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"I live by syllogisms: God is love. Love is blind. Stevie Wonder is blind. Therefore, Stevie Wonder is God. I don't know what I'd believe in if it wasn't for that."
"Logic, my dear Zoe, only allows one to be wrong with authority."
"That's enough logical fallacies to convince me. Black Mage is fine."
Logic. Every story needs it, unless you just want a series of unconnected images and no plot to speak of.
But the problem is, logic requires writers to think pretty hard about what they write, and not all writers have time for this. As such, they take shortcuts which at best can lead to Plot Holes, and at worst undermines the entire story. Most Straw Vulcans suffer from this, following the author's flawed view of what is logical.
Other times, writers deliberately invoke this, to make their characters more human, or to explain why they didn't take the best choice.
For examples of writers intentionally or unintentionally failing logic forever, see Insane Troll Logic.
Not to be confused with Logic Bomb.
Types of fallacies and examples
- False dilemma
 - Portraying a problem as having only two solutions. For example, telling someone "either you stop eating ice cream or you will get fat" - this ignores that there are other ways of losing weight, such as exercise, or giving up foods other than ice cream. Commonly invoked to set up a Friend Or Idol Decision or The Sadistic Choice. Subverted whenever a character Takes A Third Option.
- A common version of this is to assume that only the extreme ends of a scale are possible, without considering intermediate positions. For example "You're either with us or against us" or "If you're not part of the solution, you're part of the problem". See False Dichotomy.
- Argument to moderation - In some sense the opposite of a false dichotomy, assuming that the extreme ends of the spectrum are necessarily wrong and the best option must lie somewhere in the middle:
Bob wants to kill all puppies in the world.
Alice doesn't want to kill any puppies.
Therefore, we should compromise and kill 50% of all puppies.
- Best known in the Solomon solution: Two women claim a baby as theirs. Solomon judges that they each get half the baby, which would necessitate the death of said baby. The real mother retracts her claim, preferring the baby going to the other woman to keep it from being killed...which was Solomon's plan all along to identify the real mother.
- See Golden Mean Fallacy.
- Begging the question - "Proving" that something is true by assuming it.
The Bible says God exists.
The Bible is the product of divine inspiration.
Therefore God exists.
- This phrase is commonly misused as "begging the question" despite purists pointing out that this is not the original meaning; "Beg" in this instance is really a shortened version of "Beget."
- Because the phrase 'begging the question' can be used outside an accusation of fallacy, this is often referred to by its Latin name, petitio principii in more formal settings.
- Sharpshooter fallacy - A way of fiddling statistics where you change the conditions until you find what you expect. The prototypical example is of a person shooting a gun at a wall, painting a target around the bullethole, and claiming to have scored a bullseye.
- Anyone who claims to have found codes in the Bible or Shakespeare has more likely than not committed this one, since finding the codes involves trying every combination of letters until a message that suits your particular type of eschatology/authorship appears.
- Likewise with all those claims of how various people's names are secret encodings of the Number of the Beast, 666.
- The opening credits of The Number 23 suffered from this.
- Quite deliberately, as "everything is related to the number 23" is the method of insanity of the main character.
- Gambler's fallacy - believing that dice/coins have memory: if a coin has just landed on heads four times in a row, surely it's much more likely to get tails this time, to even things out... or alternatively, heads is on a roll and will appear next time too. See also Random Number God and You Fail Statistics Forever.
- A Darths & Droids strip covered this one, with one player having a carefully prepared 20-sided die that had previously rolled two ones - the chances of rolling 3 ones in a row is only 1 in 8000, so surely another one is almost impossible, right?
- Of course the chance of it rolling a 1 is still 1 out of 20 possible combinations possible.
- Which demonstrates why this fallacy is so common; isn't three-in-a-row wildly unlikely? Well, sure, but two in a row is only a bit more likely, and it's already happened.
- For that matter, the chance of it happening once more is greater than the chance of it having happened twice in a row already.
- . . .I'm pretty sure the probability of anything that's already happened having happened is 1. Assuming no time travel, of course.
- Discussed at length in Rosencrantz & Guildenstern Are Dead, in which Rosencrantz flips a coin and lands on heads over and over, and Guildenstern observes that, since heads is just as likely as tails, that it shouldn't come as a surprise. "Seventy-nine..."
- Any Warhammer gamer or tabletop roleplayer will tell you that this is absolutely true.
- Psychologically this fallacy tends to come from the fact that the odds to replicate a pattern do go up cumulatively. The odds of rolling 20 on a d20 twice is 1:400. The odds of rolling the first is 1:20, and the odds of rolling the second is also 1:20. The fallacy occurs when someone attempts apply the full cumulative odds to the next roll. Similarly the odds of rolling a 5 and a 17 on a D20 in that order are also 1:400, so that number's pretty irrelevant.
- The reverse of this can sometimes be true - if you get a lot more heads than tails, it's possible that the coin is biased.
- Correlation implies causation - claiming that because two things are correlated, one must cause the other.
- Pastafarianism claims that pirates prevent global warming, as the number of pirates is decreasing while temperatures increase.
- This xkcd strip using the rise of both Firefox and Wicca to imply that Firefox causes witchcraft.
- One episode of Justice League features a journalist claiming that since white*collar crime has risen since the League formed, the League clearly causes that crime. (In fact, given the League's style, it's entirely possible that the smarter criminals turn to white-collar rather than blue-collar crime to reduce the chances of Superman slapping them around Metropolis, but his logic still doesn't track.)
- The Simpsons has an episode where an isolated bear attack leads the Mayor to fund a massive Bear Patrol scheme. Homer claims the lack of bears proves the Bear Patrol works, at which point Lisa points out that you might as well say that a rock keeps tigers away, since she's holding the rock, and she can't see any tigers. Homer's response? "Lisa, I want to buy your rock."
- This is actually the plot of a fairly old joke: a man is spreading some substance over his garden, and his neighbour asks him what it is. "It's elephant repellent," says the man. "But there aren't any elephants around here," replies the neighbour. "That proves it works then!"
- This troper prefers "The Simpsons" version, as the eloquence of Lisa's criticism of Homer's logic makes for more intellectually compelling, thought-provoking viewing.
- This has also been used, for example, to argue that listening to country music causes higher suicide rates, since places with a higher percentage of country music listening tend to also have a higher percentage of suicide.
- Four terms fallacy: Using a standard 3-step proof-of-concept to prove your theory, but including one unconnected tenet which breaks the line of reasoning. Results from equivocation. It's better explained by example:
Nothing is better than eternal happiness.
A cheeseburger is better than nothing.
Therefore a cheeseburger is better than eternal happiness.
- This uses two different meanings of the word "nothing": the first line uses nothing to mean "there exists no such thing", while the second line uses nothing as in "an absence of food".
- An episode of Yes Minister called this fallacy "The Politician's Syllogism", specifically the form: "Something must be done. This is something. Therefore we must do this." The two different meanings of something: "A solution to this problem" and "A thing" are mixed and said to be the same.
- Reification (no, not that kind): Sometimes related to the four terms fallacy, this is treating a abstract idea as a physical object. For example: "eating ice cream makes you happy. Therefore, eating more ice cream makes you more happy". This assumes that happiness is an idea that can be measured and which is a physical property of ice cream - in fact happiness is an unmeasurable emotional response to ice cream, and would probably tail off at around the point that you started to get really sick and/or fat.
- Ad hominem fallacy: Discarding an argument based on the character of the person presenting it, rather than the content of the argument itself. For example:
Bob thinks we shouldn't buy a pool, because it would be too expensive to maintain.
Why should we listen to someone who kicks puppies?
Therefore, we should buy a pool.
- This even applies when the ad hominem attack itself is related to the argument. Even if the supposition comes from a source that is known for fallibility, that does not logically make it untrue.
Bob thinks we shouldn't buy a pool, because it would be too expensive to maintain.
Bob once went bankrupt.
Why should we listen to economic advice from someone who went bankrupt?
Therefore, we should buy a pool.
- In fact, in some cases that fallibility may be a good reason to take advice from that person. I might well want to listen to economic advice from someone who went bankrupt. Maybe he went bankrupt because of the expense of maintaining a pool. Similarly, an admonition not to smoke would have more resonance coming from a smoker than from someone who never tried it themselves. But that's just this troper.
- Reductio ad Hitlerum: Hitler did it, therefore it's bad. Sometimes extended to other evil or unpopular people. While persuasive, it's not always true, since while Hitler did a lot of evil things, he also built motorways, painted pictures, hosted the Olympics, ate sugar and breathed oxygen. Related to Godwins Law and Hitler Ate Sugar.
- Classic example: "Y'know, Hitler was a vegetarian." And therefore vegetarians are as morally suspicious as a dude with a German accent in an Indiana Jones film.
- An anti-abortion Chick Tract claims abortion is wrong because Hitler killed Jewish babies, and therefore doctors who carry out abortions are as bad as Hitler.
- In general, saying that something is bad because a bad person did/believed/approved of it is the association fallacy.
- The association fallacy, especially in its reductio ad Hitlerum form, is closely related to the ad hominem fallacy. In both cases, the goal is to link an opinion to a discredited person. The difference is that in a normal ad hominem you have to work to discredit the person you're linking the idea with, even if it's just by adding a claim that itself isn't true. But since everybody but Those Wacky Nazis hates Hitler, he comes pre-discredited.
- No true Scotsman - Redefining the terms in your argument to change the end result. The classic example works by redefining the word "Scotsman" from "person from Scotland" to "person from Scotland who doesn't eat sugar with their porridge":
Alice: No Scotsman puts sugar on their porridge.
Bob: That's not true - my Scottish uncle Scotty McScotscot puts sugar on his porridge all the time!
Alice: But no true Scotsman puts sugar on their porridge!
- The game Metal Wolf Chaos features propaganda news reports that define a true American as "anyone who supports the idea of having the families and friends of terrorist sympathisers murdered in the streets" rather than "anyone who is a citizen or long-standing resident of America". Unfortunately, only slightly less extreme examples are common in real life.
- See also instances of Fan Dumb redefining "a true fan" from "someone who likes this work" to "someone who agrees with me."
- Many questions fallacy - phrasing your question in such a way that some answers are impossible. The classic example is "have you stopped beating your wife?" - whether you answer yes or no, you automatically admit to having beaten your wife at some point.
- One episode of The Simpsons has the family being accused of being unpatriotic, but when given the chance to clarify, are asked loaded questions such as "which part of America do you hate the most".
- This is an easy way to brand someone a traitor in Paranoia- simply ask "Are you a happy communist?" If they say no, they're saying they're not happy, which is treason. But if they say yes, then they're saying they're a communist, which is also treason.
- The traditional correct answer to this type of question is Mu.
- Just asking these questions is an example of a False Dichotomy in that you assume the answer can only be 'yes' or 'no', whereas a normal person would say "I've never beaten my wife, get away from my family."
- It's a common cross-examination trick to do this, because the attorney conditions the witness to respond "Yes" or "No" to everything and then *Bam!* you trap them with this and force them to give a detailed answer. The delay as the person scrambles to shift back and think of the correct answer is what makes them look "shifty." It's the reason for the objection "Assumes a fact not in evidence."
- Or this troper's favorite, "I've never been married."
- Or just: "I'm gay."
- "And your boyfriend's name is—" "Chuck."
- Strawman: Beloved of Author Tracts the world over, this fallacy involves misrepresenting your opponent's views, or only attacking their weakest arguments, and then claiming that because you beat this "straw man", you beat their entire argument. See The War On Straw.
- Affirming the consequent - Claiming that because the result of something is true, the original statement must also be true. For example:
If a car is a Ferrari, it is fast.
My car is fast.
Therefore it is a Ferrari.
- Denying the antecedent - the opposite of the above, where you say that because the initial conditions did not happen, the result is impossible. Example:
If a person is wearing a hat, they have a head.
I am not wearing a hat.
Therefore I do not have a head.
- Sunk cost fallacy - When somebody's sacrificed/invested a great deal in a cause or project, they tend to become irrationally dedicated to it. This applies even when the costs invested can't be recovered. More of a cognitive bias than anything. Example:
If I spend seven dollars more on this contest, I can win the prize.
I can buy the prize elsewhere for five dollars.
I have already spent X dollars on the contest. Since I don't want the money to be wasted, I will continue.
- A particularly tragic example of this - with much higher stakes - can be found in The Order Of The Stick: Start of Darkness.
- Tends to afflict professional sports teams as well. Team A signs Johnny Stud Muffin to a 10-year, $200 million guaranteed contract. Turns out, Johnny's a bit of a malingerer and spends the first three years playing poorly for Team A. Many, many teams will continue to play Johnny, even after it's clear that he's a bad player, simply because they invested assloads of money in him. The reality is, Johnny's getting that $200 million no matter what, so the best move is to cut him and play the best players.
- In psychology this is generally related to cognitive dissonance theory. Which is basically the idea that when people think of themselves one way, but act in another way they will try to rationalize it. Specifically people think they make good decisions, but when the decisions aren't paying off, they throw more effort after it to make the decisions good rather than admit they made a bad decision.
- In business, this is known as throwing good money after bad.
- In poker, the term is "pot committed". The best poker players can overcome the fact that they've put half of their money into the pot to get away from a hand that has little chance of winning (rather than throwing the other half in and busting out).
- Retrospective determinism - Assuming that because something happened it was therefore bound to happen. Often brought up as justification for Hitlers Time Travel Exemption Act by saying that World War II was inevitable.
- Error of composition - generalising from part of a group to the whole group. Example:
The southern states have more Christian fundamentalists
The southern states have higher divorce rates
Therefore the southern states are full of hypocritical divorced Christian fundamentalists
- The problem being that the first two statements can be true without any overlap between Christian fundamentalists and divorced people.
This can get you into the positions the sunk-costs fallacy keeps you in.
- The cab driver's fallacy: Similar to the sunk-costs fallacy; having a particular goal in mind and refusing to give up on it, however impossible it seems, while also ignoring the possibility of doing even better. Named from the tendency of taxi drivers to have a specific amount they'll try and earn each day, when they'd do much better to go home early on slow days and keep going past their target if it's busy.
- I thought it was named after my tendency not to simply leap clean out of taxis after being taken on half-hour roundtrips by degenerates who don't know which side of the river they're on. Always have an eye on the meter, kids!
- Perfect solution fallacy: Arguing that a plan is no good because it won't completely solve the problem, even if it does make an improvement. For example, "Why bother with seatbelts? Some people will still die in car crashes."
- This is popular when answering a technical question on the internet: "There is no solution to your problem which I can guarantee to work in 100% of all cases. So I'm not going to bother telling you what will work in 99% of all cases."
- This is extremely popular with anti-vaccinationists. Their reasoning: a particular measles vaccine only protects 95% of the time, therefore it's no better than rat poison, so they'd rather take their chances with a potentially fatal disease. Oh, how I wish I were making this up.
- This has even happened with the U.S. Army. In a series of tests to find a replacement for the M-16 assault rifle, all of the competitors were disqualified for not producing a guaranteed first-round hit.
A special subset of fallacies are appeals: claims that a position is right made by making an emotional argument without referring to your reasoning.
- If you break this one down and examine it, it's actually two fallacies wrapped up in one. Bob starts with a single example — Chuck Smithenson did not drill a hole in his head and did not become famous. First he commits an error of composition, extrapolating that into the general rule that anyone who does not drill a hole in their head will not become famous. Then he Denies the Antecedent - Anyone who does not drill a hole in their head will not become famous. Alice will drill a hole in her head, therefore Alice will become famous. Bad logic all around there, Bob.
- However, this type of appeal may contain enough subtext by implication to make a complete (but not completely stated) argument, as per "you would have heard of them had they not made that decision".
Alice: The Enormous Evil Empire wouldn't destroy an entire city.
Bob: Yeah, that's what the people of Doomdia thought.
Alice: I don't think I've ever seen Doomdia on a map.
Bob: No. No you haven't. [Implied but not stated: because the EEE nuked them].
Of course, before you get too carried away with these fallacies, there's one more you should remember:
- The fallacy fallacy: Claiming that a position must be false because the argument used to get to that position used a fallacy. It may sound logical, but just see an example:
To read TV Tropes, you must be on the Internet.
You are on the Internet, therefore you must be reading TV Tropes.
But wait, I just used the "Affirming the consequent" fallacy!
Therefore my argument was wrong, and therefore you're not reading TV Tropes at all!
- Lore has a nice video on the subject.
- Common in internet debates when one person knows logical fallacies but no one else does. This troper cannot count the times he's seen a Strawman argument to for a claim of False Dichotomy when either alternatives were address originially or there otherwise really are only two options. Also claiming Appeal to Authority when someone uses a source for facts. Because when someone cites a source to establishes facts then argues from those facts there argument is not 'because X says so.'
- And don't forget the opposite of that, the "garbage in, garbage out" principle. If one of your premises is wrong, than the argument can be logically valid and still have a false conclusion.
The moon is made of cheese.
Cheese is edible.
Therefore, the moon is edible.
"What else floats in water?"
"A duck."
"If she weighs the same as a duck...then she's made of wood..."
"And therefore...?"
- You can also guess the conclusion, but use a faulty premise, and then if the conclusion is right, pretend your guess was perfectly logical. The argument is still faulty.
"The sky is blue because of fairies painting it."
The sky is blue, but because of the oxygen reacting to sunlight.
- The above is technically true but misleading. The sunlight scatters due to all components of the atmosphere. The exact hue of the sky would change slightly if the atmosphere's composition changes. When a large volcanic eruption changes the way sunsets look, that is the same process.
- A Real Life variant, common in Message Boards:
That systems wins, but because of Y and Z.
"I knew it would win!"
- And, to muddy the waters even more, your premises can be wrong and/or logic, but you can still have a true conclusion.
A human is a reptile.
All (known) reptiles live on Earth.
Therefore, all humans live on Earth.
The moon is made out of green cheese.
Green cheese is delicious.
Hitler was the political leader of Germany from 1933 to 1945.
The more massive something is, the stronger its gravity.
Your Head A Splode now.
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