Bronze-level articleLogical fallacy

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A logical fallacy is an error in logic that prevents an argument from being true. Fallacious logic, however, does not prevent people from being swayed by said argument.


[edit] Explanation

Logical fallacies come from human intuition. A logical fallacy is not necessarily a Bayesian fallacy, so given a particular circumstance, jumping to the conclusion will be more likely than not, and get baked into human thinking as a heuristic. When the heuristic is applied outside its reasonable bounds, it becomes a cognitive bias.

The problem is that this can lead to you being grievously wrong about reality. So you may form an opinion by a heuristic (System 1 thinking), but you need to show your working to make sure you haven't just said something silly (System 2 thinking).

This particularly applies to thinking about science, because scientific thinking is unintuitive for most people unless trained into it; and to arguing your points in general, because heuristics are full of glaring exceptions.

[edit] Types

[edit] Formal

A formal fallacy is a syllogism in which, regardless of the truth of the terms, the conclusion does not follow (ie, the syllogism is invalid).

There are many subfallacies:

[edit] Informal

An informal fallacy in which the conclusion would not follow if the terms were true (ie, some premise is incorrect).

There are many subfallacies.

[edit] Ad hoc

An ad hoc fallacy occurs when some idea is asserted purely to shore up some other idea.

[edit] Ad hominem

An ad hominem fallacy occurs when an arguer is attacked, rather than their idea.

  • Ad iram: When an arguer's supposed anger is used to disprove their ideas.
  • Appeal to bias: When an arguer's supposed mental bias is used to disprove their ideas.
  • Appeal to motive: When an arguer's supposed external motivations (eg, bribes) are used to disprove their ideas.
    • Shill gambit: Asserting an arguer is working for someone and spreading disinformation.
  • Argumentum ad cellarium: Accusing the arguer of still being in "mom's basement".
  • Association fallacy: When someone's associations are used as evidence against their ideas.
  • Blaming the victim: When a victim's actions are used as proof that some offense against them was justified.
  • Demonization: When one's opponent is depicted as unequivocally terrible.
  • Fallacy of opposition: When someone's opposition to your opinion is taken as proof of their incorrectness.
    • Bulverism: Arguing about how someone got such bad ideas, rather than that the ideas are bad.
    • Jonanism: Considering all people who disagree with you as the same person (or following the same ideas).
  • Genetic fallacy: When the source of an argument is attacked, rather than its merits.
  • Nutpicking: When a few extremists from a group are taken as representative of the group.
  • Tu quoque: Where a criticism is falsely dismissed because its author is also guilty of the charge.
  • Scapegoat: Using someone to take the blame.

[edit] Conditional

Conditional fallacies may or may not be fallacious, conditional on whether or not one of their premises is accepted. An informal conditional fallacy follows the form:

P1: X is Y.
P2: Being Y causes something to be true / to be false / etc.
C1: X is true / false / etc.

This is not always fallacious. For example, a good argument from authority:

P1: Global warming is agreed to exist by over 95% of climatologists.
P2: Something that is agreed to exist by over 95% of climatologists is probably true.
C1: Global warming is probably true.

The problem only occurs when Y is something stupid. For example, a bad argument from authority:

P1: Creationism is agreed to be true by about 250 assorted scientists.
P2: Something that is agreed to by about 250 assorted scientists is probably true.
C1: Creationism is probably true.

Conditional fallacies are almost always informal fallacies. However, if comparing multiple systems of logic (which would imply different valid and invalid logical structures, and thus different formal fallacies) then it may be possible for a formal fallacy to be conditional.

[edit] Appeals to authority

An appeal to authority incorrectly asserts that some authority's assertion proves a point. It has several subfallacies:

  • Anonymous authority: When a source is quoted (or supposedly quoted), but no name is given.
  • Appeal to celebrity: When a source is supposedly authoritative because of the respect people give them.
  • Appeal to confidence: When a source is supposedly authoritative because of their confidence.
  • Argumentum ad populum: When a source is supposedly because of their popularity.
    • Silent Majority: When a source is supposedly authoritative because of the popularity of their views, yet there is no evidence of the popularity of their views.
  • Invincible authority: When a source is the entirety of an argument.
  • Ipse dixit: When a source is the person making the argument.
  • Linking to authority: When a source is "cited" in-text yet the reference doesn't exist / is irrelevant / says something else.
  • Ultracrepidarianism: When a source is quoted outside their expertise, as if expertise in one field extended to another.
  • Quote-mined authority: When an authority is selectively quoted to distort their views.

[edit] Argumentative

Fallacious argument styles occur when one speaker uses certain tactics to attempt to prevent honest and useful discourse.

Fallacy misidentifications occur when one speaker doesn't know what they're talking about, while also attempting to claim something's a logical fallacy.

For example, a Gish Gallop involves spewing so many arguments out that the opponent cannot respond to them. Because the opponent failed to respond to every point, the Galloper can argue that they have won the debate. However, this is fallacious; even though not every point was responded to, this does not mean that the points which went unrebutted were true; instead, the tactic merely showed that the respondent needed more time.

[edit] Must be used in argument

One common error when first learning about logical fallacies is to fail to realise that a fallacy can only be present if it is used as part of an argument: for example, "you are an idiot" is not an ad hominem fallacy because it is simply a blank statement: "you are an idiot, therefore you are wrong" is an ad hominem because a conclusion is being drawn. Further, "you are saying idiotic things, therefore you are an idiot" is a valid argument regardless of whether the premise (the opponent is saying idiotic things) is true. However, it is only a sound argument in the event the premise is true.

[edit] Validity versus truth

Just because an argument is valid does not mean the conclusion is true. A valid argument simply means that if the premises are true, the conclusion must be true as well. A sound argument is a valid argument with the additional requirement that the premises (and thus the conclusion) are true. For instance, consider the following argument.

P1: All humans are cows.
P2: All cows are plants.
C1: All humans are plants.

Although the conclusion is false and the premises are false, this is still a valid argument because if the premises were true, the conclusion must be true as well. Since at least one premise is false, the argument is valid but not sound.

[edit] Common examples

  • Argument from ignorance — Basing the truth of a premise only on whether it has been proved to your satisfaction.
    • Argument from incredulity — Literally "that's unbelievable = that's obviously not real". This kind of thinking would quickly put an end to virtually all quantum physics.
    • One single proof — Dismissing all circumstantial evidence in favor of a single "smoking gun" that may not (and may not need to) exist.
    • Argument by assertion — The belief that if you say something enough times, it eventually becomes true and therefore you win the argument.
    • Bullshit — Placing your argument above the truth.
    • Equivocation — Deliberately substituting the meaning of a given word in one context for another context that is inappropriate in order to make your argument.
    • Extended analogy - Arguing an opponent's analogy that A is like B in one particular way is a claim that A and B are directly comparable to one another.
    • False analogy — Use of a flawed or self-refuting analogy.
    • Generalisation from fictional evidence
    • Great Beethoven fallacy
    • Red herring — A group of fallacies which bring up a fact which is irrelevant to the issue, in an attempt to distract the opponent and/or audience.
    • Straw man — Distorting an opponent's position for greater rhetorical flexibility.
    • Poisoning the well — Attempting to refute an argument based on the perceived veracity of the presenter.
    • Shill gambit - Attacking the opponent directly by accusing him of being bought, typically by a large corporation.
  • Balance fallacy — Giving equal weighting to both sides of an argument, even if one really doesn't deserve the time.
  • Composition and division — A pair of fallacies confusing what is true of a part with what is true of the whole.
    • Overgeneralisation — Taking a few specifics and making a general rule out of them, without the few specifics adequately representing the entire group.
    • Spotlight fallacy — Assuming aspects of a group from aspects from a smaller observed part of the group.
  • False cause — Presuming that a real, or perceived, relationship between things, means that one is the cause of the other.
  • False dilemma — Portraying two options as the only possibilities, with no middle ground (see Pascal's wager for an example).
  • Appeal to moderation — Presuming that the answer to a problem always lies between two extremes.
  • Fallacy of relative privation — A moral equivalence fallacy that says because B is worse than A, A is justified. Also known as the 'not as bad as' fallacy.
  • Intuition pump — Deliberately abusing a thought experiment to prove a fallacious point.
  • Loaded question — Asking a question, which has an assumption built into it, so that it can't be answered without appearing to agree to the assumption, or else appearing evasive by questioning the assumption. To be distinguished from a Leading question, which is not a fallacy, but is a way of suggesting the desired answer by how the question is phrased. "Are you still beating your wife?" is a loaded question, for it assumes that at one time you did beat your wife; while "You weren't beating your wife, were you?" is a leading question, for it suggests the simple answer no.
  • My enemy's enemy — Supporting someone because you've a mutual enemy with them.
  • Nirvana fallacy — Claiming that a realistic solution is useless because it is not as good as an idealized perfect solution.
  • Presupposition — Making an implicit assumption as part of an argument.
  • Reification — Treating an abstraction as a real thing, e.g. treating the test score called "intelligence" as an innate part of a person.
  • Reductio ad absurdum — Following a chain of thought to its absurd or contradictory conclusion. Sometimes worthwhile, but oftentimes fallacious.
    • Argument from adverse consequences — Arguing against a point based on expected negative outcome.
      • Argumentum ad baculum — A subset of arguing from adverse consequences, the negative outcomes are being actualised by the one making the argument. Also a form of the style over substance fallacy (see below).
    • Slippery slope — If event A happens, it will lead downhill to further undesirable results. For example, "if we allow gays to get married, then we'll have to let men marry little kids".
    • Continuum fallacy Asserting that a continuum of possibilities between two distinct states renders the states identical.
  • Slothful induction — Ignoring the strongest conclusion of an inductive argument to focus on a weaker one.
  • Special pleading — Demanding that your point of view is subject to different standards than all other points of view without any justification beyond "it's the only real support I have for my argument".
  • Suppressed correlative — Attempting to redefine two mutually exclusive options so that one encompasses the other.

[edit] See also

Icon fun.svg For those of you in the mood, RationalWiki has a fun article about Justification generator.

[edit] External links

[edit] Fallacy collections

There are lots of fallacy collections in the Web. Some of them promote a particular agenda, but most fallacies listed in them are real and present in arguments everyday. Unfortunately, many are deprecated.

Here is a list of websites, ordered by usefulness:

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