一、如何论证中的前提假设

GRE阅读文章的本质是考察学生对于学术论文的理解能力以及批判性思考能力。所以考试中的文本全部选自于论证类学术文章。我们知道对于一篇严谨的论证类文章（Argument）包含结论（Conclusion）以及支撑它的论据（Premise）。除此之外，一篇论证中还存在假设（Assumption）即作者明确给出，并且为推断出结论的必要条件。识别和评价这些假设对于一篇论证文章至关重要。下面我们用一篇阅读文章和一篇逻辑题分别讲解。

二、阅读文本中对于假设的考察

In 1971, hot on the heels of plate-tectonic theory's acceptance, J. W. Morgan suggested that hotspots—areas of intense volcanism such as Hawaii, Yellowstone, and Iceland—are fueled by plumes of hot material arising in the deep mantle and punching through the mobile shallow mantle and crust to the surface. Morgan's theory was developed to explain the time-progressive trails of volcanoes associated with some hotspots and the hotspots' apparent fixity relative to one another. If the sources of the volcanism were rooted in the immobile deep mantle, they would not move relative to one another and the plates at the surface would drift above, bearing away trails of volcanism. According to a recent article by geologist G. R. Foulger, however, although hotspots do exist, they do not have time-progressive volcanic trails and are not fixed relative to one another.

本文开篇给出JW观点认为hotspot是由plumes of hot material加热的。第二句说这个理论解释了火山的时间序列。第三句是论证的假设，即如果火山的扎根在不动的地幔中，那么他们将不会彼此关联并且板块也会漂移，使得火山飘走。第四句给出结论说尽管hotspot存在，但是不能解释火山的时间序列。

Which of the following best describes the function of the highlighted sentence?

A. It points out some unintended consequences of a theory.

B. It elucidates the thinking behind Morgan's suggestion.

C. It explains why the theory of plate tectonics had been accepted.

D. It expresses the author 's attitude toward Morgan's suggestion.

E. It introduces an alternative to Morgan's suggested explanation.

根据文章内容，黑题字为作者论证中的假设，而假设又是得出结论的必要条件，所以本题选B—解释了观点背后的想法。

三、逻辑题中对假设的考察

Springtown’s fire department wants the town council to buy it another ladder truck. These trucks are useful in fighting fires in high-rise buildings. The town council argues, however, that Springtown has only two high-rise buildings, and it already owns enough ladder trucks to fight any fires there. So, they say, Springtown does not need another ladder truck.

The town council's argument assumes that

A. Springtown cannot afford to buy another ladder truck

B. Springtown's high-rise buildings all meet or exceed current fire safety standards

C. the number of high-rise buildings in Springtown is likely to grow

D. at least one of Springtown's ladder trucks is not due to be permanently removed from service in the immediate future

E. it is possible to fight fires successfully in high-rise buildings without using ladder trucks.

本文论证过程：

前提1：议会认为斯普林顿只有两栋高层建筑

前提2：消防局已经有足够的车扑灭火灾

结论：不需要另一辆消防车

本题正确答案为D选项：该选项取非之后为所有的消防车都退役了，所以需要买新车。这样就使原文结论不成立。证明没有该选项作为前提条件不能够得出原文结论。

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