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Review some of the most important logical fallcies to understand how they can be used to manipulate arguments.
Logic is the study of argumentation. The purpose of an argument is to prove the validity of a conclusion, which in turn is based on the author’s premise. An argument is valid if it follows the rules of inference, which govern the proper means of inferring a conclusion from a given premise.
To determine the validity of an argument, the reader must examine whether the author’s claims and assertions have a logical relationship to his or her premise and whether those claims, in turn, imply the conclusion. An argument is invalid if the conclusion cannot be inferred from the given premise. When a logical argument includes an error in reasoning, the argument is defined as invalid, or fallacious. (The term “non sequitur” defines an argument whose conclusion does not follow from the premise and thus encompasses all kinds of fallacies.)
Fallacy describes the structure of an argument rather than the truth of its components. An author’s premise and conclusion may both be true while the argument itself is still fallacious. Invalid arguments tend to follow patterns of faulty reasoning, which are defined as logical fallacies.
The following table categorizes the most common logical fallacies according to whether the error in reasoning occurs in the author’s assertions (evidence) or actual logic (induction or deduction).
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| Appeal to authority | Relies on an authoritative source to provide evidence in an area outside his or her area of expertise. Example: “The superiority of the Gothic novel to the morality play is made evident by the catalog of Abraham Lincoln’s library, which included first editions of every contemporary example of the genre.” |
| Red herring | Presents a misleading argument in order to distract attention from the real point being argued. “Example: In judging the extent of Ezra Pound’s influence on the modern writers, it is important to remember that Pound was personally responsible for bringing the beautiful simplicity of Japanese art and literature to the attention of the West.” |
| Self-contradiction | Presents evidence that supports contradictory premises or that contradicts itself. Example: “Francis Bacon must have been the real author of Shakespeare’s work. Shakespeare’s existence can be proved from historical data; moreover, the style and vocabulary of Shakespeare’s tragedies follows a pattern found nowhere in Bacon’s other works.” |
| Confusion of fact and opinion | Mistakes a value judgment as objective fact. Example: “Because female poets are more sensitive to the painful consequences of human relationships, their work tends to focus on topics of personal significance.” |
| Equivocation | Uses a word or term according to its multiple definitions while assuming that the word means the same in each context. Example: “According to the nihilists, who believed in nothing, nothing is worse than the false assumption that life has meaning. Therefore, the nihilists held the contradictory belief that nothing was both a good and a bad thing.” |
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Fallacies of Induction |
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| Argumentum ad hominem (personal attack) |
Makes the faulty general conclusion that a person’s claims are false because there is something objectionable about the person. Example: “ Hitler’s claims that Germany suffered under the terms of reparation are contradicted by his blatant anti-Semitism.” |
| Straw man | Assumes that disproving one theory or idea as false also disproves a related theory. Example: “ The Platonic ideal is a clumsy, unworkable concept. Plato’s Republic can therefore be disregarded in terms of its value to societies interested in building a just form of government. ” |
| False analogy | Assumes that because two events or conditions are partially similar, they must be similar in other (or all) respects. Example: “ Writing a novel is like farming. The crop must be changed periodically or the earth dries up and turns to dust. Therefore, the novelist who fails to change his subject matter or take a new perspective loses all freshness and creativity.” |
| False dilemma | Concludes from the presentation of two opposing points of view that no alternatives are available and one of the given options must be true. Example: “Either Ezra Pound was a fascist and a traitor, or he temporarily lost his mind. Because he claimed after the war to still love his country, he must have been insane.” |
| Slippery slope | Infers that the existence of one event or condition inevitably leads to another. Example: If we don’t do something about the cost of gasoline now, before you know it we’ll be paying five dollars a gallon. |
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| Argumentum ad populum (appeal to the majority) or bandwagon |
From the general premise that a given belief is widely held, concludes that it must be true. Example: “ Most scholars agree that at the age of nineteen Mary Shelley lacked the intellectual maturity to write a novel like Frankenstein, which must therefore have been written by her poet husband.” |
| Argumentum ad consequentiam (appeal to consequences) | Supports a particular conclusion because of the consequences that such a claim or belief would entail. Example: “If Percy Bysshe Shelley wrote Frankenstein, the canon of female horror writers would shrink significantly. Therefore it is vital to assert that Mary Shelley was the novel’s true author.” |
| Begging the question | The premise of an argument assumes the truth of its conclusion. Example: “Emily Dickinson’s poetry is marvelous in its simplicity. Therefore, Dickinson can be called a master of the simple poem.” |