GRE逻辑单题是GRE verbal中一种特殊的题型，解题思路和技巧与我们的填空题型、阅读题型是完全不同的体系，需要同学们额外花时间学习的。
再加之新版GRE考试只会考察1道逻辑单题，对于目标分在V163以下的同学，不值得抽出大块的时间去复习逻辑单题。
因此我们青山学堂特别推出了【1篇解决1个逻辑单题知识点/题型】系列，帮助大家利用好碎片时间结合这一系列去解决逻辑单题~
本文讨论GRE verbal当中逻辑单题会涉及到的所有跟因果关系相关的论证如何进行分析评价。
形式：
原因C > 结果R [“>”表示“推出”]
例：万蜀黍最近很努力。所以他之后考试一定会成绩提高。
· 形式1：结果R > 原因C
· 形式2：(原因C + 结果R) > (原因C → 结果R) [“→”表示“导致”]
· 形式3：原因C > (原因C → 结果R)
削弱方式3：直接否定原因C（GRE考试当中几乎没有见过，LSAT考试当中偶见）
削弱方式4：否定原因结果之间关系（GRE考试当中几乎没有见过，LSAT考试当中偶见）
· 例1: 万蜀黍最近成绩提高了。这说明他之前肯定是努力了。
· 例2：万蜀黍前一段时间很努力，然后最近成绩提高了。说明肯定是努力让他成绩提高了。
· 例3：万蜀黍前一段时间很努力。所以肯定是努力让他最近成绩提高的。
削弱方式1：万蜀黍最近考试作弊了，之前没有；题目变简单了。
第③种论证难度相对较大，我们放到末位再谈。先看前面论证的例题。
In 1998 the United States Department of Transportation received nearly 10,000 consumer complaints about airlines; in 1999 it received over 20,000. Moreover, the number of complaints per 100,000 passengers also more than doubled. In both years the vast majority of complaints concerned flight delays, cancellations, mishandled baggage, and customer service. Clearly, therefore, despite the United States airline industry’s serious efforts to improve performance in these areas, passenger dissatisfaction with airline service increased significantly in 1999.
Which of the following, if true, most seriously weakens the argument?
A. Although the percentage of flights that arrived on time dropped slightly overall, from 77 percent in 1998 to 76 percent in 1999, some United States airlines’ 1999 on-time rate was actually better than their 1998 on-time rate.
B. The number of passengers flying on United States airlines was significantly higher in 1999 than in 1998.
C. Fewer bags per 1,000 passengers flying on United States airlines were lost or delayed in 1999 than in 1998.
D. The appearance in 1999 of many new Internet sites that relay complaints directly to the Department of Transportation has made filing a complaint about airlines much easier for consumers than ever before.
E. Although the number of consumer complaints increased for every major United States airline in 1999, for some airlines the extent of the increase was substantial, whereas for others it was extremely small.
论证思路：
解释原因的论证 —— 投诉增多 > 乘客不满增多（乘客不满增多是导致投诉增多的原因）
给出投诉增多的其它解释，投诉增多其实可能是因为投诉比以前更加方便造成的
旅客增多似乎也可以解释投诉增多，但只能解释投诉量增多，不能解释投诉率（单位旅客数中的投诉量）增多，但原文的投诉增多不仅仅是投诉量，也包括投诉率。
The average temperature of the lobster-rich waters off the coast of Foerkland has been increasing for some years. In warmer water, lobsters grow faster. In particular, lobster larvae take less time to reach the size at which they are no longer vulnerable to predation by young cod, the chief threat to their survival. Consequently, the survival rate of lobster larvae must be going up, and the lobster population in Foerkland’s coastal waters is bound to increase.
Which of the following, if true, most seriously weakens the argument?
A. There are indications that in recent years the fishing fleet operating off the coast of Foerkland has been taking cod at an unsustainably high rate.
B. The increase in water temperatures off Foerkland has not been as pronounced as the increase in average soil temperatures in Foerkland.
C. Because of their speeded-up growth, lobsters now get large enough to be legal catch before they reach reproductive maturity.
D. Even though lobsters grow faster in warmer waters, warmer waters have no effect on the maximum size to which a lobster can eventually grow.
E. Cod are a cold-water species, and the increasing water temperatures have caused a northward shift in Foerkland’s cod population.
论证思路：
预测结果的论证 —— 水温升高造成龙虾幼体更容易成年 > 龙虾的存活率将提高，以及龙虾数量将增多（幼体更容易成年是存活率提高，数量增多的原因）
给出其它干扰因素，人类的捕捞，将妨碍龙虾存活率、数量提高
我们平时进行前两种推理时其实背后都有常识的默认。我们能从努力推考好，或者反过来由考好推努力，是因为我们相信努力一般会提高成绩。如果没有默认的因果关系，我们是不会随便预测结果或推断原因的。
比如，我不可能看到万蜀黍今天考好了，就推测昨天晚上他是不是吃了米饭，因为除非我对他有什么特殊的认知，否则我们一般不认为吃米饭和考好有什么强关联。
同样，我也不会看到他最近非常努力学习，就预测他过几天会感染甲流，因为我们并不默认努力学习会有助于得甲流。
简而言之，前面的两种论证之所以会发生，是因为我们已有了某些因果关系的常识。
这就引出下一个问题：我们凭什么认为两种现象之间有因果关系？
形式：
一类现象A与一类现象B相关 > A会引起B
与先前解释原因的论证中的形式2的关键区别在于，刚才我们谈论的是两个具体事件R与C。现在的现象AB不是两个个体事件，而一定是一类事件。
这其实就是我们在科研当中建立因果关系的过程，我们发现两种现象之间有强相关性，于是我们才可能去推测之间他们有因果联系。比如，科学家最初发现酗酒人群当中的肝癌比例更高，于是我们推测酗酒会引发肝癌。
本质上，这种推理是在尝试对AB之间的相关性进行解释，从逻辑上，解释AB的相关性有三种方式：
1) A通过某种过程引起B；
2) B通过某种过程引起A；
3) 某个第三方因素同时引起AB。这些不同解释路径就为我们提供了削弱该论证的渠道。
例：高学历人群普遍收入较高，说明上了好学校有助于找到好工作获得高收入。
削弱1：有钱的家庭会增加教育投入，更愿意支持孩子进行进一步的教育。
削弱2：天生智商高的人通过各种考学，也更容易找到赚钱好工作。
People who watch television for an average of three hours per day are twice as likely to be overweight as those who watch television very rarely. It can be concluded that by watching television for as much as three hours each day, a person increases his or her chances of becoming overweight.
The argument is vulnerable to criticism on the ground that it
A. takes it for granted that the condition of being overweight predisposes overweight people to watch television
B. takes it for granted that no one who watches television more than three hours a day engages in regular physical exercise
C. overlooks the possibility that those who watch television for an average of three hours per day are physically active at work
D. overlooks the possibility that a dislike of physical exercise makes certain people more likely both to spend time watching television and to be overweight
E. overlooks the possibility that there are some people who are overweight and yet watch no television at all
论证思路：
看电视久与肥胖相关 > 看电视久引发肥胖
逻辑单题虽然鸡肋，但对于要冲刺verbal高分的同学是有必要系统性地了解会出现“哪些类题型”，以及每类题型得“本质考点和解题方法”
推荐大家去小破站上看万炜老师的《解密GRE逻辑单题》公益系列课程，详细讲解了每类逻辑单题的本质考点和相应的解题思路！也欢迎把这么优质的课程安利给周围也在备考GRE的小伙伴！