A fallacy is a false argument or belief. What follows is a list of the most common informal and formal logical fallacies. Learn these and be able to spot logical fallacies in your (and others') reasoning.

ARGUMENTUM AD NUMERAM

A fallacy that asserts that the more people who support or believe a proposition then the more likely that that proposition is correct; it equates mass support with correctness - the fallacy that led to Man believing that the earth is flat for centuries.

ACCENTUS

One of the Fallacies of Ambiguity, which arises from the emphasis (the accent) placed on a word or phrase.

AFFIRMING THE CONSEQUENT

An argument based on a hypothetical statement, and the truth of the consequent to the truth of the antecedent. In the SYLLOGISM below, A is the antecedent and C is the consequent:

A implies C
C is true <-- Affirming the consequent
______________
Therefore: A is true

AMBIGUITY

An argument in the course of which at least one term (such as "rights") is used in different senses. Also known as equivocation. There are several types of "fallacies of ambiguity," including REIFICATION, EQUIVOCATION, AMPHIBOLY, COMPOSITION, DIVISION, and ACCENTUS.

AMPHIBOLY

A type of Fallacy of Ambiguity where the ambiguity involved is of an "amphibolous" (equivocal, uncertain) nature. Amphiboly is a syntactic error.

The fallacy is caused by faulty sentence structure, and can result in a
meaning not intended by the author. "The department store now has pants for men with 32 waists." (How many waists do you have?)

ARGUMENTUM AD ANTIQUITAM

A fallacy of asserting that something is right or good simply because it is old; that is, because "that's the way it's always been."

ARGUMENTUM AD BACULUM

An argument that resorts to the threat of force to cause the acceptance of the conclusion. Ad baculum arguments also include threats of fear to cause acceptance (e.g., "Do this or you'll go to Hell when you die!" or "I made him an offer he couldn't refuse.").

ARGUMENTUM AD CRUMENAM

Fallacy of believing that money is a criterion of correctness; that those with more money are more likely to be right. "If he's so stupid why is he so rich?" The reverse of a. ad crumenam is a. ad lazarum.

ARGUMENTUM AD HOMINEM

An argument that attempts to disprove the truth of what is asserted by attacking the speaker rather than the speaker's argument. Another way of putting it: Fallacy where you attack someone's character instead of dealing with issues. The two basic types of ad hominem arguments: are 1) abusive, and 2) circumstantial. This is the most common form of Logical Fallacy.

ARGUMENTUM AD IGNORANTIAM

An argument that a proposition is true because it has not been shown to be false, or vice versa. Ad ignorantium arguments are also known as "appeals to ignorance." This fallacy has two forms:

1. The statement is true, because it has not been proven false.
2. The statement is false, because it has not been proven true.

ARGUMENTUM AD LAZARUM

A fallacy of assuming that because someone is poor he or she is sounder or more virtuous than one who is wealthier. -- "responsible breeders don't make money." This fallacy is the opposite of the informal fallacy a. ad crumenam.

ARGUMENTUM AD MISERICORDIAM

An argument that appeals to pity for the sake of getting a conclusion accepted (or for fundraising).

ARGUMENTUM AD NAUSEUM

The incorrect belief that an assertion is more likely to be true the more often it is heard. An a. ad nauseum is one that employs constant repitition in asserting a a statement is the truth. Dr. Goebbel's Big Lie Theory.

ARGUMENTUM AD NOVITAM

A fallacy of asserting that something is more correct simply because it is new or newer than something else. Or that something is better because it is newer. -- "we've tried the other way for a while and it's failed, let's try something (anything) ." This type of fallacy is the opposite of a. ad antiquitam.

ARGUMENTUM AD NUMERAM

A fallacy that asserts that the more people who support or believe a proposition then the more likely that that proposition is correct; it equates mass support with correctness.

ARGUMENTUM AD POPULUM

An argument that appeals to the beliefs of the multitude. Another way of putting it: Speaker deals with passions of audience rather than with salient issues. This fallacy is also known as "Appeal to Tradition" Ad populum arguments often occur in 1) propaganda, 2) demagogy, and 3) advertising.

ARGUMENTUM AD VERECUNDIAM

An argument in which an authority is appealed to on matters outside his/her field of authority. (like veterinarians dispensing medical advice). a.ad verecundiam also refers to a fallacy of simply resorting to appeals to authority (like "Doctor" Tom Regan)

BEGGING THE QUESTION (CIRCULAR REASONING)

An argument that assumes as part of its premises that the conclusion is true. Another way of saying this is: Fallacy of assuming at the onset of an argument the very point you are trying to prove. This Fallacy is also known by the Latin "PETITIO PRINCIPII"or "CIRCULUS IN PROBANDO."

BIFURCATION

Also referred to as the "black and white" fallacy, bifurcation is the presentation of a situation or condition with ONLY TWO alternatives, whereas in fact other alternatives exist or can exist.

COMPOSITION

An argument which assumes that a whole has a specific property solely because itsvarious parts have that property. -- "Because ALF is a terrorist organization (and ALF is part of PETA) > all PETA members condone terrorism." Composition is a type of Fallacy of Ambiguity.

CONVERTING A CONDITIONAL

Description: If A then B, therefore, if B then A. <<<NOT!!!

CUM HOC ERGO PROPTER HOC

A fallacy of correlation that links events because they occur simultaneously; one asserts that because two events occur together they are causally related, and leaves no room for other factors that may be the cause(s) of the events. This fallacy is similar to the "post hoc" fallacy.

DENIAL OF THE ANTECEDENT

An argument in which one infers the falsity of the consequent from the truth of a hypothetical proposition, and the falsity of its antecedent.

A implies B
Not-A
____________
Therefore: Not-B

DIVISION

An argument in which one assumes that various parts have a property solely because the whole has that same property. Division is a type of Fallacy of Ambiguity -- the inverse of COMPOSITION.

EQUIVOCATION

An argument in which an equivocal expression is used in one sense in one premise and in a different sense in another premise, or in the conclusion.
Equivocal means 1) of uncertain significance; not determined, and 2) having different meanings equally possible. Equivocation is a type of Fallacy of Ambiguity. The opposite of equivocation is "UNOVOCATION," in which a word always carries the same meaning through a given context.

FALLACY OF INTERROGATION

The question asked has a presuppostion which the answerer may wish to deny, but which he/she would be accepting if he/she gave anything that would count as an answer. Any answer to the question "Why does this event happen?" presupposes that the event does indeed happen.

FALSE ANALOGY

An analogy is a partial similarity between the like features of two things
or events on which a comparison can be made. A false analogy involves comparing two things that are NOT similar. Note that the two things may be similar in superficial ways, but not with respect to what is being argued.
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IGNORATIO ELENCHI

An argument that is supposed to prove one proposition but succeeds only in proving a different one. IGNORATIO ELENCHI stands for "pure and simple irrelevance."

ILLICIT PROCESS

A syllogistic argument in which a term is distributed in the conclusion, but not in the premises. One of the rules for a valid categorical syllogism is that if either term is distributed in the conclusion, then IT MUST BE DISTRIBUTED IN THE PREMISES. There are two types of Illicit Process: Illicit Process of the Major Term and Illicit Process of the Minor Term.

PLURIUM INTERROGATIONUM - COMPLEX QUESTIONS

A demand for a simple answer to a complex question.

NON CAUSA PRO CAUSA

An argument to reject a proposition because of the falsity of some other proposition that SEEMS to be a consequence of the first, but really is not.

NON-SEQUITUR - DOES NOT FOLLOW

An argument in which the conclusion is not a necessary consequence of the premises. A conclusion drawn from premises that provide no logical connection to it.

PETITIO PRINCIPII

The same as "Begging the Question" This argument assumes its conclusion is true but DOES NOT SHOW it to be true. Petitio principii has two forms:

1. P is true, because P is true.
2. P is true, because A is true. And A is true because B is true.
And B is true because P is true.

POST HOC, ERGO PROPTER HOC

An argument from a premise of the form "A preceded B" to a conclusion of the form "A caused B." Simply because one event precedes another event in time does not mean that the first event is the cause of the second event. This argument resembles a fallacy known as a HASTY GENERALIZATION.

QUATERNIO TERMINORUM

An argument of the syllogistic form in which there occur four or more terms. In a standard categorical syllogism there are ONLY THREE TERMS: a subject, a predicate, and a middle term.

RED HERRING

A fallacy when irrelevant material is introduced to the issue being discussed, such that everyone's attention is diverted away from the points being made, and toward a different conclusion. It is not logically valid to divert a chain of reasoning with extraneous points.

REIFICATION

To reify something is to convert an abstract concept into a concrete thing. Reification is a Fallacy of Ambiguity. Reification is also sometimes known as a fallacy of "HYPOSTATIZATION".

SECUNDUM QUID (HASTY GENERALIZATION)

An argument in which a proposition is used as a premise without attention given to some obvious condition that would affect the proposition's application. This fallacy is also known as the "HASTY GENERALIZATION." It is a fallacy that takes evidence from several, possibly unrepresentative, cases to a general rule; generalizing from few to many. NOTE THE RELATION TO STATISTICS: Much of statistics concerns whether or not a sample is representative of a larger population. The larger the sample size, the better the representativeness. Note also that the opposite of a hasty
generalization is a sweeping generalization.

SHIFTING THE BURDEN OF PROOF

The burden of proof is always on the person making the assertion or proposition. Shifting the burden of proof, a special case of "ARGUMENTUM AD IGNORANTIUM," is a fallacy of putting the burden of proof on the person who denies or questions the assertion being made. The source of the fallacy is the assumption that something is true unless proven otherwise.

SPECIAL PLEADING

Special pleading is a logical fallacy wherein a double standard is employed by the person making the assertion. Special pleading typically happens when one insists upon less strict treatment for the argument he/she is making than he or she would make when evaluating someone else's arguments.

STRAW MAN

It is a fallacy to misrepresent someone else's position for the purposes of more easily attacking it, then to knock down that misrepresented position, and then to conclude that the original position has been demolished. It is a fallacy because it fails to deal with the actual arguments that one has made.

SWEEPING GENERALIZATION

Also known by the Latin term "DICTO SIMPLICITER", a Sweeping Generalization occurs when a general rule is applied to a particular situation in which the features of that particular situation render the rule inapplicable. A sweeping generalization is the opposite of a hasty generalization.

TWO WRONGS MAKE A RIGHT (TU QUOQUE)

Two wrongs never add up to a right; you cannot right a wrong by applying yet another wrong. Such a fallacy is a misplaced appeal to consistency. It is a fallacy because it makes no attempt to deal with the subject under discussion.

UNDISTRIBUTED MIDDLE

A syllogistic argument in which the middle term of a categorical syllogism is not distributed in AT LEAST ONE of the premises.