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【邏輯入門】(九)Flaw (part 1)
Prompts for Flaw questions:
• Which one of the following most accurately describes a flaw in the argument?
• The argument above is most vulnerable to criticism on the grounds that it . . .
• The reasoning in the argument is questionable because the argument . . .
Before you look at the answer:
1. Pinpoint the main conclusion in the passage. (Read my previous Main Pointpost.)
2. Separate the premises from everything else. After you find the main point, don’t assume that all the other statements are premises; they might include opposing viewpoints, background information, and concessions.
3. Ask yourself, “Do the premises, especially as they are stated in the passage, support the conclusion, especially as it is stated in the passage?” In other words, once you discard everything else, how well do the premises support the conclusion? The reason you want to focus on exactly what the premises and the conclusion state is that you do not want to subconsciously make the argument better than it actually is. Do not help the author. Look at what he actually said and then decide whether his evidence stacks up. (See “weaken” questions)
Make sure you are not reading anything into the conclusion that is not there. (“Thus, there is no evidence that Mar has life” is very different from “Thus, there is no life on Mars.”)
Wrong answers will often describe a flaw for a conclusion that the argument never actually reached. (For the conclusion saying that there is no evidence of life on Mars, the answer choice which states the argument “presumes, without providing justification, that the lack of evidence for a claim proves that the claim is false” would be wrong because the argument does not conclude that there is no life on Mars –only that there is no evidence of life on Mars.)
4. In your own words, describe the flaw. Try to avoid looking at the answers until you have forced yourself to describe at least one flaw or weakness in the stimulus.
Then look for the answer that most accurately describes what youdescribed.
1. Focus on the active clause of each answer to help yourself move through the answers faster.
2. The correct answer must describe exactly what is happening in the passage. Make sure every word of that answer correlates with some part of the passage. In other words, translate the abstract terms into concrete terms from the passage. If there is only one example in the passage, for example, the answer choice with “examples” is probably wrong.
3. Check your answer by asking yourself, “If I remove this flaw, would that fix the argument?”
Common Flaws
1. False contrapositives
• Negating both conditions without switching them.
• Switching both conditions without negating them.
• The correct answer will usually include one of these words: necessary, required, sufficient, or ensured.
If the law punishes littering, then the city has an obligation to provide trash cans. But the law does not punish littering, so the city has no such obligation.
Which one of the following exhibits a flawed pattern of reasoning most similar to that in the argument above?
(A) If today is a holiday, then the bakery will not be open. The bakery is not open for business. Thus today is a holiday.
(B) Jenny will have lots of balloons at her birthday party. There are no balloons around yet, so today is not her birthday.
(C) The new regulations will be successful only if most of the students adhere to them. Since most of the students will adhere to those regulations, the new regulations will be successful.
(D) In the event that my flight had been late, I would have missed the committee meeting. Fortunately, my flight is on time. Therefore, I will make it to the meeting.
(E) When the law is enforced, some people are jailed. But no one is in jail. So clearly the law is not enforced.
2. Causation
i) A happens before B does not mean that A causes B.
• Might be a coincidence
• Maybe C caused B
• Maybe C caused A and B
ii) A happens with B does not mean that A caused B.
• Might be a coincidence
• Maybe B caused A
• Maybe C caused B
• Maybe C caused A and B
iii) the correct answer will usually include one these words: cause or effect.
Driver: My friends say I will one day have an accident because I drive my sports car recklessly. But I have done some research, and apparently minivans and larger sedans have very low accident rates compared to sports cars. So trading my sports car in for a minivan would lower my risk of having an accident.
The reasoning in the driver's argument is most vulnerable to criticism on the grounds that this argument
(A) infers a cause from a mere correlation
(B) relies on a sample that is too narrow
(C) misinterprets evidence that a result is likely as evidence that the result is certain
(D) mistakes a condition sufficient for bringing about a result for a condition necessary for doing so
(E) relies on a sorce that is probably not well-informed
3. Attacking the author’s motive or actions, rather than the argument itself. Keep in mind that you need to focus on the logic stated in the argument, nothing else. Whether the author is angel or devil has no effect on the argument.
4. Attacking a weaker argument instead of the main conclusion: “What you are saying is that. . .”
5. Irrelevant authority
• Popular opinion. Most CDers think I am smart. Therefore, I am smart!
• Unqualified individuals. Since Yao Mng is a great athlete, if he says GMAT is a piece of cake, it probably is.
• Emotions.
6. Irrelevant premises
Ask yourself, “Do the premises strengthen or weaken the conclusion?” If not, they are irrelevant. Often premises will use the same words and thus appear relevant even though they are not.
7. Conflicting premise or assumptions
Example: “That family of four needs a dozen eggs to make a breakfast. Hua, the dad, did not buy enough eggs for his family because he bought only three eggs for each family member.”
At present the Hollywood Restaurant has only standard-height tables. However, many customers come to watch the celebrities who frequent the Hollywood, and they would prefer tall tables with stools because such seating would afford a better view of the celebrities. Moreover, diners seated on stools typically do not stay as long as diners seated at standard-height tables. Therefore, if the Hollywood replaced some of its seating with high tables and stools, its profits would increase.
The argument is vulnerable to criticism on the grounds that it gives reason to believe that it is likely that
(A) some celebrities come to the Hollywood to be seen, and so might choose to sit at the tall tables if they were available
(B) the price of meals ordered by celebrities dining at the Hollywood compensates for the longer time, if any, they spend lingering over their meals
(C) a customer of the Hollywood who would choose to sit at a tall table would be an exception to the generalization about lingering
(D) a restaurant's customers who spend less time at their meals typically order less expensive meals than those who remain at their meals longer
(E) with enough tall tables to accommodate all the Hollywood's customers interested in such seating, there would be no view except of other tall he support
8. Circular
Ask yourself, “Could the conclusion support the premises?” If so, then the argument is circular –it assumes what it is trying to prove.
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