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要确定正确的因果关系，至少要排除两类错误：一是因为两件事情是同时或先后发生就认为它们之间一定有因果关系；二是因为两件事情有紧密联系，就默认A是B的原因，而事实上B才是A的原因，即将因果倒置。在GRE写作中主要有两类因果关系方面的逻辑错误：一类是因果论述不通畅，一类是因果关系达不成。
因果论述不通畅
因果论述不通畅是指虽然一个因果关系可以达成，但是这条因果线上其实存在很多原因和结果，它只走了其中的一条线。而事实上，在没有给出足够信息的时候，不能简单地去判定一个因素是唯一的原因或最重要的原因，一个结果是唯一的结果或影响最大的结果。
具体结合例题如下（Day 14）：
The following appeared as part of an article in a business magazine.
“A recent study rating 300 male and female Mentian advertising executives according to the average number of hours they sleep per night showed an association between the amount of sleep the executives need and the success of their firms. Of the advertising firms studied, those whose executives reported needing no more than 6 hours of sleep per night had higher profit margins and faster growth. These results suggest that if a business wants to prosper, it should hire only people who need less than 6 hours of sleep per night.”
更高的利润与更快的增长这两个结果真的是由于更少的睡眠导致的吗？真的“只是”由于更少的睡眠导致的吗？它们之间的因果关系是否完整且唯一？是否有其他要素的影响？例如员工能力更强、经营策略更好、与更多知名品牌合作等。再进一步说，是否可能因果倒置：正是由于公司增长快、工作增加才导致广告经理睡眠减少呢？
因果关系达不成
因果关系达不成是指一个因果关系中的因果走不通。在前面的情况中，因果关系是可能达成的，这条线是可以走通的，而因果关系达不成的情况则是这条线彻底走不通。例如：Violence in movies has bad effect on children, therefore no one should see violent movies.（暴力
电影对儿童有负面的影响，所以所有人都不应该看暴力电影。）在此论述里，因果关系中出现了对象的不一致，前言不搭后语。暴力电影对孩子有负面的影响，那么成年人为什么也不应该看呢？
具体结合例题如下（Day 165）：
Humans arrived in the Kaliko Islands about 7,000 years ago, and within 3,000 years most of the large mammal species that had lived in the forests of the Kaliko Islands were extinct. Previous archaeological findings have suggested that early humans generally relied on both fishing and hunting for food; since archaeologists have discovered numerous sites in the Kaliko Islands where the bones of fish were discarded, it is likely that the humans also hunted the mammals.Furthermore, researchers have uncovered simple tools, such as stone knives, that could be used for hunting. The only clear explanation is that humans caused the extinction of the various mammal species through excessive hunting.
发现了被丢弃的鱼骨头，能证明当时人类可能从事捕鱼活动，但是能证明人类猎取哺乳动物吗？这之间有联系吗？有人发现哺乳动物的骨骼吗？这个因果关系能达成吗？