Logical Fallacies

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Irrelevant Premises An irrelevant premise is one which, regardless of whether it is true or false, does not pertain to the argument.
Fallacy	Description	Examples	Comments
appeal to belief	Implies that an idea is true because most people believe it.	"All Christendom knows the earth is flat, Señor Columbus!"	That many (or even all) people believe an idea does not make that idea true.
appeal to common practice	Asserts that something is correct or acceptable because "everyone" does it.	"Everyone cheats on their taxes, so there's nothing wrong with it."	The fact that a practice (slavery, for example) is widespread does not make it correct or morally acceptable.
appeal to consequences of belief	Asserts that an idea is true, either because belief in it has desirable effects, or because lack of belief in it has undesirable effects.	"Nuclear war can't be possible. If I believed it was, I'd be a nervous wreck."	The truth or falseness of ideas is seldom related to whether the consequences of believing them are desirable or undesirable.
appeal to emotion	Elicits response to an idea by associating it with an emotion (very common in politics, advertising, and personal relationships).	A TV commercial shows a person being bullied, then later smiling and confident after buying a huge SUV. The idea: "Buying this SUV will make you an in-charge person to be respected." "That was a superb lecture, Professor. By the way, can I get full credit for the paper I turned in late?" [appeal to flattery] "Professing the doctrine that the earth moves around the sun is punishable by torture." [appeal to fear / force (argumentum ad baculum)] "I can't support my family without this job, so you simply must hire me." [appeal to pity (argumentum ad misericordiam)] "Susan didn't nominate me for treasurer, so despite her demonstrated ability and commitment to the club, I'm not voting for her for president." [appeal to spite] "Only squares listen to Beethoven, Mr. Toscanini. Swing is King!" [bandwagon]	Emotions and ideas may be positive, such as enhancement of self-esteem, or negative, such as hatred of an ethnic group. But emotions aren't evidence of anything except emotional response itself, and thus do not provide logical support for ideas. Variants include appeals to flattery, fear or force, pity, and spite, as well as the bandwagon fallacy, which implies that acceptance or rejection of an idea is tied to social acceptance or ostracism, and is thus a form of appeal to fear. Emotional appeals and responses generally have no logical bearing upon whether an idea is true or false.
appeal to popular opinion (argumentum ad populum)	Implies that an idea is confirmed by popular acceptance.	"The crackdown on crime is beneficial, since most people approve of the results, even though police have sometimes resorted to brutal methods."	That many people accept something does not make it correct or good. Similar to appeals to belief, common practice, and emotion. (Often equated with bandwagon fallacy.)
appeal to ridicule	Substitutes ridicule or mockery of an idea for evidence against it.	"It's absurd to think that repealing laws against victimless crimes might restore respect for law and order!"	Mockery is not evidence, and ridicule is not reason.
argument against the person (argumentum ad hominem)	Attacks the person making a statement, rather than challenging the statement itself.	"The witness is a prostitute, so her testimony about the theft isn't credible." [poisoning the well] "Of course Jones criticizes our product. She's a stockholder of our competitor." [circumstantial] "Commissioner Smith is a fine one to talk about the importance of education, considering he didn't even finish 8th grade." [tu quoque]	Variants of ad hominem include abusive ad hominem (direct personal attack or "poisoning the well"), circumstantial ad hominem (charge of bias), and tu quoque (charge of hypocrisy).
red herring	Diverts attention from the issue in question.	"I can't be held responsible for the damage my dog caused, because there's no leash law in my neighborhood."	A red herring is often the mark of an inattentive arguer, but it can also be a deliberate attempt to divert attention, either from an opponent's line of reasoning, or from the weaknesses of one's own position.
straw man	Distorts a viewpoint in such a way that the distortion is easily refuted.	"Evolution relies on random change. But it would be impossible for the human eye to assemble itself at random, so evolution can't be true."	Refuting a misrepresentation of a position does not refute the actual position itself.
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Unwarranted Assumptions An unwarranted assumption is either a premise that is poorly based or a conclusion that is inadequately supported.
Fallacy	Description	Examples	Comments
appeal to ignorance (argumentum ad ignorantium)	Proposes that something must be true if not proved false, or false if not proved true.	"The existence of fairies has never been proved, so fairies do not exist." "The existence of God has never been disproved, so God exists."	That something has never been observed or proven might give some grounds for bias against it, but by no means does it lead to a conclusion.
appeal to novelty	Implies that something is better because it is new. (Compare to appeal to tradition.)	"Buy new and improved Slosh detergent for a cleaner wash."	If what is "new" about Slosh turns out to be only that it uses cheaper ingredients to reduce costs, the alleged "improvement" will likely be confined to the profit margin.
appeal to tradition	Implies that something is better because it is old. (Compare to appeal to novelty.)	"You can't possibly expect your new electric light to replace tried-and-true gaslight, Mr. Edison."	The mere fact that a thing or idea has endured for a long time does not mean it cannot be improved upon, or abandoned if it becomes obsolete.
appeal to unreliable authority (argumentum ad verecundiam)	Attributes authority to a source whose expertise in the field in question is doubtful.	"Isaac Newton believed in astrology, so there must be something to it."	We don't cite the Pope as an authority on theoretical physics, just as we don't cite Hawking as an authority on Christian theology, though each is highly capable and respected in his own field.
begging the question (petitio principii)	A.k.a. "circular argument," uses some form of its own conclusion as one of its key premises.	"God's word is always true, the Bible is God's word, and the Bible says God exists. So God exists."	Sometimes the circularity is subtle, or even unstated but implied.
biased sample	Uses a skewed sample of individuals to draw conclusions about an entire group.	"A study taken in Florida and Maine shows 55% of the people spend much of their time near the ocean. So 55% of Americans spend much of their time near the ocean."	Unrepresentative samples yield unreliable conclusions. (Compare to "spotlight.")
complex question	Asks a question with an unjustified presupposition.	"Have you stopped beating your wife yet?"	Although questions are not generally considered logical statements, the presupposition contained in a complex question constitutes a statement.
confusion of cause and effect	Assumes that one thing causes another, when in fact what is assumed to be the cause may be the effect, and vice-versa.	"The bread is going bad, so it's getting moldy."	In fact, it's the mold that causes the bread to go bad, not the reverse.
contradictory premises	Contains a self-contradictory premise, or two or more mutually exclusive premises.	"If God can do anything, he must be able to make a stone that he can't lift."	Mutually contradictory premises cannot both be true at the same time and in the same way.
false (questionable) cause	Assigns a particular cause to an event, despite reason to consider other possible causes.	"John brought that layoff on himself by walking under a step-ladder." [post hoc] "Establishing equal rights for women will require men and women to share the same public rest rooms." [slippery slope]	Variants of false cause include post hoc ergo propter hoc ("after this, so because of this") and slippery slope fallacies.
false dilemma	Sets unrealistic limits on possible options.	"Elizabeth must be either at home or at work." [false dilemma] "Anyone who does not stand with us stands against us." [false dichotomy / missing middle]	The example ignores many possibilities for Elizabeth's whereabouts, such as at the shopping mall, at the theater, on vacation, visiting a friend, or in transit between locations. A special kind of false dilemma, in which there is a range of alternatives between two stated extremes, is called the "false dichotomy" or "missing middle" fallacy.
gambler's fallacy	Assumes the repeated occurrence of an event that departs from the expected norm indicates that the event will cease soon.	"I've lost $500 in this slot machine already, so it's bound to hit the jackpot any time now."	Events can be reliably predicted only when they (or other related and observable events) occur in some recognizable pattern or sequence. Because random events exhibit no particular sequence, we can predict only their statistical chances, not their specific occurrences.
genetic fallacy	Infers that information must be false if the reputation of its source is questionable.	"The way to tell when a politician is lying is that his lips are moving."	It is quite possible, even for sources that are frequently in error or even routinely dishonest, to make true statements.
guilt by association	Implies that an idea is bad if it is associated with harmful ideas or practices.	"Religion is bad, because it's sometimes used to justify persecution, terrorism, and warfare."	Even the best of humanity's institutions have been used to excuse atrocities, and even the worst can put forward some constructive ideas.
hasty generalization	Makes an unjustified general inference from an insufficient sample.	"The only two Italians I ever knew belonged to the Mafia. So if you ask me, all Italians are crooks."	There are millions of Italians. That the two individuals I happen to know are in the Mafia is not enough to generalize that all Italians are crooks—any more than it would justify a generalization that all the people I know are crooks.
middle ground (golden mean)	Assumes that a mean between two extreme alternatives must be the correct course.	"A storm blew my tree down onto my neighbor's shed, and he demands that I pay for it. While I don't think I ought to pay, it should be fair if I pay him half of what he demands."	Although compromise is sometimes a realistic way to settle differences, in cases where the alternatives are justice and injustice, or truth and falsehood, a halfway measure is as devoid of justice and truth as the extreme.
missing middle	(See "false dilemma.")
non sequitur	Does not logically follow from the argument presented.	"Marriage is traditionally a union of one man and one woman, so legitimizing homosexual unions would destroy traditional marriage."	A non sequitur misrepresents an unsupported or inadequately supported opinion as a conclusion.
relativist / subjectivist fallacy	Denies that what is objectively true for others is true for oneself.	"I read that geologists have dated some rocks at 200 million years. Maybe that's true for them, but not for me."	The relativist fallacy confuses opinion with reality, supposing that objective truth can vary from one person to another.
special pleading	Bids for exemption from standards assumed to apply to everyone else.	"Hey, I naturally get angry when other drivers cut me off in traffic. So why are you writing me a citation for reckless operation, officer?"	Under some circumstances, special pleading is matter of adjusting balance and fairness. But it can also be an unfair bid for special privilege or exemption from just obligation.
spotlight	Infers that highly conspicuous individuals are typical of a group.	"On TV I saw a crowd of Palestinians chanting 'Death to Israel!' All Palestinians are murderous fiends."	Similar to "biased sample" fallacy.
undistributed middle	Makes an unjustified inference about some members of a group from information about some other members of the group.	"Some Arabs are wealthy. Some Arabs live next door to me. So my next-door neighbors are wealthy."	The statement that some Arabs are wealthy provides no definite information about the wealth (or lack thereof) of the particular Arabs who live next door.
unqualified generalization (dicto simpliciter)	Makes an assumption about all individuals in a group, overlooking pertinent differences.	"Exercise is healthful, so everyone ought to exercise."	The example overlooks that, for people in poor health, exercise might be harmful or even fatal.
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Ambiguities An ambiguity is a word, phrase, or statement that, without adequate clarification, could be interpreted in different ways.
Fallacy	Description	Examples	Comments
equivocation	Relies on the double meaning of a term to make a questionable point.	"Christians, Hindus, Jews, and Muslims all believe in God, so they all believe in the same thing."	In the example, each religion's concept of "God" is different, e.g., Christians don't believe in gods with four arms, Hindus don't believe in a god with three manifestations, and Muslims and Jews don't believe in gods who approve of eating pork.
amphiboly	Relies on an ambiguity due to sentence structure.	"Professor Hefner lectures on sexual promiscuity in room 210? They ought to post a guard there to discourage that sort of behavior!"	The structure of the sentence makes it unclear whether what goes on in room 210 is the lecture about sexual promiscuity, or the sexual promiscuity itself.
composition	Ascribes the nature of one or more parts to the whole.	"Each of this computer's components is reliable, so the computer itself must be reliable."	Composition is not always fallacious. Consider if we were to substitute "unreliable" for "reliable" in the example.
division	Ascribes the nature of a whole or a group to specific parts or individuals.	"The orchestra plays superbly. Each member must be a virtuoso."
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Formal Errors A formal error is a violation of accepted rules of formal logic.
Fallacy	Description	Examples	Comments
affirming the consequent	Tries to confirm the antecedent of a conditional statement from a true consequent.	"If it is raining, then the ground is wet. The ground is wet. Therefore it is raining."	We cannot conclude anything about the cause of wet ground, since there are things besides rain (e.g., sprinklers, a burst water main) which can cause the ground to be wet.
denying the antecedent	Tries to refute the consequent of a conditional statement from a false antecedent.	"If it is raining, then the ground is wet. It is not raining. So the ground is not wet.".	We cannot conclude anything about the results of no rain, since here are things besides rain (e.g., sprinklers, a burst water main) which can cause the ground to be wet.
Two rules, modus ponens and modus tollens, govern the conditional IF-THEN relationship. In the symbolic example "If A, then B," A is the antecedent and B is the consequent. Modus ponens states that if A (the antecedent) is true, then B (the consequent) is necessarily true. But if A is false, that tells us nothing about B (except that if B happens to be true, it is for some reason other than A). Modus tollens states that if B (the consequent) is false, then A (the antecedent) is necessarily false. But if B is true, that tells us nothing about A (because there might well be other causes of B). Note: In this example, the antecedent "it is raining" is the cause, and the consequent "the ground is wet" is the effect, though this is not always the case. Consider another example, "If it is raining, then there are clouds in the sky," where the antecedent "it is raining" is the effect, and the consequent "there are clouds in the sky" is the cause. There can even be situations in which neither antecedent nor consequent is either cause or effect, for instance, "If we have fish for dinner, then we drink white wine." Regardless, the IF-THEN logic works exactly the same way with modus ponens and modus tollens—and malfunctions exactly the same way with either fallacy.
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Terms: fallacy (an error in reasoning) ambiguity (an idea that can be interpreted in different ways) amphiboly composition division equivocation formal error (a violation of the principles of formal logic) affirming the consequent (of a conditional statement) denying the antecedent (of a conditional statement) irrelevant premise (a claim having no logical bearing on the conclusion) appeal to belief appeal to common practice appeal to consequences of belief appeal to emotion appeal to flattery appeal to fear or force (ad baculum) appeal to pity (ad misericordiam) appeal to spite appeal to ignorance (ad ignorantium) appeal to popular opinion (ad populum) argument against the person (ad hominem): circumstantial ad hominem poisoning the well tu quoque bandwagon red herring straw man unwarranted assumption (a claim without adequate support of evidence or logic) appeal to novelty appeal to tradition appeal to unreliable authority (ad verecundiam) begging the question (petitio principii) / circular argument biased sample complex question confusing cause and effect contradictory premises false / questionable cause post hoc (ergo propter hoc) slippery slope false dilemma false dichotomy missing middle gambler's fallacy genetic fallacy guilt by association hasty generalization middle ground / golden mean non sequitur relativist / subjectivist fallacy special pleading spotlight undistributed middle unqualified generalization (dicto simpliciter)