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Logic. Every story needs some of it, unless you just want a [[MindScrew series of unconnected images]] and [[GagSeries no plot to speak of]].

The problem is that logic requires writers to think pretty hard about what they write, and not all writers have time or inclination to do so. So they take shortcuts, creating fallacies which at best can lead to {{plot hole}}s or, at worst, undermine the entire story.

Fallacies are common errors in logic. By strict standards, fallacies don't address the truth of the premises or syllogism; they only address the ''validity of the logic'', and as the SoundValidTrue rule demonstrates, "truth" and "validity" are not the same thing when speaking of formal logic. There is a reason there are Critical Thinking classes.

Where deductive logic is valid, the conclusion must be true if the premises are true. "If it rains, then the sidewalk will be wet" is valid, so if you know that it rained, you know that the sidewalk will be wet. If you simply reverse the terms and say "if the sidewalk is wet, then it rained" this would not be valid (likewise, negating the terms, yielding "if it did not rain, then the sidewalk is not wet", is also invalid. To correct this, you need to construct a "contra-positive," where you reverse the terms as well as negating them to get "if the sidewalk is not wet, then it did not rain").

However, inductive logic involves reasonable inferences of what might be true, but not necessarily. A sidewalk could be wet due to a passing street sweeping vehicle or neighbours carelessly watering their lawns. Seeing a wet sidewalk and concluding that there was rain is fallacious--not deductively valid--but it is not necessarily false, nor is it necessarily an unreasonable inference to make.

Logical fallacies are faulty deductive reasoning. This doesn't mean that they aren't effective at persuading. Many of them are extremely effective tools of persuasion. The key is that there are two primary routes of persuasion: the central (logical) route and the peripheral (emotional) route. To persuade someone using the central route, you ''need'' logic; a logical fallacy will make your argument fall flat on its face. To persuade someone using the peripheral route, you don't need logic; you simply need to play on their emotions. Some people are impassive to emotional appeals, and so you must use logic to persuade them; others are confused by logic, and so must be persuaded through emotion.

However, one must keep in mind that--depending on the surrounding circumstances--a deductively fallacious argument may still, none the less, [[RightForTheWrongReasons be a reasonable and (inductively) logical argument that has decent prospects of being true despite the deductive logic being invalid]]. A classic example is if someone were to examine a million swans and note that all of them were white. It would be a (deductively) logical fallacy to conclude that "all swans are white". You could not make that conclusion unless you know that you had examined all swans in the universe. That doesn't make it illogical, however. If no one had ever seen a black swan, it might be rather sensible. Plus, this whole type of analysis is complicated when you talk about statistical trends. For these kinds of special cases, see FallacyFallacy.

For examples of characters falling into these fallacies (intentionally on the writer's part), see InsaneTrollLogic and ChewbaccaDefense. Not to be confused with LogicBomb.
* AdHoc: Mistaking an argument for an explanation.
* AdHominem: Attacking the arguer or the argument's presentation instead of the actual argument.
* AnecdotalFallacy: Using a personal example as empirical evidence.
* AppealToAuthority: Assuming something is true because an authority said it to be so OR calling someone an expert (and therefore correct) when they are not an actual expert.
* AppealToConsequences: Assuming something is correct/incorrect because of the positive/negative effects that will arise if it is implemented.
** AppealToForce: Threatening anyone who disagrees with you, and therefore claiming what you say is true. A species of the Appeal to Consequences.
** AppealToFear: Saying bad things will happen to anyone who disagrees with you, and therefore what you say is true. A species of the Appeal to Consequences.
* AppealToFlattery: Claiming that a certain conclusion reflects well on anyone who agrees with it, or poorly on anyone who does not.
* AppealToIgnorance: Claiming that something is true because it apparently can't be proven false (or vice-versa).
* AppealToInherentNature: Claiming something otherwise unacceptable is acceptable because it is within the nature of the doer to do it.
* AppealToMorality: Claming anything that is morally desirable is true/natural, and anything that is immoral is false/unnatural.
* AppealToNature: Claiming anything that appears naturally is good, and anything that appears unnaturally is bad.
* AppealToNovelty: Claiming something is superior to something else because the first is newer.
* AppealToObscurity: Attributing an argument to someone the other party doesn't know and using the fact that they aren't known as evidence.
* AppealToPity: Claiming an argument is valid because either the arguer or an involved party deserves sympathy.
* AppealToPopularity: Claiming something is true because many or most people believe it.
* AppealToPossibility: Claiming that if something could possibly happen, then it will/must happen.
* AppealToRidicule: Claiming an argument is false by presenting it in an absurd fashion.
* AppealToTradition: Claiming something is superior to something else because the first is older.
* AppealToWealth: Claiming something is good because the rich or famous support it.
* AppealToWorseProblems: Claiming an argument isn't valid because there are bigger problems than it.
* ArgumentOfContradictions: An argument that consists of nothing more than a shouting match -- each side loudly repeating their side in turn.
* ArgumentumAdNauseam: Repeating an argument over and over until no one wants to dispute it anymore, then claiming it to be correct.
* ArgumentumAdLapidem: Dismissing an opposing argument as absurd without any sort of support.
* AssociationFallacy: Claiming "X is a Y. X is also a Z. Therefore, Y is a Z."
** HitlerAteSugar: Claiming something is bad because an evil person (like Hitler) liked it. A species of the Association Fallacy.
* BandwagonFallacy: Accept or adopt something simply because the majorities have already done so, regardless of actual validity or desirability.
* BeggingTheQuestion: Mistaking the argument for the evidence. "People who use X are in danger of Y, because X can Y."
* {{Bulverism}}: Rather than proving a statement wrong, assuming that it is wrong and then explaining why your opponent holds it.
* CabDriversFallacy: Being so devoted to meeting a quota that one tries too hard when there is little reward to be gained or doesn't try hard enough when great rewards are available.
* ChewbaccaDefense: Using non-sequitur arguments to ''prove'' a point, relying on distracting and confusing the opposition.
* CircularReasoning: Any argument in which the conclusion is used as a premise: for instance, "A is true because A is correct."
* ConfirmationBias (also known as cherry-picking): Presenting only data that supports your predetermined position and ignoring data that damages your position.
* ConverseError: Concluding that a certain set of results can only come from one set of circumstances. "If A, then B. B, therefore A."
* CorrelationImpliesCausation: Believing that any two variables that appear to work in tandem actually do work in tandem. "When X occurs, Y goes up. Therefore, Y must always go up when X occurs."
* ExtendedAnalogy: Comparing two issues as direct analogs, regardless of their relation. "You support X, which means you support Y."
* FallacyFallacy: Because someone used a fallacy to argue a point, their premise must be incorrect.
* FallacyOfComposition: The properties of the parts are applied to the whole. "A is made of B. B is X, so A is X."
* FallacyOfDivision: The properties of the whole are applied to the parts. "A is made of B. A is X, so B is X."
* FalseCause: Assuming that because one event came after another, that the first event must have caused the second.
* FalseDichotomy (Either/Or Reasoning): Offering a choice between two extremes, usually one desired and one not, and ignoring the possibility of [[TakeAThirdOption other options.]]
** WithUsOrAgainstUs: Assuming that not openly supporting one side means you oppose them.
* FourTermsFallacy (False Syllogism): [[Creator/StephenColbert "God is love. Love is blind. Stevie Wonder is blind. Therefore, Stevie Wonder is God."]]
* FrozenAbstraction: An argument implicitly assumes that a subset of a wider class is the wider class. "Anarchism cannot be a political ideology because it denies the state."
* GamblersFallacy: Thinking that previous random events will have an effect on future random events. "Odds of winning are 1:20, I've played 19 times, I'm ''due'' for some good luck."
* GeneticFallacy: Dismissing or accepting something entirely on the basis of its origin.
* GoldenMeanFallacy: Thinking that the "middle ground" between two points is the best option, even when it shouldn't apply.
* HardWorkFallacy ("If I can do it, so can you."): The argument that the desired outcome is ''purely'' the result of the effort put in by the individual, regardless of any other factors.
* HistoriansFallacy: When one assumes that a decision-maker had the same information and perspective as those analysing their decision(s) with the benefit of hindsight.
* {{Hypostatization}}: Treating an abstract idea as a physical object. "Eating ice cream feels good. Therefore, we should give ice cream to criminals, so they become good."
* InsaneTrollLogic: A conclusion drawn on irrelevant or nonsensical postulates.
* IrrelevantThesis: Not refuting the opposing position at all, but acting as though you did. "Well, that's just your opinion."
* LetsSeeYouDoBetter: Thinking that because one is not an authority on whatever he/she is criticizing, they're in no position to do so.
* LoadedWords: Using words which appeal to emotions rather than to logic.
* ManyQuestionsFallacy: A question is asked that assumes the answer to one or more additional questions, and a demand is made that it be answered without qualifiers. "Yes or no: have you stopped beating your wife?"
* MovingTheGoalposts: Continually changing the requirements for a reward so that it is never obtained.
* Nirvana / PerfectSolutionFallacy: Comparing actual things with unrealistic, idealized alternatives. "If we can't fix it perfectly, we shouldn't try at all."
* NonSequiturFallacy: Coming to a conclusion which is not supported by the facts or even has no relationship to the facts.
* NoTrueScotsman: Redefining a category to not include something that the speaker doesn't want it to include, even though it does in fact include that thing.
* OriginalPositionFallacy: Claiming something is good on the assumption that one ''will'' gain benefits from it, ignoring the possibility that they may not.
* OvenLogic: Assuming that any one condition can still produce a valid result if a second condition is altered 'proportionately', such as by baking something for half the time at double the temperature.
* {{Presentism}}: [[ValuesDissonance Projecting present-day ideas and perspectives are anachronistically introduced into depictions or interpretations of the past.]]
* ProofByExamples: Generalizing a category to match the properties of given examples. "3, 17, and 97 are prime numbers; all odd numbers are prime."
* ProsecutorsFallacy: Rejecting an explanation on the basis that it relies on exceptional circumstances in favor of an equally exceptional, but personally desired, explanation.
* RetrospectiveDeterminism: Assuming that because something happened it was inevitable.
* SharpshooterFallacy: Claiming that a conclusion is inevitable after the specific results have already been witnessed: "Painting the target around the bullet hole."
* ShiftingTheBurdenOfProof: Arguing that something must be true (or false) because it has not been proven false (or true).
* SlipperySlopeFallacy: Claiming that an action will inevitably lead to another, very unacceptable action. "If X, then eventually Y."
** ReverseSlipperySlopeFallacy: Arguing that since the "Slippery Slope" hasn't happened yet, it's safe to continue down the slope.
** SemanticSlipperySlopeFallacy: Arguing that two things are so similar that they are "practically the same thing."
* SpecialPleading: Demanding an exception be made without justification or for a non-logical reason ("I can park in the handicapped spot because I'm a movie star!")
* SpotlightFallacy: "I've been hearing a lot about event X in the news lately, so event X must happen a lot..." when an event only appears in the news because it's ''unusual.''
* SpuriousSimilarity: It is suggested that some relatively superficial resemblance is proof of a relationship.
* StolenConcept: Making an argument that rests upon (and conveniently ignores) contradictory, intrinsically self-refuting concepts.
* StrawmanFallacy: Deliberately misrepresenting an opponent's argument by constructing it in such a way that it can be easily defeated.
* StyleOverSubstance: When the arguer embellishes the argument with compelling language or rhetoric, and/or visual aesthetics.
* SubjectivistFallacy ("That's just your opinion"): When someone resists the conclusion of an argument not by questioning whether the argumentís premises support its conclusion, but by treating the conclusion as subjective when it is in fact objective.
* SunkCostFallacy ("Throwing good money after bad"): Assuming that because one has already invested time or money into something, it is worth continuing to do that thing even if it produces no gains.
* SurvivorshipBias: Over-playing a small number of successes of a given example, while ignoring a large number of failures.
* TautologicalTemplar: Self-identifying as ''definitively'' good or right, then using it as a supposition for argument. "I'm a good guy so everything I do is good because I say so."
* TwoNegativePremises: Identifying what something isn't doesn't identify what it is. "No dogs are reptiles. No reptiles are magenta. Therefore dogs are magenta."
* UndistributedMiddle: When the middle term of a standard three-step syllogism is not distributed in either premise. "Penguins are black and white. Some old TV shows are black and white. Therefore some penguins are old TV shows."

... The following are relevant argumentative concepts that are ''not'' fallacies:

* OccamsRazor: The simplest explanation is most often correct. (Not to be confused with "always correct.")
* HanlonsRazor: Don't assume malicious intent when stupidity could do.
* SoundValidTrue: "Truth" means factually accurate. "Valid" means logically constructed. "Sound" means both valid and true.

"A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines." (Ralph Waldo Emerson)