Logical fallacies are mistakes in reasoning. They may be intentional or unintentional, but in either case they undermine the strength of an argument. Some common fallacies are defined below.
1. Hasty Generalization: A generalization based on too little evidence, or on evidence that is biased.
Example: All men are testosterone-driven idiots. Or: After being in New York for a week, I can tell you: all New Yorkers are rude.
2. Either/Or Fallacy: Only two possibilities are presented when in fact several exist.
Example: America: love it or leave it. Or: Shut down all nuclear power plants, or watch your children and grandchildren die from radiation poisoning.
3. Non Sequitur: The conclusion does not follow logically from the premise.
Example: My teacher is pretty; I'll learn a lot from her. Or: George Bush was a war hero; he'll be willing to stand tough for America.
4. Ad Hominem: Arguing against the man instead of against the issue.
Example: We can't elect him mayor. He cheats on his wife! Or: He doesn't really believe in the first amendment. He just wants to defend his right to see porno flicks.
5. Red Herring: Distracting the audience by drawing attention to an irrelevant issue.
Example: How can he be expected to manage the company? Look at how he manages his wife! Or: Why worry about nuclear war when we're all going to die anyway?
6. Circular Reasoning: Asserting a point that has just been made. Sometimes called "begging the question."
Example: She is ignorant because she was never educated. Or: We sin because we're sinners.
7. False Analogy: Wrongly assuming that because two things are alike in some ways, they must be alike in all ways.
Example: An old grandmother's advice to her granddaughter, who is contemplating living with her boyfriend: Why should he buy the cow when he can get the milk for free?
8. Post Hoc, Ergo Propter Hoc: The mistake of assuming that, because event a is followed by event b, event a caused event b.
Example: It rained today because I washed my car. Or: The stock market fell because the Japanese are considering implementing an import tax.
9. Equivocation: Equates two meanings of the same word falsely.
Example: The end of a thing is its perfection; hence, death is the perfection of life. (The argument is fallacious because there are two different definitions of the word "end" involved in the argument.)