关于取非的一点疑问

snow_mountain

因为没有上过XDF的课，对于XDF的很多技巧都是一知半解的。特别是“取非”。不知道为什么做assumption的时候，如果有意识的用取非反而很糊涂。不知道是不是使用不当。请指导。以下题为例：

11.         When a person is under intense psychological stress, his or her cardiovascular response is the same as it is during vigorous physical exercise. Psychological stress, then, must be beneficial for the heart as is vigorous physical exercise.fficeffice" />

The argument above relies on which of the following assumptions?

(A) Exercise is an effective means of relieving psychological stress.

(B) The body’s short-term cardiovascular response to any activity indicates that activity’s long-term effect on the body.

(C) Cardiovascular response during an activity is an adequate measure of how beneficial the activity is for the heart.

(D) Psychological stress can have a positive effect on the body.

(E) Vigorous exercise is the most reliable method of maintaining a healthy heart.



这题答案是C。我选的也是，而且也很明白C为什么对。但是当我回头再看这题时，顺便对D进行取非，觉得D也能weaken. 虽然我还是认为C对。


我的想法是：假如ps cannot have a positive effect on the body. 即：PS根本就不可能有对身体良性效果，那么怎么也不可能对心脏有好处呀。问，我这种取非错在哪里呀？

睡熊

First off, think about this:



If Ps cannot have a positive effect on the body, can Ps still have a positive effect on the heart?


If you answer "NO", then this is the reasoning fallacy you commited, though I am not a medicine expert, I do know lots of medicine do good to heart while doing harm to the    nervous    or    digestive system, thus do harm to the whole body.


Now back to your question, Feifei's "Negate Option" rule applies to an assumption only when the    neaged option can thoroughly smash the argument, NOT JUST weaken it.


BTW, Feifei's "Negate Option" approach is really awkward when in practical exam, though it becomes a kinda his earmark in GMAT & LSAT cr.

[此贴子已经被作者于2004-3-20 23:59:51编辑过]

cranberry

hehe,就是搞清对身体好不一定对心脏好。如果修改为“psychological stress can do good to the heart.”对不对呢？我觉得也不对，因为这个假设已经包含着结论了，不属于我们讨论范围的假设了，如果对它取非的话，题目的结论当然不成立。

睡熊

如果修改为“psychological stress can do good to the heart.”，则这个选项一定是一个假设题的答案。假设题的答案除了包括重复原题论述以外（原文改写型），还包括原题的一个Must be型推论，原论述的逆否命题，排除它因型和它因不唯一型等等。


“因为这个假设已经包含着结论了，不属于我们讨论范围的假设了”实难苟同。

snow_mountain

谢谢楼上两位。搞清楚了取非的问题。


关于改成ps can do good to the heart. 是否算assumption, 迷惑中。

cranberry

如果假定psychological stress 是对心脏有益的，那么所谓结论就不需要任何其它的条件支持了，这也就不是此题的假设了。例如：如果假设元宵是黑的，那么元宵不是白的就不是一个结论，而只是一个推论，因为其已经包含在假设之中。因为假设是不须证明的，而如果其包含结论的话，那么结论就不需要证明了，这个逻辑就不是一个完整的判断，因而就是错误的。

snow_mountain

同意这种观点。谢谢。

睡熊

Thanks for the discussion. I agree what mentioned above in terms of how we are    reasoning in daily life, but LSAT/Gmat is not all about that. Let's    look at    some questions from LSAT and into how their keys are come out. (keys are highlighted)

CR-90.10-Section IV-    No. 13 (yellowbook 1-4-13)

                        That gadget I bought for the kitchen last week has already broken. It's just another example of the shoddy products that we are seeing more and more of these days. The thing was probably manufactured in East Golo.

                     Which one of the following is the best expression of an unstated premise that underlies the author's reasoning in the passage?
(A) If a manufacturer uses shoddy materials to make a gadget, the gadget is likely to break quickly.
(B) If a gadget breaks quickly, it was probably manufactured in East Golo.
(C) If a kitchen gadget was manufactured in East Golo, it should not be sold in this country.
(D) If everything that is manufactured in East Golo breaks quickly, then kitchen gadgets manufactured in East Golo are likely to break quickly.
(E) Nothing that is manufactured in East Golo can be expected to last more than a week

                         LSAT 2000.10-section I-No.5
In some countries, there is a free flow of information about infrastructure, agriculture, and industry, whereas in other countries, this information is controlled by a small elite. In the latter countries, the vast majority of the population is denied vital information about factors that determine their welfare. Thus, these countries are likely to experience more frequent economic crises than other countries do.

                     The conclusion follows logically if which one of the following is assumed?
(A) It is more likely that people without political power will suffer from economic crises than it is that people in power will.
(B) Economic crises become more frequent as the amount of information available to the population about factors determining its welfare decreases.
(C) In nations in which the government controls access to information about infrastructure, agriculture, and industry, economic crises are common.
(D) The higher the percentage of the population that participates in economic decisions, the better those decisions are.
(E) A small elite that controls information about infrastructure, agriculture, and industry is likely to manipulate that information for its own benefit.

                    

                         Either conclusion of the above is constituted in the assumption, respectively, which holds my opinion ...

                            

snow_mountain

谢谢睡熊。不过我认为这条题目的例子不大一样呀。作者的推理是：


PS和 physical exercise 一样产生A物质。因为physical exercise is good for heart, 所以PS 也对心脏有好处。这里应该有一个analogy. 假如我们能够说明A物质并不是说明phyisical exercise is good for heart 的标志，这个analogy 就是false analogy. 假如我们assume PS can be good to heart. 不是连整个论证都不需要了？

睡熊

thanks bryony and cranberry for your inputs. It is way off the track of the original "Negate Option" question. Though still believe what I believed, let's keep the discussion open, and I will try to put together some more examples from the real tests on a separate note in a while...nice discussion!