GMAT 逻辑知识：Assumptions
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Assumptions
An assumption is an unstated premise. In other words, an assumption is a piece of evidence
that strengthens the author’s argument but that the author never sates explicitly in the stimulus.
Sufficient vs. Necessary Assumptions
Imagine that you were accused of cheating during a GMAT test. At the hearing before a panel
of disciplinary committee, only one piece of evidence is presented – your one-sentence testimony:
“I did not cheat at the GMAT exam.” After hearing your testimony, the panel concludes that
you did not cheat. You are definitely elated. But logically, the panel’s conclusion is on shaky
ground because they assume, for better or worse, that you are telling the truth! In fact there
are at least three different assumptions the panel could be making.
1. You are telling the truth.
2. You told the truth at the hearing.
3. It is possible for you to tell the truth.
The first assumption that you always tell the truth is a sufficient assumption because it
is sufficient or enough to justify the panel’s conclusion. If you always tell the truth, then
the panel must be right. However, this assumption is not a necessary assumption or an assumption
that we have to make, because the panel could still be right even if you do not always tell
the truth. As long as you told the truth at your hearing, the judge is still right. In other
words, the panel’s conclusion does not depend on the assumption that you always tell the truth.
The second assumption that you told the truth at your hearing is both a necessary and sufficient
assumption. It is sufficient because if you told the truth at the hearing, then the panel is
right; it is also necessary because if you did not tell the truth at the hearing, then the panel
is wrong. In other words, the panel’s decision depends on this assumption being true.
The third assumption that it is possible for you to tell the truth is a necessary assumption
because if it were not possible for you to tell the truth, then the panel would be wrong. However,
this assumption is not sufficient because it alone does not justify the panel’s conclusion.
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Just because you can tell the truth does not mean you actually tell the truth at the hearing.
Therefore, the panel could still be wrong, even if the last assumption is true.
As this example illustrates, sufficient assumptions typically have strong language (always,
never, none, all), while necessary assumptions typically have weak language (possible, likely,
often, many).
Sufficient Assumption
Prompts for sufficient assumptions
• Which one of the following, if assumed, would justify the conclusion?
• The author’s conclusion would be properly drawn if which one of the following was true?
Sufficient assumption prompts almost always use the word IF, while necessary assumption
prompts never use IF. Also, sufficient assumption prompts avoid qualifiers such as MOST. MOST
JUSTIFIES, for example, is a strengthen question, while JUSTIFY by itself is a sufficient
assumption question.
Before you look at the answers:
1. Pinpoint the main conclusion in the passage. (Read my previous Main Point post.)
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. Are there any jumps between the premises and the conclusion or between each premise?
To find these jumps, look for any concepts that are discussed only once. There are usually two.
The correct answer usually fills in the gap in the argument by linking those two concepts together.
Alternatively, slowly read each word of the conclusion. Which phrase was not used in the premise?
That word or phrase will likely reveal where the argument jumped to the conclusion: at the gap!
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