GMAT逻辑推理错误解析（一）

GMAT逻辑推理中的假设题型作为一类常考题型有着非常重要的意义，往往在考试时，考生却不知道应该如何有效找到可以使得整个推理成立的必要条件，或者考生往往出于使推理一定正确的心理，在各个选项中游走不定，最终导致考生无法清楚到底哪个选项才是“真正”的符合题干要求的推理条件。其实我们在解题的过程中把握一个宗旨，也许我们就会发现其实假设题型并非我们想象的那样难，这个宗旨就是：只需要找到一个能使得推理成立的条件或者说是必要条件，我们不需要去保证通过这个条件推理一定做到正确。下面就让我们通过实例对考生在假设推理的过程中容易出现的错误进行解析。

一、无法找出题干中讨论对象之间的本质关系

例：A thorough search of Edgar Allan Poe’s correspondence has turned up not a single letter in which he mentions his reputed morphine addiction. On the basis of this evidence it is safe to say that Poe’s reputation for having been a morphine addict is undeserved and that reports of his supposed addiction are untrue.

Which of the following is assumed by the argument above?

(A) Reports claiming that Poe was addicted to morphine did not begin to circulate until after his death.

(B) None of the reports of Poe’s supposed morphine addiction can be traced to individuals who actually knew Poe.

(C) Poe’s income from writing would not have been sufficient to support a morphine addiction

(D) Poe would have been unable to carry on an extensive correspondence while under the influence of morphine

(E) Fear of the consequences would not have prevented Poe from indicating in his correspondence that he was addicted to morphine.

上述题干中提到：一、研究发现Poe的信件中未提及他患有吗啡瘾；二、Poe没有吗啡瘾

本题极力在让考生从“对Poe的信件调查[证据]→Poe未患有吗啡瘾[结论]”找出两者的内在联系，5个选项中也给出了不同的假设，那么我们只需通过反推，来找出一种能够使得推理成立的假设条件即可，但考生往往会抛开“信件调查”去思考其他使得上述结论正确的条件（假设），换句话说考生往往会认为除了对信件的调查这样一个证据以外，还可以有其他证据能够推出Poe未患有吗啡瘾。通常来讲，当一个推理是从一个我们已知的，已经发生的事实推断出一个结论的时候，我们需要记住的就是，其中的隐含假设多为这个结论的唯一原因，换句话说，在本题中要想对Poe的信件做出说明，那么我们只能假定Poe不会因害怕后果而不在其信件中提及对吗啡的嗜好，本题的正确假设选项为E.

二、无因就无果

在假设题型里，如果想要找出段落推理的一个必要条件，那么我们首先就应明确，这个必要条件就是使得推理必然成立的一个假设，假设必为支持性假设。

例：

The population of peregrine falcons declined rapidly during the 1950’s and 1960’s and reached an all-time low in the early 1970’s. The decline was attributed by scientists to the widespread use of the pesticide DDT in rural areas

Which of the following, if true, gives the strongest support to the scientists’ claim?

(A) DDT was not generally in use in areas devoted to heavy industry

(B) In the time since the use of DDT was banned in 1972, the population of peregrine falcons has been steadily increasing

(C) Peregrine falcons, like other birds of prey, abandon eggs that have fallen out of the nest, even if the eggs remain intact

(D) Starlings, house sparrows, and blue jays-birds the peregrine falcon preys on were not adversely affected by DDT in their habitats

(E) Other birds of prey, such as the osprey, the bald eagle, and the brown pelican, are found in the same areas as is the peregrine falcon.

题干是由果推因，游隼数量下降由使用杀虫剂（DDT）所致，我们不妨将“游隼数量下降”视为B, 而将“使用DDT”视为A, 如果由B→A正确,那么我们就可以得出没有A就不会存在现在的B,因此我们就可以推断两种情况，DDT禁用前与DDT禁用后游隼在数量上的比较，如果在DDT禁用后，游隼数量开始增加的话，那么我们就会非常肯定的是确实是杀虫剂的使用导致了游隼数量的减少。这种类型的题目出错的主要原因有两点：一：考生无法把握题干要求，在段落内容之外进行推理；二：无法将题干给出的原因视做唯一原因。

建议：

考生应能在平时的不断练习的过程中，学习找出论点和结论的能力，并依据两者的关系进行判断，只要弄清论点和结论也就相当于找出了答案。在实践练习中，考生也应善于从题干所描述的对象中找出核心关键词语，一般来讲，我们可以通过在选项中加入Not 来判断推理成立的有效性并排除那些误选项。