10 Apr 2010
03 Oct 2013

Informal Fallacies


How Reasoning Goes Wrong
(The  COMICS  Page!)

The rules for basic logic are fairly few, mostly simple, and to some extent intuitive.  However, most people aren't well acquainted with these rules and their limits, and thus mistakes in reasoning happen.  A lot.  When they do, sometimes we get lucky and the results are of little or no consequence.  But when the consequences are significant, they can range from mildly embarrassing to catastrophic.  It's therefore in our interest to be able to identify the most common kinds of reasoning errors, both in order to keep them from messing up our own thinking, and also to avoid being taken in by the flawed reasoning of others, whether through innocent errors or through deliberate attempts to mislead.

An error in reasoning is called a fallacy, and reasoning that contains such errors is described as fallacious.  A fallacy is a defect or fault that in some way disrupts a line of reasoning or diverts it from its proper objective.  There are many specific kinds of fallacies, but most of these fall into one of four general categories: ambiguities, irrelevant premises, unwarranted assumptions, and formal errors.  In this section we'll discuss the first three of these categories, which are fairly easily understood with little background—and the reader will discover why we call this "The Comics Page."  We'll discuss formal errors later, after we've gone into a bit more detail about rules of logical form and function.

Although we'll specify some particular fallacies as common examples, our objectives here are merely (1) to determine whether or not something is a fallacy, and (2) if it is a fallacy, to identify it with only enough precision to decide in which general category it belongs.  For our purposes, we need only be able to tell when something is wrong with a line of reasoning; we'll leave the nitpicking to the experts and the fanatics.  (Some people find fallacies fascinating.  Readers with such an interest are invited to click here for a more extensive listing.)



An ambiguity is something that can produce a confusion of meaning.  These often arise from words or expressions with more than one common meaning in a given context, from unclear syntax or sentence structure, and from confusion of a whole with its parts.

 Equivocation  A word or expression that can have significantly different meanings in the same context constitutes equivocation.
"Only man can reason abstractly.  Ruth is a woman, not a man.  So, Ruth cannot reason abstractly."
(It's likely that "man" in the first premise refers to "humans," while in the second premise it clearly signifies "a male human."  The shift in meaning gives rise to an erroneous conclusion.)  

 Amphiboly  An ambiguity resulting from sentence structure is called amphiboly.
"Professor Hefner lectures on sexual promiscuity in room 210."
(So, if we go to room 210,  what should we expect to observe?  A lecture, or an orgy?)

 Composition  The fallacy of composition attributes to a whole the qualities of one or more of its parts.
"An all-star team would be unbeatable."
(The star quality of each player doesn't necessarily translate into well coordinated team performance, so it's quite possible that an all-star team could be beaten by a well organized and practiced team of non-stellar players.)

 Division  The fallacy of division attributes the qualities of a whole to its parts.
"Each of this computer's components is highly reliable.  So, this computer is highly reliable."
(A computer built entirely of the most reliable components might still be an utter failure if those components are improperly connected or not well matched.)

 Reification  Reification is the confusion of an idea of a thing with the thing itself.
"God is the greatest concept that can exist, and existence is greater than non-existence; so, God exists."
(The existence of a concept of God does not enable us to conclude the existence of God as anything other than a concept.)

Optional: List of Common Fallacies – Ambiguities


Irrelevant Premises

A premise is irrelevant if it has no logical connection to the claim that it's presumably intended to support.  A majority of irrelevant premises fall into a few major subcategories.

 Appeals against the person (ad hominem)  Ad hominem (Latin for "against the person") is a criticism against an arguer rather than against his or her argument.  In most cases, ad hominem is fallacious because it has nothing to do with the truth or falseness of the issue being discussed.  In a few cases in which the arguer's credibility is a genuine issue (e.g., if he or she is known to be an unreliable authority or a compulsive liar), an ad hominem appropriately targeted at that specific shortcoming would be relevant.

Particular kinds of ad hominem include:

  • "poisoning the well" / abusive ad hominem:
    "The incumbent can't be trusted because he once had a consensual affair with an aide."

  • claim of bias / circumstantial ad hominem:
    "We'd expect O'Hara not to be critical of P&Q; after all, she's a P&Q employee."

  • claim of hypocrisy / tu quoque:
    "Dad speeds when he drives, so he's in no position to tell me not to speed."

 Appeals to emotion  Appeals to hope, fear, love, hate, loyalty, vanity, patriotism, prejudice, pity, and spite typically have no actual bearing on the truth or falseness of the issue at hand.

  • appeals to fear:

    • of force (ad bacculum):
      "If you don't agree to cooperate in the cover-up, you'll be terminated."

    • of ostracism (bandwagon): 
      "Play along with our plan, and you'll find yourself in the inner circle.  (Or if you don't, you won't.)"

  • appeal to flattery:
    "I'll bet a smart, in-charge type like you could open a few doors for someone like me."

  • appeal to pity (ad misericordiam):
    "Please, boss, you mustn't fire me.  I'll lose my house and car."

  • appeal to spite:
    "I don't care if Iacocca is the best qualified applicant.  He used to work for our competitor, and I refuse to hire him."

  • appeal to consequences of belief:
    "I have to believe in God.  Otherwise, there's no reason to live, and I'd commit suicide."

 Appeals to popular opinion  Even if true, a claim that everyone thinks a certain way is no guarantee that the way everyone currently thinks is best.  Opinions about reality might be subject to majority vote, but reality itself is not.  Reality simply is what it is, and truth hangs on reality, not on opinion.

  • appeal to popular belief (ad populum):
    "Why shouldn't you believe the earth is the center of the universe, Galileo?  Everyone else does."

  • appeal to the people:
    "You need to acquire a more individualistic outlook, like everyone else in our class."

  • appeal to novelty:
    "We can never progress unless we take some risk.  Our next new car will have tailfins."

  • appeal to tradition:
    "Things have always worked fine just the way they are.  We're not giving up our horse and buggy."

 Miscellaneous irrelevant premises  Sometimes extraneous ideas are introduced into a discussion in order to distract attention, either from a weak point in one's own argument or from a strong point in an opponent's argument.

  • "red herring": Like the smell of fish diverting a cat, this fallacy serves no purpose but to draw attention away from the central issue of discussion.
    "I couldn't possibly go to the polls tomorrow.  I have a 9:00 a.m. appointment at the hairdresser."

  • appeal to ignorance (ad ignorantium): This amusing fallacy bounces both ways, proposing that an idea must be true if it hasn't been shown false—or that an idea must be false if it hasn't been shown true.
    "Reincarnation has never been ruled out, so there is reincarnation."
    "Reincarnation has never been proved, so there's no reincarnation."

Optional: List of Common Fallacies – Irrelevant Premises


Unwarranted Assumptions

As we noted in the previous section, a great many ideas we deal with are not known for certain, and some may even be entirely speculative.  These are not facts or evidence, but rather assumptions.  Still, not all assumptions are equal.  Some are "safer"—more probable—than others.  If you're reading this, it's safe to assume that you exist.  It's also fairly safe to assume that what you're reading was produced by a human being.  However, it's less safe to make assumptions about the writer's ethnicity, beliefs, or preferences.

An assumption about which there's significant room for doubt is said to be unwarranted.  That is, to be considered persuasively credible, the assumption needs to be justified by additional information, or warrants.  If adequate justification can't be supplied, then any further argumentation that depends on the unwarranted assumption collapses.

 Dubious Generalization  

  • biased sample:
    "Sixty percent of Packard owners said they'd choose another Packard as their next car.  So, Packards are very popular."

  • hasty generalization:
    "I've visited six states, and they were all densely forested.  So, all the other states must be just the same."

  • "spotlight":
    "I see in the news that an investment broker was convicted of fraud.  All investment brokers are crooked."

  • unqualified generalization (dicto simpliciter):
    "Vigorous physical exercise is healthful.  You should get more physical exercise, Dr. Hawking."

 False or questionable cause  

  • confusion of cause and effect: This error misinterprets the nature of a causal relationship.
    "The trees sway whenever the wind blows.  So, the swaying of the trees obviously causes the wind to blow."

  • post hoc ergo propter hoc, "after this, so because of this": A post hoc fallacy assumes that because one event precedes another, the first event causes the second.  It's very popular among the superstitious and conspiracy theorists.
    "The H.M.S. Titanic was prominently advertised as unsinkable.  That's why she sank on her maiden voyage."

  • "slippery slope": A slippery slope argument predicts that a specified condition will cause a certain sequence of events.  It's fallacious if there's little reason to suppose that the ensuing sequence would be inevitable.
    "If we give women the right to vote, pretty soon they'll be demanding to use the men's restroom."

  False choice  Both false dilemma and false dichotomy are attempts to impose a limited range of options, when in fact there are more available.  False dichotomy / missing middle is a special case, in which only two extreme options are presented, when in fact intermediate options (perhaps including compromise) are viable.

  • false dilemma:
    "Give me liberty, or give me death!"  (Or how about a frosty beer to mellow out, Pat?)

  • false dichotomy / missing middle:
    "If you aren't for us, you're against us!"  (Would you mind too much if we review the facts before deciding?)

 Miscellaneous unwarranted assumptions

  • appeal to unreliable authority (ad verecundiam): Appeals to authority are fairly common, but are fallacious if the authority's field of expertise is something other than the issue at hand.  The Pope and Einstein are both trusted authorities, but in different fields of expertise; we don't defend Catholic doctrine by citing Einstein, and we don't support claims about relativity by quoting the Pope.

  • begging the question / circular argument (petitio principii): The phrase "begging the question" refers to the question of whether an argument's conclusion is true or false.  An argument is said to be circular or to beg the question if it uses an assumption that its own conclusion is true as a premise to support that same conclusion.  Usually the circularity is camouflaged by using a paraphrase of the conclusion, or even better a subtle implication, rather than a direct quote, as the key assumption.

  • complex question: A complex question is a dubious insinuation cleverly phrased as a question.
    "Have you stopped pilfering office supplies yet?" clearly insinuates that one has been pilfering, but technically stops short of making the accusation outright.

  • contradictory premise: An argument is fallacious if its premises make simultaneous, mutually exclusive claims.
    "Being omnipotent, God can lift any stone, even one he's made so heavy that it's impossible to lift."

  • non sequitur ("it doesn't follow"): A questionable assumption that isn't made adequately credible by explanatory warrants is characterized as not logically following from the reasons given.
    "I've filed the required papers with the election board, so I'll be elected king in November."

  • "straw man": A straw-man fallacy is an easily refuted misrepresentation of an opponent's claim.  The error lies in the fact that refutation of a misrepresentation does not constitute refutation of the actual claim.
    "Evolutionists claim that new species spring up completely at random."  (Actually, evolutionists make no such claim.  While the causes of mutation appear to be random within certain limits, natural selection and speciation are strictly governed by viability in the existing environment.) 
    (Straw-man fallacies are categorized by some as irrelevant premises.  It's a borderline case, which hinges on whether we fault the unwarranted claim itself or the irrelevant inference drawn from it.)

Optional: List of Common Fallacies – Unwarranted Assumptions





In this section we've learned to identify three general categories of reasoning errors: ambiguities, irrelevant premises, and unwarranted assumptions.  With this knowledge and some practice, we should be able to make our own reasoning cleaner and more solid, and to take notice when others use faulty reasoning to mislead us.

This concludes the Informal Fallacies study section.  However, if interested, you're welcome and encouraged to look over our more detailed list of individual fallacies, which includes descriptions, examples, and comments.

List of Common Fallacies

In the next section, we'll use a system of symbolic representation to assist in our further investigation, testing, and application of basic logic principles, and in identifying a fourth category of common fallacies: formal errors.

Next Section: Symbolic Logic