A logical fallacy
is, fundamentally, an error in logic
. This means that an argument that uses one certainly doesn't hold if you're using logic and reason as your source of decision making - although it doesn't stop people from using them or being swayed by them. People who want to sound smart tend to use the Latin (or some cases, Dog-Latin) names to refer to them. Others, who don't care about being too diplomatic when bringing them up, will just cry bullshit
and carry on. There are two forms of a logical fallacy, formal and informal.
Formal fallacies are those fallacies that violate a particular rule of propositional calculus, such as modus ponens. These fallacies can be determined to be invalid simply by the inspection of the form or structure of the argument - at heart, a formal fallacy contains some sort of non sequitur. This doesn't necessarily mean that a conclusion is wrong, but it does mean that we will need a better reason or argument to derive the conclusion.
Informal fallacies are arguments that, while not violating propositional calculus rules, are invalid because of the content of their argument. Informal fallacies are often characterised by the fact that there is a disconnect of somekind between their premises and conclusions. Inappropriate generalisations and argument from ignorance are forms or informal fallacies. These tend to be more numerous and slightly more interesting than formal fallacies.
- 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.
- Negative proof -- Arguing that something must exist because there is no evidence it does not exist.
- One single proof -- Dismissing all circumstantial evidence in favor of a single "smoking gun" that may not (and may not need to) exist.
- Argumentum ex culo -- Various forms of making things up or lying.
- Ad hoc -- Using an off-the-cuff explanation with no evidentiary support.
- Argument by assertion -- The belief that if you say something enough times, it eventually becomes true and therefore you win the argument.
- Bullshit -- deceit through obfuscation.
- Equivocation -- Deliberately substituting the meaning of a given word in one context for another context that is inappropriate in order to make your argument.
- False analogy -- Creating an analogy or metaphor, then extending it to prove one's point.
- Generalization from fictional evidence
- Moving the goalposts -- Changing evidential requirements in an argument once they have been met, "what I really meant was..."
- Non sequitur -- Giving an evasive or nonsensical answer to a challenge.
- Red herring -- a group of fallacies which bring up a fact which is irrelevant to the issue.
- Straw man -- Falsifying an opponent's position for greater rhetorical flexibility.
- Argumentum ad hominem -- Attacking the opponent directly rather than addressing the opponent's idea.
- Argumentum ad cellarium -- attacking the opponent directly by accusing him of still being in "mom's basement"
- Poisoning the well -- Attempting to refute an argument based on the perceived veracity of the presenter.
- Tu quoque, where a criticism is falsely dismissed because its author is also guilty of the charge.
- Argumentum ad verecundiam -- The Appeal to Authority; because someone famous/powerful/respected believes it, it must be true.
- Appeal to ancient wisdom -- It's right because the Maya/Chinese/Hebrews said it thousands of years ago!
- Appeal to tradition -- Just because it's always been that way doesn't mean it's the right way.
- Argumentum ad Hitlerum -- Saying something is bad because Hitler did it.
- Argumentum ad populum or bandwagon -- Arguing for a point based on popularity rather than merit.
- The Galileo gambit -- This states that if someone is going against the tide of popular thinking, they must be right because the likes of Galileo were right, while in reality, Galileo was right for the more simple reason that he was right.
- Balance fallacy -- Giving equal weighting to both sides of an argument, even if one really doesn't deserve the time.
- Begging the question -- Assuming the conclusion as part of the premise (similar to circular reasoning).
- Composition and division -- A pair of fallacies confusing what is true of a part with what is true of the whole.
- Association fallacy -- Associating the values of one group with the values of another due to superficial or coincidental similarities.
- No True Scotsman -- Excluding an inconveniently misbehaving member of a class to defend the class as a whole.
- Overgeneralization -- 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.
- Intuition pump -- Deliberately abusing a thought experiment to prove a fallacious point.
- 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 conclusion. Sometimes worthwhile, but usually fallacious.
- Argument from adverse consequences -- Arguing against a point based on expected negative outcome.
- 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".
- Special Pleading -- Demanding that your point of view is subject to different standards than all other points of view.