一． 循环论证定义
循环论证（又称为乞词魔术等），用来证明论题的论据本身的真实性要依靠论题来证明的逻辑错误。如证明“鸦片能催眠”，所用的论据是“它有催眠的力量”。而“鸦片有催眠的力量”，又要借助于“它能催眠”来证明。这就是犯了循环论证的错误。
这是论证谬误的一种，当辩士为支持某项主张所提供的根据，其实是同一主张换汤不换药的重复时，就是犯了“循环论证”的谬误。换句话说，在循环论证中，论证的前提就是论证的结论，因此又称为“先定结论”。
二． 两个简单例子
一个瘦子问胖子：“你为什么长的胖？”
胖子回答：“因为我吃的多。”
瘦子又问胖子：“你为什么吃的多？”
胖子回答：“因为我长的胖。”
士兵许三多：
他说：“人要做有意义的事。”可什么是有意义呢?他说：“有意义就是要好好活。”可怎样才算是好好活呢?他说：“好好活就是要做很多有意义的事。”
三． 论证结构
最简单的循环论证是以下结构的。对一些命题p：
p 蕴含 p
假设 p
所以， p成立。
可是这种结构更加常见：
p 蕴含
q
q 蕴含 r
r 蕴含 p
假设 p
所以， q成立
所以， r成立
所以， p成立。
四． GMAT题举例
1．大全-c-14
14. Dr. A: The new influenza vaccine is useless at best and possibly dangerous. I would never use it on a patient.
Dr. B: But three studies published in the Journal of Medical Associates have rated that vaccine as unusually effective.
Dr. A: The studies must have been faulty because the vaccine is worthless.
In which of the following is the reasoning most similar to that of Dr. A?
(A) Three of my patients have been harmed by that vaccine during the past three weeks, so the vaccine is unsafe.
(B) Jerrold Jersey recommends this milk, and I don’t trust Jerrold Jersey, so I won’t buy this milk.
(C) Wingzz tennis balls perform best because they are far more effective than any other tennis balls.
(D) I’m buying Vim Vitamins. Doctors recommend them more often than they recommend any other vitamins, so Vim Vitamins must be good.
(E) Since
答案为C
原文：studies说疫苗有效－－Dr. A认为studies是错的－－也就是“疫苗无效”。即studies是错的＝疫苗无效
题干中最后一句话：Dr. A：因为疫苗无效，所以studies是错的。
换言之：Dr. A：因为疫苗无效，所以疫苗无效。因就是果，果就是因，犯了循环论证的错误。
C：因为比任何都好，所以是最好的。即比任何都好 = 是最好的，因就是果，果就是因，犯了循环论证的错误。
因此，二者等同。
循环论证表现在：
因为疫苗无效，所以studies是错的。因为studies是错的，所以疫苗无效。
因为比任何都好，所以是最好的。因为是最好的，所以比任何都好。
2．大全test A-15
15. Alba: I don’t intend to vote for Senator Frank in the next election. She is not a strong supporter of the war against crime.
Tam: But Senator Frank sponsored the latest anticrime law passed by the Senate.
Alba: If Senator Frank sponsored it, it can’t be a very strong anticrime law.
Which of the following identifies the most serious logical flaw in Alba’s reasoning?
(A) The facts she presents do not support her conclusion that Senator Frank is soft on crime.
(B) She assumes without proof that crime is the most important issue in the upcoming election.
(C) She argues in a circle, using an unsupported assertion to dismiss conflicting evidence.
(D) She attacks Senator Frank on personal grounds rather than on he merit as a political leader.
(E) In deciding not to vote for Senator Frank, she fails to consider issues other than crime.
答案为C
Tam 说Senator Frank支持了一个反犯罪法律，Alba认为Senator Frank不是一个反犯罪运动的支持者——If 反犯罪法律是效果不明显（not a very strong）的，那么Senator Frank将支持它
而题干：If Senator Frank sponsored it, it can’t be a very strong anticrime law.其中，
If A,B同时If B, A，陷入循环论证
五． 判定思路
以下两点本质上是一样的：
1． 问自己，因为A，所以B中，是否可以根据以知信息得出A=B.（如大全-c-14）
2． 问自己，因为A，所以B中，是否可以根据以知信息得出因为B，所以A（如大全test A-15）
如果以上两点中任何一点是的话，则犯了循环论证的错误。
补充：今天又发现一题
大全-3-20
20. The following proposal to amend the bylaws of an organization was circulated to its members for comment.
When more than one nominee is to be named for an office, prospective nominees must consent to（同意） nomination and before giving such consent must be told who the other nominees will be.
Which of the following comments concerning the logic of the proposal is accurate if it cannot be known who the actual nominees are until prospective nominees have given their consent to be nominated?
(A) The proposal would make it possible for each of several nominees for an office to be aware of who all of the other nominees are.
(B) The proposal would widen the choice available to those choosing among the nominees.
(C) If there are several prospective nominees, the proposal would deny the last nominee equal treatment with the first.
(D) The proposal would enable a prospective nominee to withdraw from competition with a specific person without making that withdrawal known.（E）
(E) If there is more than one prospective nominee, the proposal would make it impossible for anyone to become a nominee.
以下来自携隐：
解释一下那个proposal：
当不止一个人被提名为某个office的负责人的时候，潜在的提名者必须首先同意自己被提名（然后才能成为一个被提名者），且在他说同意之前，必须告诉他其他的被提名者是谁。
问题：如果在被提名者同意之前，我们都不知道哪些人会最终被提名，那么，以下哪个答案符合原文的逻辑？
原文有一个条件：被提名者必须知道其他的提名人是谁，才能同意自己被提名。而问题中又有一个条件：在被提名者同意之前，我们不可能知道哪些人最终会被提名。也就是说，条件一中的那个条件，变成了条件二中的结果，而条件二中的结果，就是条件一中的条件，这是一个死循环，如果要同时遵循这两个条件的话，唯一的结果就是，谁也不可能被提名，因为谁也不知道其他的提名者，于是就不可能同意自己被提名。答案E正确。
以下关于循环论证部分摘自lawyer的文章：FLAW IN THE REASONING QUESTIONS
3. CIRCULAR REASONING（the premise and the conclusion are identical in meaning）
Assumes as true what is supposed to be proved.
It assumes what it seeks to establish
Argues circularly by assuming the conclusion is true in stating the premises
Presupposes the truth of what it sets out to prove
The argument assumes what it is attempting to demonstrate
It takes for granted the very claim that it sets out to establish
It offers, in place of support for its conclusion, a mere restatement of that conclusion
今天又发现一题，不过是充要条件，贴出来以区别：
大全Text-7-20:
20. One analyst predicts that Hong Kong can retain its capitalist ways after it becomes part of mainland China in 1997 as long as a capitalist Hong Kong is useful to China; that a capitalist Hong Kong will be useful to China as long as Hong Kong is prosperous; and that Hong Kong will remain prosperous as long as it retains its capitalist ways.
If the predictions above are correct, which of the following further predictions can logically be derived from them?
(A) If Hong Kong fails to stay prosperous, it will no longer remain part of mainland
(B) If
(C) If there is a world economic crisis after 1997, it will not adversely affect the economy of
(D) Hong Kong will be prosperous after 1997.（B）
(E) The citizens of Hong Kong will have no restrictions placed on them by the government of mainland
capitalist Hong Kong is useful—>Hong Kong can retain its capitalist ways
Hong Kong is prosperous—>a capitalist Hong Kong will be useful
it retains its capitalist ways—>Hong Kong will remain prosperous
综合起来：
prosperous—>retains capitalist
prosperous<—retains capitalist，即
prosperous<—>retains capitalist，互为充要条件。
所以，如果到97年为止能保持资本主义，那么HK就会持续繁荣；持续繁荣又导致能够保持资本主义。所以选B。
rpch2004 发表于 2008-5-7 23:35
补充：今天又发现一题大全-3-2020. The following proposal to amend the bylaws of an organization was c ...
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