Disclaimer: The opinions, real or imagined, expressed in the examples are not necessarily mine or those of Kalamazoo College. Some are plain dumb for purpose of explanation. You were warned. Common Logical Fallacies1 Propositional Fallacies Affirming a disjunct concluding that one disjunct of a logical disjunction must be false because the other disjunct is true; A or B; A, therefore not B. Affirming the consequent the antecedent in an indicative conditional is claimed to be true because the con- sequent is true; if A, then B; B, therefore A. Denying the antecedent the consequent in an indicative conditional is claimed to be false because the an- tecedent is false; if A, then B; not A, therefore not B. Informal Fallacies Appeal to the stone argumentum ad lapidem Dismissing a claim as absurd without showing it is absurd. “The idea that a police officer would shot a 12 year–old boy is stupid!” Appeal to Ignorance agrumentum ad ignorantiam Assuming a claim is true because it cannot be shown to be false. or vice versa. Appeal to common sense “I can’t see how this can be true, so it must be false.” Related to argumentum ad populum Appeal to the crowd argumentum ad populum Appealing to what everyone “knows” to be true must be true. Often hidden well in the argument by someone who wants to convenience the audience whether the point is proved or not. Advertisements and political messages use this frequently. “All true Ameri- cans know the military budget must be increased.” “We all know the Second Amendment gives us the right to carry handguns.” Argument by repetition argumentum ad nauseam, argumentum ad infinitum Stating the argument over and over until no one wants to talk about it anymore. Not to be confused with proof by assertion. Proof by Assertion Simply asserting a fact with no proof. “It is a fact.” “The world is flat.” Argument from silence The conclusion is based upon the absence of evidence rather than the existence of supporting evidence. Begging the questionor Circular reasoning petitio principii Using what is essentially the same as the con- clusion as a premise. Burden of ProofOnus probandi From the Latin onus probandi incumbit ei qui divit, non ei qui negat” The burden of proof is on the person making the claim, not the one disputing it. If you claim that global warming is a fact, you must provide the proof, not someone disputing the existence of global warming. Gambler’s fallacy The false belief that the occurrence of a series of independent results can affect the like- lihood of the next independent result. “This coin has come up heads ten times so the next time is more likely to be tails.” Sometime called the “Law of Averages” or the “Law of Large Numbers”; neither of which has any bearing upon the exact question. 1Mainly from Wikipedia If–by–whiskey An argument that supports both sides of an issue by using terms that are selectively emo- tionally sensitive. Missing the point ignoratio elnchi A valid argument that never addresses the proposed conclusion. The argument leads one astray and sometimes “proves” a different conclusion. Gracie Allen (Burns and Allen), Goldie Hawn (Laugh-in), Lisa Douglas(Green Acres),, and many other comedy stars made a carrier of this. False cause and effect post hoc ergo propter hoc or “after this, therefore because of this” Asserting a prior even must be the cause of a later event simply because on happened after the other. “I washed my car this morning so it will rain this afternoon.” Wrong direction Cause and effect are reversed, that is, the effect is said to cause the cause. Hasty generalization What is true of a small sample is said to be true of all. “John Doe an John Q Public are left–handed criminals. Therefore all left–handed men are criminals!” Red Herring Fallacies2 Argument to the man Ad hominem Arguing that flaws in the person (real or alleged) make the argument false. Also attacking the person not the argument. “Al Gore lost his election for the Presidency, so how could he be right about global warming?” Appeal to authority Argument based upon the persons authority rather than the evidence supporting the statements. This is often due to the person’s looks, good works, intelligence, or authority in a different filed. “We should all follow Linus Pauling’s advice to mega–does on vitamin C because he is a Noble Prize Laureate.” Excuse me, his Prize was for Physics, not Biology. Appeal to emotion An argument designed to be accepted because of fear of the out–come, flattery, love, hate, or some other emotion. “Noah was an idiot for taking those two ticks on the Arc.” “We have to set aside this land that is the home of a large warren of cuddly rabbits.” Appeal to tradition Claiming an argument is true because it has long been believed to be true or it has always been done that way. Sadly, this has been used too many times in science. “What is good enough for Aristotle is good enough for us.” *sigh* Association fallacy (Guilt by association) Incorrectly assuming that since two premises share, or are thought to share, a common characteristic they are both true or both false. The “K” Basketball team is good so the “K” baseball team must be good as well. While this may, or may not, be the case this is not a valid point. Pooh-pooh Dismissing an argument as being unworthy of consideration. “That’s stupid. Next point.” Vacuous truth A claim that is technically true because it does not apply to any real case. “All pink elephants are left–handed.” Appeal to self–evident truth A claim that a statement is obviously true and therefore does not need any supporting evidence. This is not necessarily stating a fact such as “1 + 1 = 2”. “No one could survive a fall like that.” 2I don’t like this title from Wikipedia. I am open to a better suggestion..