A Class-Room Introduction to Logic

May 4, 2009

Unit-XIV: Fallacies (Formal & Informal)

Filed under: Logic,Uncategorized — Dr. Desh Raj Sirswal @ 7:37 am

From a psychological point of view, a fallacy is often defined as a mistake in reasoning used for deceptive purposes; however, many fallacies are, in fact, not deceptive to most persons. Even so, many of the informal fallacies are often used in the manipulation of opinion. Many of these mistakes in reasoning occur so often they deserve special study. This section investigates informal fallacies—those dependent upon language. An informal fallacy is one that arises from the content of an argument (the meaning what is said, not the grammar in terms of how the argument is expressed).

Our account of fallacies is in the tradition of I. M. Copi’s presentation: he reveals that some mistakes in reasoning arise from appeals to irrelevant factors and others from unsupported assumptions.

Nevertheless, as Joseph says in his Introduction to Logic (569): “Truth may have its norm, but error is infinite in its aberrations, and they cannot be digested in any classification.”

Not all irrelevant appeals and unsupported assumptions are fallacies. Fallacies occur in argumentative discourse. Thus, if no argument is offered, no fallacy is present


A fallacy is a type of mistake in argumentation that might appear to be correct, but which proves upon examination not to be so.

Let us classify two basic types: 1. Informal Fallacy : those dependent upon language– i.e., a fallacy that arises from the content of an argument (the what is said, not the how it is said). 2. Formal Fallacy: those outside the content of language–i.e., a fallacy that arises from an error in the form of an argument; it is (usually) independent of content.

Fallacies of No Evidence

Argument Against the Person (argumentum ad hominem) This fallacy is committed when you attack a person s character or personal circumstances in order to oppose or discredit their argument or viewpoint. Also:

Tu Quoque Fallacy (you re one, too ) A type of abusive ad hominem that attempts to discredit a person s viewpoint or position by charging the person with hypocrisy or inconsistency. Essentially, the charge is, We don t need to take his argument seriously because he doesn’t practice what he preaches.

Guilt by association Fallacy A type of abusive ad hominem in which one person attacks a second person s associates in order to discredit the person and thereby his view or argument.

Appeal to Force (argumentum ad baculum, literally argument from the stick ) A fallacy committed when an arguer appeals to force or to the threat of force to make someone accept a conclusion.

Appeal to Pity (argumentum ad misericordiam) A fallacy committed when the arguer attempts to evoke pity from the audience and tries to use that pity to make the audience accept a conclusion.

Appeal to the People (argumentum ad populum) A fallacy committed when an arguer attempts to arouse and use the emotions of a group to win acceptance for a conclusion.

Snob Appeal Fallacy This is committed when the arguer claims that if you will adopt a particular conclusion, this will place you in a special, elite group or will make you better than everyone else.

Fallacy of Irrelevant Conclusion (ignoratio elenchi, meaning ignorance of the proof ) A fallacy in which someone puts forward premises in support of a stated conclusion, but the premises actually support a different conclusion.

Begging the Question Fallacy (petitio principii, meaning postulation of the beginning ) This is committed when someone employs the conclusion (usually in some disguised form) as a premise in support of that same conclusion.

Appeal to Ignorance (argumentum ad ignorantium) In this fallacy, someone argues that a proposition is true simply on the grounds that it has not been proven false (or that a proposition must be false because it has not been proven true).

Red Herring Fallacy A fallacy committed when the arguer tries to divert attention from his opponent s argument by changing the subject and drawing a conclusion about the new subject.

Genetic Fallacy A fallacy committed when someone attacks a view by disparaging the view s origin or the manner in which the view was acquired.

Poisoning the Well The use of emotionally charged language to discredit an argument or position before arguing against it.

Fallacies of Little Evidence

Fallacy of Accident A fallacy committed when a general rule is applied to a specific case, but because of extenuating circumstances, the case is an exception to the general rule and the general rule should not be applied to the case.

Straw Man Fallacy A fallacy committed when an arguer (a) summarizes his opponent’s argument but the summary is an exaggerated, ridiculous, or oversimplified representation of the opponent s argument that makes the opposing argument appear illogical or weak;

(b) the arguer refutes the weakened, summarized argument; and (c) the arguer concludes that the opponent s actual argument has been refuted.

Appeal to Questionable Authority Fallacy (argumentum ad verecundiam) When someone attempts to support a claim by appealing to an authority that is untrustworthy, or when the authority is unqualified, or prejudiced, or has a motive to lie.

Fallacy of Hasty Generalization A fallacy committed when someone draws a generalization about a group on the basis of observing an unrepresentative sample of the group.

False Cause Fallacy A fallacy involving faulty reasoning about causality. Also:

In a Post Hoc Ergo Propter Hoc fallacy ( after this, therefore, because of this ) someone concludes that A is the cause of B simply on the grounds that A preceded B in time.

In a Non Causa Pro Causa fallacy ( not the cause for the cause ) someone claims that A is the cause of B, when in fact (1) A is not the cause of B, but (2) the mistake is not based merely on one thing coming after another thing. One version of this fallacy is the fallacy of accidental correlation: the arguer concludes that one thing is the cause of another thing from the mere fact that the two phenomena are correlated.

Slippery Slope Fallacy (or domino argument ) In this fallacy, someone objects to a position P on the grounds that P will set off a chain reaction leading to trouble; but no reason is given for supposing the chain will actually occur. Metaphorically, if we adopt a certain position, we will start sliding down a slippery slope and we won t be able to stop until we slide all the way to the bottom (where some bad result lies in wait).

Fallacy of Weak Analogy A fallacy committed when an analogical argument is presented but the analogy is too weak to support the conclusion.

Fallacy of False Dilemma A fallacy committed when someone assumes there are only two alternatives, eliminates one of these two, and concludes in favor of the second, when more than the two stated alternatives exist, but have not been considered.

Fallacy of Suppressed Evidence In this fallacy, evidence that would count heavily against the conclusion is left out of the argument or is covered up.

Fallacy of Special Pleading In this fallacy, the arguer applies a principle to someone else s case but makes a special exception to the principle in his own case.

Fallacies of Language

Fallacy of Equivocation In this fallacy, a particular word or phrase is used with one meaning in one place, that word or phrase is used with another meaning in another place, and what has been established on the basis of the one meaning is regarded as established with respect to the other meaning. As a result, the conclusion depends on a word (or phrase) being used in two different senses in the argument. The premises are true on one interpretation of the word, but the conclusion follows only from a different interpretation.

Fallacy of Amphiboly A fallacy containing a statement that is ambiguous because of its grammatical construction. One interpretation makes the statement true, the other makes it false. If the ambiguous statement is interpreted one way, the premise is true but the conclusion is false; but if the ambiguous statement is interpreted the other way, the premise is false. The meaning must shift if the argument is going to go from a true premise to a true conclusion. If the meaning is not allowed to shift during the argument, either the argument has false a premise or it is invalid.

Fallacy of Composition A fallacy in which someone uncritically assumes that what is true of a part of a whole is also true of the whole.

Fallacy of Division A fallacy in which someone uncritically assumes that what is true of the whole must be true of the parts.


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