GMAT逻辑的取非方法有特定的适用范围，且在方法的运用和对取非的理解上都有讲究。如果大家还没有太明白这种方法，就来读一下小编的分享吧。
一、GMAT逻辑取非的概念
1、逻辑中取非的定义。
取非实际是句子的主项和谓项取非。谓项已解决，是取非变不是，不是取非变是。主项的取非可用画圈法去理解。圈内为主项，则圈外为取非项。如本题，圈内为SOME，则圈外为SOME之外，可理解为：大多数。B项取非后为：那个时候，多数人吃M。如果这样，原文的结论就推不出。
2、逻辑中取非和数学中取非的区别。
大家在学习数学命题时一定都听过命题中有一种概念叫作取非，但是在逻辑解题中，对命题取非不是找命题的否命题。“部分A是B”的非命题是“全部A都不是B”。这是逻辑上的对当关系。
(详情请看 GMAT逻辑假设题型讲解 )
二、GMAT逻辑取非的应用范围
1、逻辑中取非的应用范围。
逻辑中的取非并不是在每种题型中都会用到的，它主要是应用于假设题型——这是由假设题型的特点决定的。
2、逻辑取非为何应用于假设题。
1)逻辑中assumption是结论成立的必要条件，必要条件不成立(假命题)，则结论也不成立。这就是为何ASSUMPTION的正确选项取非，结论必不成立的原因。一句话，选项取非的含义就是找选项的假命题(或者叫假判断)(因为原文是假设选项是真)
2)从逻辑上讲，假命题和真命题是矛盾关系(四种对当关系中的一种)。逻辑有两种矛盾关系：全称肯定判断和特称肯定是矛盾关系，全称否定和特称肯定是矛盾关系。
(详情请看 GMAT逻辑取非的注意事项 )
三、GMAT逻辑取非例题分析
例一：
Exports of United States wood pulp will rise considerably during this year. The reason for the rise is that the falling value of the dollar will make it cheaper for paper manufacturers in Japan and Western Europe to buy American wood pulp than to get it from any other source.
Which of the following is an assumption made in drawing the conclusion above?
美元贬值会使日本和西欧造纸商购买美国木桨而不是从其它渠道购买，所以美国木桨出口量会大幅上升。
(A) Factory output of paper products in Japan and Western Europe will increase sharply during this year.
(B) The quality of the wood pulp produced in the United States would be adequate for the purposes of Japanese and Western European paper manufacturers.
(C) Paper manufacturers in Japan and Western Europe would prefer to use wood pulp produced in the United States if cost were not a factor.
(D) Demand for paper products made in Japan and Western Europe will not increase sharply during this year.(B)
(E) Production of wood pulp by United States companies will not increase sharply during this year.
1、例题分析：
如果不考虑价格因素，日本和西欧造纸商会比较喜欢美国木桨。
关键对原文逻辑的把握。原文的逻辑是：美国货便宜，所以日本和西欧会购买。B说美国货质量必须满足要求。将它取非后，结论就推不出了(不合要求，再便宜也没用)。C的核心是J，WE更爱美国货。J，WE买美国货是因为便宜，更爱不爱无关。C很容易混，但方向偏了。其实原文只要能搭起便宜就会买的桥就可以。
2、取非的方法：(当选项的取非问题较复杂时，还是慎用，会扰乱自己的思路)
1)C的谓语是WOULD PREFER TO，更喜欢的意思，是主观问题，“取非”后变成不喜欢，还是主观问题。而原文的推理是建立在利益基础上，是客观问题。
2)取非是直言判断才能取非，假言判断没说到取非问题。
3)C的意思为：如果不考虑价格因素，J和WE更愿意买美国货。而原文却说价格便宜才买，价格是个决定因素。这个考虑角度不一样，价格是个决定因素需要不考虑价格因素这样的假设吗。
例二：
Historian: We can learn about the medical history of individuals through chemical analysis of their hair. It is likely, for example, that Isaac Newton’s psychological problems were due to mercury poisoning; traces of mercury were found in his hair. Analysis is now being done on a lock of Beethoven’s hair. Although no convincing argument has shown that Beethoven ever had a venereal disease, some people hypothesize that venereal disease caused his deafness. Since mercury was commonly ingested in Beethoven’s time to treat venereal disease, if researchers find a trace of mercury in his hair, we can conclude that this hypothesis is correct.
Which one of the following is an assumption on which the historian’s argument depends?
(A) None of the mercury introduced into the body can be eliminated.
(B) Some people in Beethoven’s time did not ingest mercury.
(C) Mercury is an effective treatment for venereal disease.
(D) Mercury poisoning can cause deafness in people with venereal disease.
(E) Beethoven suffered from psychological problems of the same severity as Newton’s.
1、正确答案：B
2、例题分析：
1)前面提到的从逻辑上讲，假命题和真命题是矛盾关系(四种对当关系中的一种)。逻辑有两种矛盾关系：全称肯定判断和特称肯定是矛盾关系，全称否定和特称肯定是矛盾关系。
2)该文中，B选项为特称否定命题，其矛盾关系的命题为：EVERYONE DO(全称肯定)
3)B取非为：那时候，每个人都吃贡。如果这样，则结论不成立。
(详情请看 GMAT逻辑例题分析 )
GMAT逻辑假设题的对于取非这一问题的理解小编就为大家简单讲解这些，取非的方法很容易扰乱大家的思路，所以如果大家很难理清思路建议采用更适合自己的方法。
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