal Alchemist

Power is consistent. Power is functional. Power is not free.

For magic to be performed, for something to come into existence, for a wish to be granted, or for a loved one to be healed or returned to life, another thing of equal value must be given up. It has a simple dramatic purpose: It avoids the dangers of giving heroes Story Breaker Powers while adding conflict and sacrifice to an already hard moral choice.

Are the gifts, loves, and life you possess truly worth losing for what you desire? Beware Desire, s/he is a fickle thing.

The object or goal to be traded with Equivalent Exchange must be of equal value to a petitioner. However, how this "equivalency" is determined can be vague. Is the value preset? How valuable it is to the giver? The value to the receiver? Is it purely emotional, or will monetary, legendary, or rare things do? The same penny, handed down from father to son, would be worthless to someone else who just found it on the ground. If a wizard, say, ritually sacrifices a cat to get some magical mojo, which is used: the value of the cat's life to the wizard, or the value of the cat's life to the cat? What measure for value is changed to the whim of the plot.

Much like King Midas's gift of the Golden Touch was a great boon, it ultimately robbed him of his greatest treasure, his daughter. Wishers may not truly understand the full effects of what they desire, and the gift itself can become its own cost. This is Truth in Television, given the First Law of Thermodynamics. Building in the cost to the boon usually results in a Fantastic Fragility.

Compare Balancing Death's Books and Mutual Disadvantage.

Anime and Manga

 * Fullmetal Alchemist is the Trope Namer and arguably also the Trope Codifier for anime and manga, and makes a big deal out of the principle in its plot. In regards to alchemical transmutation, the law is basically the law of conservation of matter—matter and energy cannot be created or destroyed in a reaction. In addition, the energy used to perform Amestrian alchemy  in the anime, while in the manga it is said to come from tectonic shifts. Xingese alkahestry is powered by reading and directing the Earth's natural energy.
 * Ed gets a truly wonderful chance to explain the principle in the very first episode of the anime (same as the first chapter of the manga and the third episode of Brotherhood). He lists, in order, all the materials existing in the human body (and just to be cruel, points out that they're cheap—you could buy a body with pocket money), and then explains that even with all those materials gathered together, you still can't make a human life. Something's missing, and the exchange isn't equivalent yet.
 * In fact Ed himself comes to this conclusion in the manga: you need a soul, and the right one at that.
 * The Philosopher's Stone was said to allow bypassing of Equivalent Exchange.
 * However, one of the central tenets of the manga and a sign of the development of Alphonse and Edward as people is that
 * The first anime has this exact same thing happen, though it is applied differently. Almost everything Ed and Al try to do only makes things worse, instead of better, because of their belief in the theory of Equivalent Exchange, due to the fact that the many variables in life always affect the outcome. This is driven even further by . Unlike the manga, though, the lesson from it is slightly different.
 * Fate/stay night (and its multiverse) contains this—energy must be taken from somewhere before it runs through the Magic Circuit, though in most cases, Prana is either generated by the magus (Od) or taken from the environment (Mana). Rin can launch A-rank attacks in seconds because she taps directly into her jewels (where she has been saving Prana for ten years) instead of gathering it slowly for a minute or three.
 * This is a plot point for all three scenarios in the game, as protagonist Emiya Shiro has to fight, but the only magecraft he is skilled at is far beyond his capabilities. Whenever he uses it excessively, blood loss (and death) is inevitable.
 * A double subversion occurs when Shirou gets the ability to use ; his mana cost for Gradation Air (Projection) becomes zero while inside it, but he still needs to gather mana in order to cast it. No points for guessing where he gets that mana.
 * Another subversion is Rin’s Jewel Sword Zelretch, which lets her use strong attacks indefinitely by absorbing mana from alternate realities.
 * In Sci Fi Harry, the main character may perform telekinesis, but someone nearby will die as a result.
 * In the multiverse of Tsubasa Reservoir Chronicle and ×××HOLiC, the price for wishes granted by Yuuko works this way. Some characters are much more wary of the possible implications of this than others. In particular, Kurogane and Doumeki will only ask for Yuuko's help if it is flat-out impossible for them to solve the problem under their own power. This is prudent, as false-hearted or impulsive wishes usually result in nasty karmic payback. Yuuko also has rules about not dealing in wishes of value equal to a human life, as that would implicate the wish-granter in murder.
 * For the record, Yuuko accepts items (or intangibilities) that have both real and emotional power: Kurogane's sword is both a precious reminder of his family and a powerful weapon; Fai's tattoo is imbued with powerful magic for his own safety; and Syaoran and Sakura pay by giving up a portion of Sakura's memories - those which contain Syaoran. This means their previous relationship is sacrificed forever. Syaoran has been edited out of the memories the feathers contain. For example, Sakura can remember the events of her last birthday party, but one seat is mysteriously empty, and there's an odd pause in the dialogue whenever someone would have otherwise said Syaoran's name. One time Sakura almost manages to put two and two together, but passes out and loses all memory of her deduction due to this price. It's not flat-out impossible for Sakura and Syaoran to have a relationship after paying the price, but it has to be rebuilt from the ground up.
 * Interestingly, you apparently don't need someone to trade with..
 * Witch Hunter Robin had a witch that was a doctor (though not a Witch Doctor) and had the power to transfer Life Energy from one person to another. He used this to heal patients, taking the needed energy from mob bosses. He ultimately decides to save his human partner and doctor by sacrificing his own life. Considering that the witch hunters kill most witches they go up against, with those captured alive, he definitely made the right decision.
 * That episode's Aesop was a bit odd: sure it's bad to kill, even if it's for a good cause, but the heroes go in guns and flames blazing against someone who had next to no offensive ability (and barely defensive ability to save himself from being char-broiled), and Robin gives him the moral lecture equivalent of the third degree when in previous episodes they fight much eviler witches like the "suck the life out of younger witches to extend my own" witch.
 * That episode was actually meant to be a Broken Aesop. The entire point is that they do not hunt evil Witches, they hunt all Witches, regardless of whether they can morally justify the use of their powers or not. The whole episode is about Robin learning the holes in her philosophy by going on a less defensible, fairly nasty hunt.
 * Hunter X Hunter has Nen abilities. It's basically a special ability invented by a user by using Nen, and it can range from simple super strength to shrinking cloths to memory reading to vacuum cleaners and books and bank accounts. Each of these abilities have unique powers that the user make up in his mind during conception. The more powerful you are, the more powerful ability you can create. The thing is, the more powerful the ability is, the more restrictions the user must needs impose on it as well. Examples are:
 * Kuroro's Skill Hunter ability allows him to steal the Nen abilities of others and record them into his book, thus robbing them of that ability forever. It's an awesome move, but there are many rules. First, he must see the ability in action personally. Then, he must know every property of the ability, from its main use to the rules of using it, all of which must be told him by the victim. Third, the victim's hand must touch the palm print of the cover of the book. Lastly, all of the above must be done within only one hour. Otherwise, it's back to step one. Now, if you get the ability into the book, it's all well and good. But another rule is that if the original user dies, the ability vanishes from the book forever, and he can't use it anymore.
 * Kurapika's Chain Jail is a long chain used to wrap around the opponent's body. Not only that, it shuts down their aura, and prevents them from using Nen abilities. However, because it is an overpowered ability, he had to impose the condition that he could only use it on a member of the Genei Ryodan. If he uses it on a person who is not a member of the group, he dies. This causes problems because there are only at most thirteen members of the Genei Ryodan at any given time.
 * Alluka Zaoldyk's wish-granting powers are an especially sadistic take on this since the price of the wish is passed on to someone else instead of the person making the wish. If that person can't fulfill Alluka's requests (and the requests can be as severe as ripping out your own brain if the preceding wish was significant enough), then that person and that person's loved one die. Depending on the severity of the wish, everyone that person has ever met is also at risk of dying instantly.
 * Hibiki no Mahou. Oh dear, Hibiki no Mahou. If you want to use magic properly, you need to sacrifice an aspect of your being, like ability to dream, age, or memory. No wonder why magic practitioners are declining.
 * Although it's not too much of a regular magic as it's more of a one-use, people at Hell Girl, when contracting Enma Ai, can send one person they dislike immediately to Hell - with the price of their own souls going to Hell when they die. That's the reason why many people hesitate in first place: going to Hell is a really high price. But then, out of spite, they send their victims into Hell anyways. The lesson is to learn to control your emotions and hatred? or You Can't Fight Fate? Who knows?
 * It is practically the premise of The Law of Ueki.
 * C.M.B. has the rather interesting case in which Shinya requires a price for solving a mystery; luckily he's very easily bought off so long as it is interesting.
 * Code Breaker: It turns out that
 * The second arc of Naruto started off with Gaara dying, but he was an immensely popular and critical character so he needed to be revived. Enter Chiyo, who has a ninjutsu capable of reviving the dead. To avoid this being used to prevent every death in-series, the jutsu was equivalent exchange: the user sacrifices their own life to resurrect another.
 * Ah! My Goddess has the Law of Conservation of Happiness. Eventually, granting too many wishes within a short period of time means that someone ends up suffering some form of bad luck.
 * This is also how wishes granted by demons run. They usually come with a myriad of attached strings that cause grief and misfortune to the person making the wish.
 * In Puella Magi Madoka Magica, the wishes that get fulfilled in exchange for the girls becoming magical girls are guaranteed to cause an amount of despair equal to the hope they bring.
 * In A Certain Magical Index, once one becomes an esper, they can never use magic, and vice-versa, without suffering extreme damage to their bodies.
 * To gain the incredible powers of God's Right Seat, a magician must give up his or her ability to use normal spells.

Comic Books

 * In the British Transformers comic, if someone travels back in time, someone from the destination period vanishes into Limbo while the traveler is there. Attempts to avoid this will eventually get you eaten by a time warp.
 * In the Pre-Crisis DCU, the Guardians of the Universe purged themselves of evil, only to find that the evil had to go somewhere. They sealed it in the same universe where they sent most of the magic to ensure that the Golden Age Green Lantern would be around to deal with it.
 * Storm's weather-controlling powers in the X-Men comics are given a similar limit to explain why she can't turn the world into a paradise, or at least bring relief to disaster-stricken areas. The first time she tried that, she successfully ended the drought in her village - only to discover that, since the moisture she'd drawn upon had to come from somewhere, she'd caused even worse droughts to strike the rest of the world.

Fan Works

 * Played with in The Teraverse. In the story that inspired the 'Verse, The Secret Return of Alex Mack, it's made clear that Alex's powers (and those of other supers) burn immense amounts of calories and require her to eat huge and frequent meals.  However, it's quickly noted that most super powers still expend more calories than their users can actually consume.  Much later in the history of the timeline, it's determined that many supers cheat the Second Law of Thermodynamics by being able to tap into "Bendix Space", a nearby dimension consisting of nothing but energy, and the calories they expend are nothing more than the "control cost" of accessing and directing that energy.
 * In The Arithmancer, Dumbledore brings up this topic by name in his private lessons with Hermione in sixth year. Hermione keeps coming back to it when researching horcruxes and how to get the one in Harry out. And in chapter 5 of the sequel story, Annals of Arithmancy, she explicitly cites this principle as the operational basis of Ritual Magic.
 * Final Stand of Death, The Undertaker had to explain this as Melanie, i.e. Sporty has to do in order to unleash her ultimate power to deal with Marilyn Manson, after Geri is left some explaining to

Film

 * In Conan the Barbarian, Conan is mortally wounded by his crucifixion on the Tree of Woe, so his lover Valeria convinces the Old Wizard to work a healing spell to save him from the brink of death. The Wizard warns that the gods demand a price for this sort of thing, and she says she'll pay it. When she's later shot with a snake arrow by Thulsa Doom, she decides that her death is her payment. Whether that's true is not revealed, but it seems logical given the bleak universe of the film.
 * The magic time traveling scepter of Teenage Mutant Ninja Turtles III required an equal number of people (of roughly the same weight, which didn’t come up) to travel in each direction. While the movie is not all that popular, it did make for some interesting events going on while the Turtles were back in Feudal Japan.
 * Rumplestiltskin's "Ogre for a Day" contract in Shrek Forever After works by taking a day from the signer's past (in this case, Shrek). The day Rumple chose was.
 * In the first part of Merlin's Shop of Mystical Wonders, the main character (a jerkass Straw Critic) gains magical powers at the expense of his life force.
 * A relatively benign example of this trope occurs in The Secret of the Magic Gourd. Rather than a life for a life, the Magic Gourd uses magic to swap Wang Bao's blank failure of a test paper with the high-scoring test of a fellow classmate. Being his typically dumb self, Bao Hulu doesn't even bother to change the name at the top of the test, which gets Wang Bao in trouble for cheating.

Literature

 * Christopher Paolini's Inheritance Cycle: The entire magic system of the series is wrapped up in this. Doing something with magic takes the same amount of energy as doing it without magic would have done. And if you try casting a spell that takes up more energy than you have, you die.
 * This is what he says, but at one point, Eragon breaks the calves of dozens (maybe hundreds) of soldiers and doesn't die. And how much energy does it take to make a blue fireball without magic?
 * And certain spells are just flat-out impossible, presumably because they take far too much energy, or simply cannot be done. For example: Eragon is warned to never try to bring people back from the dead; beyond death, there's just something that magic has no effect on; an attempt would drain the mage of all his life in one go and accomplish nothing. Trying to see the future or the past is a bad idea as well.
 * Although in the second book, the young woman running the Varden finds some wiggle room in the rules: doing something with magic takes less time than it would otherwise, therefore magic-users can outperform in tasks which are complicated but low-energy. And that's the story of how the Varden climbs out of a financial hole by.
 * In the earliest Incarnations of Immortality book, On A Pale Horse, Death is NOT scheduled to take . She is going to be burned alive by a dragon, but performs a Heroic Sacrifice. This gets her on Death's schedule, and Gaea performs an Equivalent Exchange by providing a lamb for the dragoness to eat instead. But, Satan wants her dead, so he tries finding other ways to force her demise.
 * This also comes up a lot with magic in the Discworld books, where it is referred to as the Law of Conservation of Reality. For example, to teleport someone from one side of the disc to another, you may need to have an equivalent weight to teleport back to where they came from. This is mainly to deal with conservation of momentum; because the Disc rotates, different points on its surface move at different velocities relative to the Hub. Teleporting without such a counterweight means that if you move very far, your velocity relative to your immediate surroundings tends to kill you. But you can do it!
 * This is still an imperfect science, and when the wizards of Unseen University try it to retrieve Rincewind in Interesting Times, he's hit on the head by all the crap they piled up to equal his weight, going the other way.
 * It comes up more often with conjuration than with transportation. If you wanted to make, for example, a loaf of bread appear, the casting thereof would have to expend all the energy that went into making the bread—so, growing the grain, grinding it into flour, mixing the dough, all the heat it took to bake it—or else you'd have a loaf of bread for about half a second and then it would vanish again. So mostly they just don't bother.
 * In the non-Dragonlance book The Death Gate Cycle by Margaret Weis and Tracy Hickman, the same problem came up. When you brought someone back to life, be it a full resurrection or just as a zombie, somebody else, somewhere, died. One of the two competing races nearly wiped themselves out this way.
 * Vurt. Things are swapped between the real world and the vurt world on the basis of their value. The characters are keeping a weird tentacled creature because it was somehow switched with the protagonist's sister, and they're trying to figure out how to get her back. At one point, he wants to bring a object back to reality, so he leaves something of sentimental value behind.
 * Blood-magic in George Martin's A Song of Ice and Fire works this way;
 * Near the end of the first book, —and gets badly screwed by the exchange, since the mage had played upon her desperation  . Later, she uses the same principle to.
 * Melisandre kills . Althouh the details are never quite revealed, it seems to involve getting herself pregnant and sacrificing the life of the unborn child. There are also hints that the spell was at least partially Cast from Hit Points.
 * The magic system in Eric Nylund's Pawn's Dream works through a variation of Equivalent Exchange, where opposite elementals must be present, but it varies whether users need to trade them or simply summon or banish both. Either way, most of the skill in magic is based on letting both elements flow freely.
 * The Recluce Saga by L. E. Modesitt Jr. are set in a world of Chaos and Order Magic, both of which must be carefully balanced—at times, overuse of either, or just too much Order or Chaos concentrated in one area, has shifted the entire planet's weather patterns, caused volcanic eruptions, and other disasters.
 * Despite that the balance is well-known in-universe, it didn't stop people from trying to cheat. Recluce itself, for example, was protected by a navy of Order-infused ships...and every time they replaced one, it was with a larger, more powerful ship that required more Order.
 * The novels in the Star Wars Expanded Universe emphasize that the Sith path is one of sacrifice, i.e., demolishing attachments instead of the Jedi path of avoiding attachments. Since the Sith draw power from pain and rage, easy or pleasant trade-offs aren't in the picture.
 * Which makes the Emperor's plan of either 'Luke kills his father' or 'Anakin kills his son' at the end of Return of the Jedi make a lot more sense.
 * Orson Scott Card is very fond of this trope and said that it is a practical necessity for a fantasy story driven by magic, writing in How to Write Science Fiction and Fantasy that without it, the "characters become gods...and there is no story". In that book, he proposes some grotesque systems of magical exchange, including killing (a human grants you more power than an animal; a child grants more power than an adult) or losing body parts for power (your own body parts fall off; or someone else's fall off, but they must be given willingly; or body parts of the one you truly love the most fall off; or a random person's fall off, but they tend to have a connection to the spellcaster). Nightmare Fuel indeed.
 * And as he mentioned in How to Write Science Fiction and Fantasy, Card actually wrote a novel in such a setting: Hart's Hope.
 * Deryni magic is physically taxing, with more powerful workings (say, Calling to another Deryni over a great distance, particularly when the other Deryni doesn't expect it and isn’t helping to bridge the gap from the other end) causing more fatigue.
 * Fatigue-banishing spells exist, but they cannot be re-used indefinitely, and the rejuvenation they provide can be quickly lost if the person experiences more stress.
 * Training also seems to extend endurance within limits, much as people who physically train increase athletic endurance to a point. Tiercel makes this analogy: “...you're flexing abilities you've never used before. You have to build up your endurance. I'll bet you've got a headache just from this afternoon's work.”
 * In the Coldfire Trilogy, magical energy is released by sacrifice. The amount of energy gained is directly proportional to how much the sacrificed object was worth to the sorcerer. This is one of the facts used to show the Magnificent Bastardness of Gerald Tarrent, who sacrificed his entire family to gain immortality. The spell wouldn't have worked if he hadn't loved his wife and children very, very much - and he killed them anyway.
 * It's later explained that the sacrifice which made him immortal wasn't actually his wife and children. It was his own humanity, which he lost through the expedient of killing his wife and children.
 * And if he ever tried to act like a compassionate human being again by engaging in an act of life or Healing, his immortality would be forfeit.
 * Another huge example in the backstory explains why humanity is stuck on Erna without any of the technology that got them there in the first place.
 * In the early Anita Blake stories, one must kill a living thing to create undead. Anita routinely uses goats for that purpose. Of course, most magic is swallowed up by Deus Sex Machina as the series moves forward. (She has kept the goats out of that part.)
 * In Wolf Speaker, the second book in Tamora Pierce's Immortals tetralogy, Numair turns a major villain into a tree and states that somewhere, a tree is now human.
 * Pierce eventually wrote a story (in Bruce Coville's anthology Half Human) about that tree that became a human.
 * Words of Power differ from regular use of The Gift in needing equivalent exchange. You can do more minor stuff rather easily. However, for example, destroying a powerful magical barrier with Words would have caused a major natural disaster elsewhere.
 * Additionally, we find out in Protector of the Small that the use of the Dominion Jewel to prevent an earthquake (at the end of Song of the Lioness) cost the fertility of all the grain in the kingdom. They had to import all their bread and seed wheat the next year, and the kingdom was in debt for years paying it off.
 * In the Young Wizards series, "spells" are essentially equations written in a magical language and equations always have to balance, one way or another. In one book, a character holds off the Big Bad with a shielding spell which is later explained to have been fueled by a year of the character's life per strike. (The Devil hits hard.) In another, that same Big Bad is sealed away by a ritual that requires a willing sacrifice; one character attempts to take the place of the intended victim . It's implied that this event might have come to pass because of events in the prior book: to cast a spell they couldn't have powered alone to seal away an Artifact of Doom they used a "blank check" spell in which they essentially promise that the power they use will be repaid at some unknown date in the future.
 * The "death magic" in Lois McMaster Bujold's Chalion series is like this: it's a way to murder someone at a distance, but as part of the spell's workings, the person casting the spell will die, too. In addition to this little drawback, it's very difficult, and requires animal sacrifice as well. Plus, the God of Executioners has to be invoked and
 * The "Barry Trotter" parody of the Harry Potter series mentions this in the sequel, where everyone has graduated and started families . It's only then that Barry learns that to summon something with magic is actually just teleportation, thus “created” items always comes from somewhere else. This coincidentally results in a poor sap whose possessions always go missing on Barry's birthday.
 * Broken Sky has "spirit stones", which grant people special abilities but drain their energy when used, placed into the spines of characters. Ryushi, for example, can emit a powerful blast of energy, but it drains him to the point of collapsing to do so.
 * We're later introduced to people with healing spirit stones. They can heal the wounds of others by taking them on themselves, although the healers have a slightly improved healing rate.
 * In The Neverending Story, AURYN grants humans the ability to make their wishes come true, by rewriting reality in Fantastia so that it always was so. In exchange, AURYN takes away a memory of the Outer World from the human with each wish. They start off reasonable enough (trading the memory of being fat and scared for a more heroic figure), but eventually descends into taking more and more precious memories, no matter how selfish the wish is (the memory of being from the outer world, the memory of one's parents, the memory of one's own name). In the book, the wishes don't even have to be spoken, either. If you want something badly enough, AURYN will sense it, grant that wish and take a memory with no effort on your part.
 * A system of Equivalent Exchange is enforced in the Night Watch books. Basically, the forces of good and evil have a treaty regulating and limiting their actions. When one side uses their power to interfere with humanity, the treaty demands that the other side receive an equal intervention. Hence a Light mage can heal someone, but that gives a Dark witch the right to curse someone. The system works overall, with most "Others" (the series term for supernatural beings) willing to go through the proper channels to get licensing for using their powers. (For example, vampires annually receive a license to feed on a living human, though not all use them.) If an Other breaks the rules, the Watches (the police of the Others) will locate and punish them (with most crimes being sentenced to death). If a member of the Watches breaks the rules to a relatively small extent, they can offer the other Watch an equal intervention as a compromise.
 * In Robert Silverberg's early novel The Time Hoppers, time travel is done by exchanging matter between the present and the past; when a human is sent back, an equivalent mass of air has to be brought forward.
 * In Cornelia Funke's The Inkworld Trilogy, characters can be read out of books into the real world, but not without someone or something from the real world taking their place
 * The three magic systems from Mistborn work this way.
 * Allomancy requires the wielder to consume fragments of metal and "burn" them in their stomachs to power certain effects; some of these metals are considerably rarer than others, and the metal duralumin acts as a catalyst for burning whatever other metals are in the user's stomach very, very fast.
 * Feruchemy allows the wielder to temporarily drain him or herself of a certain quality or ability - memory, skill, age, health, senses - store it in a piece of metal, and retrieve it later as needed (for example, a Feruchemist could temporarily become very weak, and then later "tap" the stored energy to acquire superhuman strength).
 * Hemalurgy does not require fuel, but it does require leeching abilities from Allomancers and Feruchemists, and often leads to great insanity.
 * The entire basis of Devon Monk's Allie Beckstrom novels. Magic always exacts a price, usually in pain. The user can choose between an hour-long migraine or a week-long head cold, but the price will be paid. For the protagonist, magic also takes her memories. For small spells, she'll forget what her stepmother's name is. For large spells, she'll wake up with a three week hole in her life. She carries a journal with her everywhere, with her name, address, etc. written on the first page, just in case.
 * In the Bras and Broomsticks series by Sarah Mlynowski, if someone uses magic to get something, that thing will be taken from wherever it comes from. For example, the main character's sister makes oranges to give to the homeless, but there is an orange shortage in stores in the area.
 * At the end of Changer's Moon, The price? . (Although that was less a function of the magic itself—she'd previously —than of using it on an opponent of such power.)
 * Roger Zelazny's Changeling follows this logic for moving items between universes.
 * Stephen Donaldson's second Chronicles of Thomas Covenant has the mere mortals of the land required to shed blood (both their own and the blood of others is used) in order to manipulate the Sunbane.
 * The first chronicles has Mhoram realizing
 * In China Mieville's Iron Council, a character is beholden to an interesting version of this. They are a monk of the God of Secrets; part of this means that they can ask the god for knowledge about a secret: a secret path, what the enemy is planning, and so on. But they have to give up some of their own knowledge to do so. The character in question lost knowledge of her gender as a result of this.
 * Sympathy in The Kingkiller Chronicle follows the actual First and Second Laws of Thermodynamics, so that heat and work applied to a target have to be taken from somewhere and transfer of energy can be rather inefficient. In practice, this means that people have killed themselves trying to raise winds with their diaphragms and light candles with their body heat.
 * In the Stephen King novella Fair Extension, George Elvid offers to cure Dave Streeter's cancer at the cost of inflicting it on someone else. Since that someone else happens to be his ex-girlfriend who cheated on him with his ex-best friend, Streeter isn't exactly bothered.
 * In Ian Irvine's Three Worlds Cycle, use of magic (or "the Secret Art") is balanced by "aftersickness": debilitating headaches and nausea. Unskilled "mancers" may even get the aftersickness without the magical effect they were going for. This, however, is nothing compared to the risks of trying to draw too much power from the field: you get "anthracised", where the sheer power you're trying to channel burns you alive from the inside out. Of course, it's lovingly described in detail in one of the books.
 * Vorkosigan Saga: A Civil Campaign; Cordelia Naismith-Vorkosigan points out that her home planet of Beta Colony is extremely sexually liberal, yes, but at the cost of being reproductively conservative. All women, by law, have a contraceptive implant, and people wanting to become parents are subject to a battery of tests before being licensed. Very few families are allowed more than two kids. This is not something the writer just made up, mind you; both the sexual liberality and the implants had been mentioned throughout the series, but it is the first time the connection was explicitly explained.
 * The Wheel of Time: While use of saidar is more or less free, the Aelfinn and Eelfinn operate on this basis, and from most non-main characters important things like valuable knowledge, cooperation and items must be bargained for, often with extensive negotiations over the price the heroes must pay.

Live Action TV

 * Ben's healing talent in Carnivale worked by drawing Life Energy out of the surrounding area. Cure a little girl of polio, the crops wither as she skips away through the cornfield. Heal a broken arm, a bunch of fish go belly-up in a nearby pond.
 * Chloe Sullivan on Smallville got a similar power in the sixth season finale (in this case, she died and came back to life). In the Seventh season, it is explained that she can heal non-fatal wounds so long as she herself takes on that wound (i.e. to heal a paper cut on Jimmy Olsen's finger, her body compensates by receiving a wound of similar size on the same spot on her body).
 * One episode of Forever Knight featured a mystic healer that could take darkness out of people. However, said mystic happened to be a novice at her craft, and didn't know that this darkness had to be put somewhere, (usually into an inanimate object of some sort), and wound up absorbing it herself and being overwhelmed by it. The episode had a really sad end to it,  Her grandfather alluded that she might have been capable of fully healing Nick (or at least making his gains permanent) if she had been more skilled.
 * Ned's talent on Pushing Daisies works the same way, by killing one thing of equal magnitude to whatever was brought back from the dead if the dead thing's alive for longer than 60 seconds. So, if a person's brought back from the dead, then another person is going to die to keep them alive. (See also: Balancing Death's Books)
 * Quantum Leap: Every time Sam leaps, the person he's replaced ends up in 1999.
 * In Buffy the Vampire Slayer, when Willow sends Buffy back in time to meet the sages who created the first Slayer, a demon is brought forwards to take her place. Willow says that this is to avoid violating the First Law of Thermodynamics.
 * Similarly, though perhaps a more accurate example of this trope, when Buffy is brought back to life at the start of Season 6, the spell creates a brand new demon through a process Willow calls "Thaumogenesis". As she explains, the universe doesn't like you getting things for free, there has to be a cost and they asked for a huge gift; Buffys life back, so the universe said "Ok, but you have to have this evil demon too". This fits the trope as its a Good Thing/Bad Thing equal parts deal but as Anya rightfully points out "That's not a cost, that's a gift with purchase".
 * And to get the ingredient that Willow needs to cast the spell to bring Buffy back, Willow.
 * Same thing happened with Jonathan in "Superstar," although in that case the existence of the demon was required for the spell to keep working.
 * The demon brought forth to take Buffy's place happens once again in the Season 8 comic when Buffy time travels.
 * In Torchwood, the Resurrection Gauntlet in the first series could bring someone back from the dead, but usually only for about one or two minutes. However, with enough empathy from the gauntlet's user, a person could be brought back completely from the dead - but the person who used the glove would slowly give up their life (including any fatal wounds the formerly deceased had) while the previously dead person would get healthier and healthier.
 * Whenever Frank Parker steps back in time in Seven Days, his self at that time period vanishes from the time stream. It's explained that this is because the same set of molecules cannot exist in two places at the same time.
 * This is the sort of thing that makes sense in magic, but not if you try to science it up. His body is not the same set of molecules that it was last week, so the results should be a little bit messy. I'd expect that to maintain secrecy, he wouldn't be allowed to eat anything that hadn't been on the base for a week.
 * This almost blows the secret once, when Parker steals something the first time through, when he's investigating the problem, and takes it back with him. So it 'magically' vanishes from villain out of air-tight security, out of a locked briefcase, and the villain twigs that that shouldn't be possible, and starts investigating this 'Backstep' project.
 * In Power Rangers there are some examples of The Sixth Ranger being limited due to their awesome power. The most notable were the Green Ranger and the Titanium Ranger. The Green Ranger’s powers were damaged, such that every time he used them, they would weaken, putting physical strain on himself. The Titanium Ranger was branded with a cursed tattoo of a cobra on his back. Every time he morphed, the cobra would move up a little. If it got to his neck, he would die.
 * This was also used as a plot point in the BBC Series Merlin.  There was even a reveal in this scene because
 * This reappeared in the season finale, where
 * In the Supernatural episode "Faith", a Reaper can restore a dying person's life, but only at the cost of another's. The woman who was holding the Restraining Bolt uses this to set her husband up as a faith healer, while using the exchange to murder various of her faith's bugaboos. When the brothers break the leash, she learns the hard way that True Neutral is not a toy.
 * In the episode "Criss Angel Is a Douchebag", real magic is used to save a magician from lethal escape tricks. The cost of saving his life is that another person dies in the same way the magician would have been.
 * In an episode of Big Wolf on Campus, Merton was given a magical watch that could turn back time. Unfortunately, every time he uses the watch, he loses some of his knowledge.
 * This appears to be how the Phoenix Talisman works in Warehouse 13, though its behavior is not consistent. It will resurrect its user after his or her death, and then the trauma of that death will be transmitted to a random person in proximity. However, it's not always a one-to-one substitution; during Mac Pherson's demonstration of the Talisman, two random mooks die in exchange for the single demonstrator. It may be proportional to the amount of trauma the user suffers.
 * In House of Anubis, the Cup of Ankh will grant immortality to the person who drinks the Elixir of Life out of it, but someone else will die to replace the life that would otherwise have ended eventually.
 * In Kaizoku Sentai Gokaiger, the Greatest Treasure in the Universe has the power to change history and reality, and confirms that it could be used to erase The Empire from history...but doing so would also retroactively erase all of the Super Sentai from existence. The Gokaiger end up deciding not to do this, since the Sentai mean too much to humanity and it isn't their right to make that call for the entire planet.

Mythology & Religion

 * The concept of Karma in many religions.
 * In Euripides's Alcestis, Admetus' wife, Alcestis, offered to die in his place. And then Heracles showed up and beat up Death to save her so that there could be a happy ending. Honestly, didn't these Greeks read Orpheus?

Tabletop Games

 * Dungeons & Dragons
 * In Dark Sun both Defilers and Preservers need to drain life forces in order to power arcane magic, but defilers drain it away without worrying about the consequences, while Preservers are careful not to drain enough to kill the plants and animals nearby that are contributing (and thus get less power and slowly). Considering that Dark Sun is a gigantic desert world, it's not hard to deduce the popularity of defiler magic.
 * In Ravenloft curses can be invoked by ordinary people, but attempting to do so invites a Powers check. If failed, the curse-layer will suffer karmic retribution from the Dark Powers. In an Equivalent Exchange Of Payback, the curse is actually more likely to work if the curse-layer fails this check. Of course, it has a much better chance in the first place if the cursed party did something to specifically deserve it.
 * Most magick in Unknown Armies works this way. The "value" of certain actions varies based on what kind of adept you are. Typically an adept gets a "minor charge" for some kind of ritual that's easy enough to be performed every day or so, a "significant charge" for doing something very difficult and painful, and a "major charge" for doing something nigh-impossible. And even the magick that seems to be free usually isn't. In Unknown Armies, ain't nothin' come for free.
 * There's even an in-game term for the concept in UA: The Law of Transaction.
 * The Vampire: The Masquerade version of the Tremere House and Clan is practically built around this trope. Originally mages, the Tremere turned themselves into magical creatures when magic began weakening, out of a supposition that said creatures would last a while longer during magic's decline. However, in doing so, they lost the essence that made them mages in the first place, and got what was, in essence, a surrogate. The price of power, indeed.
 * Present in the TCG Magic the Gathering to an extent. Every spell has a cost. Most are simply Mana drawn from the land, but others require a life (yours or your creatures), the land itself, or even time (skipping a turn). Several cards will actually kill you if the cost is too much for you to afford.
 * Perhaps the most famous example of this is Necropotence. You don't draw normally near the beginning of your turn; instead, you may pay X life to draw X cards at the end of your turn. However, this particular exchange turned out not too equivalent: both cards with this effect proved to be broken beyond imagination.
 * In Warhammer Fantasy Battle Fantasy, the species known as the Dragon Ogres succeeded at making a deal with Tzeentch that rendered their entire species immortal and able to subsist on lightning alone as energy—but the spell also struck them all sterile, so no new Dragon Ogres has been born since. Tzeentch is kind of a dick like that.
 * In Geist: The Sin Eaters, a Sin-Eater can come back from the dead easily (yes, more than once). Problem is, in addition to the act knocking a chunk off your Karma Meter, your geist is going to draw that life force directly from someone else. And when you wake from your brief dirt nap, your face is covered in an ectoplasmic caul that conveys all the details of that person's death.
 * The magic system in "The Valdorian Age" (a setting for Fantasy Hero) can be summarized as "you convince otherworldly/extraplanar beings to do something for you". However, they're doing it as a favor, and eventually they will require a favor from you in exchange ... which, depending on how big your debt is when it gets called in, could involve things like killing all the inhabitants of a village in one night. No one ever claimed those otherworldly beings were nice.
 * Several of the magic systems in Legend of the Five Rings work this way. Shugenja do not so much manipulate the elements themselves as convince elemental spirits to do them favors, and much of their duties involve making sure said spirits are happy. Maho is directly powered by the shedding of blood - but good news! It doesn't have to be your blood.
 * In Shadowrun magic is paid for by Drain imposed on the caster's system. If the caster doesn't manage to resist Drain outright, it can range anywhere from minor headaches to death.  And for truly vast workings like the Ghost Dance which brought magic back to the world in the early 21st century, the Drain can be measured in the number of magic-users who died during the casting.

Video Games
""With hell's fire, you make a bargain. It costs a little of yourself. The warlock's way was quicker, more effective, or so it seemed. But there comes a time when a price must be paid, and sometimes, it is dear indeed.""
 * The Monkey Island games have this as an element of Voodoo magic. In The Curse of Monkey Island, Guybrush needs to find need a diamond as big as or bigger than the one that turned his fiancé to stone. In Escape from Monkey Island, he needs to make an Ultimate Insult talisman as large as or bigger than the original in order to counter its effects. (No explanation is given on why his enemies can counter Guybrush’s insults even when he has the larger talisman, although it is only the second biggest monkey head Guybrush has ever seen...)
 * Oracle of Tao has a variety of these. Most magic uses MP as its price, but some abilities are Cast from Hit Points. On the other end of the scale, you have a Mana Shield. And then there's Elias's alchemy, which aside from making cool items, has certain alchemy spells. But in order to learn each spell or make a super-rare item (like gold), you sacrifice anywhere from a level to 10 levels (and you can combine spells together, meaning there's a chance you might end up making the same spell more than two times).
 * Whenever the Nameless One from Planescape: Torment.
 * Subverted, however, with the Nameless One being able to shrug off almost any price that doesn't kill him. Thus, what would be a painful exchange for a Squishy Wizard—learning secrets of fire magic from an insane pyromaniac—is nothing more than the loss of a few hit points, and you regenerate, so a few hit points won't hurt.
 * Then, double subverted with the and a particular (semisentient and evil) book of magic, which soon begin to ask for things aside from and greater than, just for an example, a taste of your flesh or you to commit some minor evil act.
 * The ritual of Soul Exchange in Valkyrie Profile allows someone to sacrifice their own life to bring someone else back. However, it won't work on someone who died by using the same ritual, and one character ends up undead for attempting this.
 * This was a main part of the old DOS action-adventure game The Immortal. Your character starts with a magic amulet that, you eventually discover, has the power to kill a dragon (the creature that awaits you at the end of your adventure). However, it turns out the amulet also kills the person who uses it (you could easily discover this at any point by using the damned thing), and the whole game is an Evil Plan by the Big Bad to get you to kill the dragon (sacrificing your life in the process) because he needed the dragon dead, but obviously couldn't use the amulet himself without dying.
 * In The Legend of Zelda Twilight Princess, Link can acquire a magical suit of golden armor which will prevent him from taking any damage. However, whenever he wears it, his wallet is slowly drained of money, and he loses extra money every time he takes a hit. Once he runs out of Rupees (the coin of the realm), the armor weighs him down considerably, making his movements slow and awkward.
 * In Final Fantasy games, to use magic, you must sacrifice MP, the amount of which is proportional to the spell's power, most of the time.
 * An aversion of this is the geomancer from Final Fantasy III. They have the ability to summon up elemental attacks depending on the environment, i.e. an earth elemental earthquake in a cave, a whirlpool in the ocean; to compensate, they have no control over the type of attack used. There are usually at least three or four attacks depending on where you are, so you're largely relying on luck to give you a good attack. However, they are one of the most powerful classes in the game, mainly because you can use their terrain attack indefinitely FOR FREE. This makes them far more effective in combat than black mages, as players usually save their spell points for boss battles.
 * In addition, Word of God says that materia, the conduit in which magic is cast, in Final Fantasy VII shortens the life of the Planet whenever it's used, specifically the mass-produced materia through Shinra's Materia Factory. Normally, Materia is made by the Planet and extremely rarely. But with the overcreation of Materia by Shinra, it is slowly draining the Planet of its Lifeforce... literally.
 * The world Kartia revolves around magical cards, Kartia, that can be used to create basically anything, from living battle-creatures to spells to specific kinds of food and drink. There are a number of Original Kartia (such as "Life" "Death" and "Human"), the use of which is forbidden and will automatically cause the death of the caster. For most of the game, the player and the characters believe that the Kartia creates things from nothingness
 * In World of Warcraft, warlock magic is said to work like this.


 * Don't expect to ever have to pay that price yourself though.
 * Examples include the Life Tap spell (converts health into mana points), Health Funnel (heals your pet demon at a cost of your own health), Hellfire (which burns you as well as your enemies), and Ritual of Doom (summons a powerful demon; originally one party member would die upon completion, now one member is simply badly wounded). Other spells are fuelled with Soul Shards, gained by killing foes with a particular spell.
 * Although now, Warlocks have a low-level spell that lets them restore all their soul shards by apparently breaking off little pieces of their OWN SOUL (at no real cost in gameplay terms, though it is interruptable and thus best used out of combat/between fights).
 * Doomguards don't even require the ritual of summoning or sacrifice of health anymore. Just hit a button, and BOOM he's there fighting on your side for 45 seconds.
 * Lore example: In the Ashbringer comic, Darion Mograine stabs himself with the titular sword to free his father Alexandros's soul from the Scourge's hold, becoming a Death Knight himself. "I love you, Dad..."
 * In Persona 3, to revive  under this principle.
 * From the same game, the skill Recarmdra fully heals all your allies' HP in exchange for yours becoming 1. It costs only 1% of your total SP, but it is still terribly risky given how it's game over if the leader is KO'd.
 * The various Dark Powersets from City of Heroes tend to work like this, Dark Miasma in particular. Dark Miasma has some highly potent healing spells (Including the only rezz in the game capable of reviving multiple people.) but in order to use them you have to tap into the life force of your enemies. This is just icing on the cake, really...
 * The only real downside of these powers was that you had to have an enemy to engage to use them. So you couldn't heal or rezz BETWEEN fights.
 * In Guild Wars, many Necromancer spells, mostly under the Blood Magic attribute, require the Necromancer to sacrifice a certain percentage of their maximum health, in addition to a small amount of MP, to cast. In addition, Necromancers also have many spells that require there to be a fresh corpse somewhere nearby for them to "exploit". These spells include raising undead servants, creating a "well" of energy that performs various effects, healing, or simply making the corpse explode.
 * Team Fortress 2 has the Crit-a-Cola for Scout. For six seconds, he can do mini-crit damage in exchange for having all damage he takes be mini-crits as well. Basically, it makes him more of a Glass Cannon than usual. Bonk! Atomic Punch could also fall under this trope as he cannot take or deal damage under the effects, but he can still do one attack: The Sandman's taunt.
 * Similarly, the Scout's Boston Basher weapon allows him to inflict the bleed status ailment on each successful hit, but each swing also inflicts bleed on himself to keep the weapon from being all upside.
 * This goes meta with each additional weapon released. Next to none of the weapons released since the game's launch do something significantly different than their original counterparts. Rather, they tend to have one incredible upside over the original (faster rate of fire, health gained from damage dealt, bigger clip size, etc) for some sort of equally incredible downside (slower movement speed, lower default health, less damage dealt, no random critical hits, etc.) Thus, most of the items aren't upgrades as much as they are sidegrades.
 * Dragon Age mages can inadvertently perform minor displays of magic effortlessly (Wynne discovered she was a mage when she accidentally set a bully's hair on fire when she was only nine years old). This is usually how young potential mages are discovered by the Templars. More powerful spells and rituals require outside sources of energy. Lyrium, the Green Rocks of the setting that are the Dragon Age setting's source of magic, is usually distilled into a consumable liquid form for this purpose. Blood Mages can use Life Energy (theirs or others' it doesn't matter) instead; this also lets them perform blood manipulating feats that non Blood Magic cannot, such as Mind Control (controlling blood in the brain?) and ripping the blood out of their enemies' pores.
 * Also, another case with Blood Magic is that for a mage to first be able to use it, they have to make a bargain with a Demon to gain the power. Now, what each bargains requires changes from person to person, but most cases end with the Demon possessing the Mage and turning them into an Abomination.
 * Phantasy Star III allows you to visit a "technique distribution" shop to alter the potency of a magic-wielding character's techniques. With the use of a square-shaped grid, at the expense of one, another can be strengthened. In practice, thanks to Useless Useful Spells, you'll usually end up maxing out Gires and utterly bottoming out Rever or Anti, since those had a high probability of failing anyway.
 * In Quest for Glory I, "Every curse has an equal and opposite countercurse."
 * Equivalent Exchange is one of the major rules of magic in the Nasuverse both in the practical (a spell needs to take its energy from somewhere else) and the social sense (a magus will never do someone a favor without expecting something in return).
 * The "Pandora" feature of Street Fighter X Tekken allows you to sacrifice your partner to give your primary character eight seconds of unlimited super meter and amped-up strength. To make this even more of a desperation move, if those eight seconds elapse before you defeat your opponent, you lose.
 * In Final Fantasy Crystal Chronicles: Ring of Fates the main characters Yuri and Chelinka has access to enough magic power for a Cosmic Retcon, if they need to. However, using that power needs something in return. Chelinka pays by losing her soul for a longer period of time, Yuri, pays with part of his life.
 * In Final Fantasy XII: Revenant Wings summoning Yarhi siphons part of your Anima, the part of you that feels, which is why all the Aegyl who regularily use Yarhi to protect them selves for random monsters appear rather emotionless.

Web Original

 * The perfume in Erika's New Perfume seems to function like this, though given she got Fake Memories out of the deal it's debatable if Sarah gave up anything.
 * New York Magician: Michel has to use energy from things like fired bullets and flashbulb capacitors to power his magic. One favorite trick is to fire one bullet, and use the energy from that to do magic to the next one he fires.
 * In the Axis Powers Hetalia fan game Heta Oni, England reveals that he can destroy the Grays all on his own, using his magic. However,
 * Fey of the Whateley Universe is a massively powerful mage who can, among other things, pull magical energy from ley lines. It turns out there's living things at the other end of those ley lines, and over the course of the first year she unknowingly caused several ecological disasters.
 * Equivalent Exchange by JohnSu on deviantART. Remember all those anime cat ears? Well...

Western Animation

 * The fact that all magic has a price is used repeatedly in the DCAU. In one notable case in Justice League Unlimited, Circe's price for releasing a Baleful Polymorph curse she had on Diana is something from Batman he can never regain once lost... his dignity.
 * It was still a Crowning Moment of Awesome, though. Because even when giving up his dignity, Batman's that much of a Badass.
 * Given there didn't seem to be any cost associated with turning Wonder Woman into a pig (or most of the other magic on display) this may have been less about the magic itself having a price, and more about Circe not wanting to undo the spell without getting something oyyut of it.
 * Kim Possible has Monkey Fist, who agreed to walk the path of the Yono in exchange for the Yono's power. It was granted to him...until he lost..
 * Futurama uses this trope when regarding time travel in Bender's Big Score. Their theory states that two of the same people in different timelines can freely interact with each other. However, to correct the issue of having two of the same person, one will be "doomed" to perish in one way or another. The paradox is gone and the time stream is fixed.
 * And then
 * Eric gets a lecture to this effect from Dungeon Master in the "Day of the Dungeonmaster" episode of Dungeons and Dragons.
 * The ending of X Men the Animated Series' take on The Dark Phoenix Saga has Jean Grey dying to stop the Phoenix threat as per source, but since here it was the real Jean posessed by a cosmic force, instead of the force itself taking her form, she really does die. The force itself is okay though, but it realises that it was wrong, and offers to ressurect Jean, requiring someone sacrificing their own Life Energy. Cyclops and Wolverine have a More Expendable Than You moment before Phoenix informs them that the necessary amount of life energy can be obtained from several donors, without anybody dying.

Real Life

 * As seen in the description, the First Law of Thermodynamics makes this trope not just Older Than Dirt, but Older Than Everything But The Universe Itself. The second law of thermodynamics states that you can't really get even equivalent exchange—you'll always "lose" some of the input energy to waste heat.
 * There's actually quite a few of these. Some are played straight, but others are subverted and only hold true under most circumstances.
 * Noether's Theorem shows that there's a one-to-one correspondence between conservation laws and physical symmetries. Conservation of energy is due to time-invariance, so as long as physical constants don't change, it will exist.
 * Technically, increasing entropy is a strict conservation law. If you kept track of all possible outputs for a given set of inputs, the total entropy would remain constant. It's just that it's effectively impossible to keep track of it all, so you have some set of "possible" outputs that includes the real set, and has higher entropy.
 * Most of chemistry is this. It's even been worked out mathematically, much to the annoyance of undergraduate chemistry students & those grading their papers.
 * 6 CO2 + 6 H2O + Energy -> C6-H12-O6 + 6 O2
 * For the less chemistry-inclined: 6 Carbon Dioxide + 6 Water + Energy -> 1 Sugar + 6 Oxygen.
 * And the Rubisco, which sucks at its job as a catalyst of that overall reaction, but is the only one that works.
 * C6-H12-O6 + 6O2 -> 6 CO2 + 6H20 + Heat. Explosion in a custard factory.
 * Also on a physical level, athletes, especially Olympic-level. They can do what seem like superhuman feats of strength, agility, endurance, etc., but the cost is devoting massive amounts of time training for a specific event, and burn out later in life.
 * More generally, while any human is capable of performing similarly superhuman feats, untrained Muggles can only do so under great stress and at the cost of shutting down certain vital functions temporarily, as well as directly damaging their bodies.
 * In a meta example partially related to the above, to achieve anything in life it is necessary for certain things to be used to reach this point. To breathe you must expend the energy needed to inhale oxygen into your lungs, to become successful at something (without relying on luck->chaos->entropy) you must devote a large amount of time to study your chosen field et cetera.