Help with this logic!

sangr
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Help with this logic!

Postby sangr » Sat Feb 20, 2010 10:41 am

Hey guys i understand why this logic could be right, but its kind of tricky to see why a certain answer choice would be wrong.

here it is:

if john goes to school, then he is a good boy.

Unfortunately, john does not go to school, thus he is not a good boy.

the argument's reasoning is flawed b/c:

this is wrong because the author assumes that not going to school prevents him from being a good boy.
mistaken negation
Right.

but heres another AC:

ignores the possibility that if john is not a good boy, then he does not go to school.

this is the contrapositive of the first premise.

why exactly is this wrong? is it because the author's error is more of an incorrect presumption, rather than ignoring the first premise? or does his conclusion not go against it?

eudaimondaimon
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Re: Help with this logic!

Postby eudaimondaimon » Sat Feb 20, 2010 11:19 am

sangr wrote:Hey guys i understand why this logic could be right, but its kind of tricky to see why a certain answer choice would be wrong.

here it is:

if john goes to school, then he is a good boy.

Unfortunately, john does not go to school, thus he is not a good boy.

the argument's reasoning is flawed b/c:

this is wrong because the author assumes that not going to school prevents him from being a good boy.
mistaken negation
Right.

but heres another AC:

ignores the possibility that if john is not a good boy, then he does not go to school.

this is the contrapositive of the first premise.

why exactly is this wrong? is it because the author's error is more of an incorrect presumption, rather than ignoring the first premise? or does his conclusion not go against it?


While the author does in fact neglect to mention the correct contrapositive, that is not why the argument is flawed. Just because an answer choice contains a true statement does not mean it answers the question.
For all intents and purposes that choice could read "Some people say cucumbers taste better pickled;" while that is true it is still wrong.

In other words, while (false statement) ==> (wrong answer), not(false statement) =/=> not(wrong answer).

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TheLuckyOne
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Re: Help with this logic!

Postby TheLuckyOne » Sat Feb 20, 2010 11:22 am

Whatever... I've just accidentally deleted my post... :roll:
Last edited by TheLuckyOne on Sat Feb 20, 2010 11:29 am, edited 3 times in total.

Shrimps
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Re: Help with this logic!

Postby Shrimps » Sat Feb 20, 2010 11:25 am

sangr wrote:but heres another AC:

ignores the possibility that if john is not a good boy, then he does not go to school..

this is the contrapositive of the first premise.

why exactly is this wrong? is it because the author's error is more of an incorrect presumption, rather than ignoring the first premise? or does his conclusion not go against it?


His reasoning led him to the conclusion that John is not a good boy. He doesn't in any way ignore "the possibility that if John is not a good boy, then he does not go to school", in fact, this statement cannot disprove his flawed reasoning.

Remember, you're asked to find flaw in his reasoning. His reasoning in no way depended on finding the right contrapositive. It's just a mistaken negation. Just because one of the answer choices has correct information, does not mean it's a correct choice. You need to ask yourself whether it is relevant to the question. It's particularly important with "reasoning" questions, where many seemingly correct answers will question the premises or even present an occasional valid sounding ad hominem attack, and so on. Remember, test makers love to throw them at you. If they ask for a flaw in reasoning, or an assumption - always be careful to weed out plausible but irrelevant answer choices. You're attacking nothing but the reasoning.

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TheLuckyOne
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Re: Help with this logic!

Postby TheLuckyOne » Sat Feb 20, 2010 11:37 am

Attempt #2.

S --> G. In other words, every boy that goes to school is good, BUT that does NOT mean that those who decided to skip school and became, say, monks are bad. Here's our flaw.

Now, -G --> -S. This is in fact contrapositive -> another way of saying the same idea. The author cannot possibly assume /neglect to assume something he explicitly stated. Orginal claim = contrapositive.

Looks like you do not have a clear picture what you're looking for in flaw questions. You seek info that the author overlooked, took for granted, not airtight logic etc. You are looking for a gap/problem with the argument that provides you a way to attack/weaken it.

sangr
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Re: Help with this logic!

Postby sangr » Sat Feb 20, 2010 11:56 am

thanks for teh replies.

i mean i dont know, when i saw this i knew for sure what the correct answer was.

its just that when i saw that other answer choice i couldnt pinpoint exactly why it was wrong.

thanks!

sangr
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Joined: Sun Jun 07, 2009 4:45 pm

Re: Help with this logic!

Postby sangr » Sun Feb 21, 2010 2:32 am

help with one more ? this one is stumping me. i could choose the right answer because the others were wrong but
i dont understand anymore.

stimulus states: a--> b

thus NOT a --> NOT b

ok.mistaken negation.

why is this argument flawed?

correct answer. because it overlooks that there is the possibility that there are A's that are not Bs.
doesnt this completely go against the original statement that all a's are b's
what the heck?
can you guys please clear htis up? thanks

tomwatts
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Re: Help with this logic!

Postby tomwatts » Sun Feb 21, 2010 4:20 am

sangr wrote:correct answer. because it overlooks that there is the possibility that there are A's that are not Bs.
doesnt this completely go against the original statement that all a's are b's

At first glance, I'd say you're correct. Where is this from?

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autarkh
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Re: Help with this logic!

Postby autarkh » Sun Feb 21, 2010 5:19 am

"If john goes to school, then he is a good boy.

Unfortunately, john does not go to school, thus he is not a good boy."

Diagram:

John Goes To School ---> Good Boy
Therefore:
[strike]John Goes To School[/strike] ---> [strike]Good Boy[/strike]
(INVALID)

This is the “fallacy of the inverse.”

A conditional statement does not imply its inverse, or opposite.

John going to school is sufficient to guarantee that he is a good boy, but it is not necessary. Thus, the fact that he does not go to school does not exclude the possibility of him being a good boy for other reasons, or that he is just inherently a good boy. All the original condition tells us is that if he is not a good boy, then he certainly does not go to school.

In other words: a good boy may not go to school, but a bad boy definitely does not go to school. Going to school is "enough" to make him a good boy, but not going to school is not enough to make him a bad boy.

thunder 85
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Re: Help with this logic!

Postby thunder 85 » Wed Feb 24, 2010 12:16 am

sangr wrote:help with one more ? this one is stumping me. i could choose the right answer because the others were wrong but
i dont understand anymore.

stimulus states: a--> b

thus NOT a --> NOT b

ok.mistaken negation.

why is this argument flawed?

correct answer. because it overlooks that there is the possibility that there are A's that are not Bs.
doesnt this completely go against the original statement that all a's are b's
what the heck?
can you guys please clear htis up? thanks



Are you sure you translated that correctly? Put up the original language, especially if it uses an "unless" or something.

thunder 85
Posts: 14
Joined: Wed Nov 04, 2009 5:17 pm

Re: Help with this logic!

Postby thunder 85 » Wed Feb 24, 2010 12:19 am

sangr wrote:Hey guys i understand why this logic could be right, but its kind of tricky to see why a certain answer choice would be wrong.

here it is:

if john goes to school, then he is a good boy.

Unfortunately, john does not go to school, thus he is not a good boy.

the argument's reasoning is flawed b/c:

this is wrong because the author assumes that not going to school prevents him from being a good boy.
mistaken negation
Right.

but heres another AC:

ignores the possibility that if john is not a good boy, then he does not go to school.

this is the contrapositive of the first premise.

why exactly is this wrong? is it because the author's error is more of an incorrect presumption, rather than ignoring the first premise? or does his conclusion not go against it?



To put this simply.

In this question you are given a premise and a conclusion. The premise is taken for granted as truth, the only flaw can be in the conclusion, which must be derived from the premise.

The contrapositive of the premise is simply restating the premise, it will never be the flaw.

sangr
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Re: Help with this logic!

Postby sangr » Wed Feb 24, 2010 8:59 am

yeap i know.

restating the contrapositive of the premise is not right.

which is exactly why im asking this.

the answer choice says it IGNORES the premise.

hes ignoring this and saying the wrong thing?

no?

i do understand that the real answer choice is a MUCH better choice either way.

but i still dont get a firm grip on why this answer choice is WRONG.

it says it "ignores the possibility of the contrapositive of the premise"

so far the explanations have been telling me why restating the contrapositive is not wrong, which i know the contrapositive is not going against the stimulus. is it wrong simply because the main mistake is that he just did the mistaken? so nothing else really matters since this mistake is SO apparent?

Shrimps
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Re: Help with this logic!

Postby Shrimps » Wed Feb 24, 2010 11:30 am

Good God, Sangr.

The guy's conclusion is "John is NOT a good boy".

Does this conclusion ignore the possibility that if "John is NOT a good boy, he does not go to school"?

No, it doesn't. The contrapositive leads to the premise that John, possibly, is NOT a good boy, and thus would in no way disprove the author's faulty reasoning (he would only say that John is not possibly, but definitely NOT a good boy - but this does not violate the contrapositive).

The contrapositive does NOT disprove the author. It plays right into his hands.

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MysticalWheel
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Re: Help with this logic!

Postby MysticalWheel » Wed Feb 24, 2010 6:28 pm

Hi Sangr; the reason I can give you for why the AC "Ignores the possibility that if john is not a good boy, then he does not go to school" is incorrect is because it is automatically implied by means of the original conditional statement, i.e., it is that statement's contrapositive, as our colleagues have stated above. This means that it goes hand in hand with the original statement. Given this, there are 2 possibilities we can consider:

1.) The person making the conclusion DOES NOT take account of the contrapositive, as the incorrect AC states, or
2.) The person making the conclusion DOES take account of the contrapositive.

Let's consider #1: If the person making the conclusion based on the original conditional statement DOES NOT take account of the contrapositive, does that change the logic of the original conditional statement? No, it does not, because the original conditional statement says "If school, then good boy," which does not exclude the possibility of other things being sufficient for John to be good. For example, imagine the statement "If John eats apples, he is a good boy." It does not say anything about school, yet if John eats apples, regardless of school attendance, he is good, according to the statement. We cannot discount such possibilities because nowhere in the premise is it stated that going to school is the ONLY condition or a NECESSARY condition for being good. It is simply something that satisfies something else, i.e., school attendance satisfies the attainment of a characteristic of goodness (reference the post by autarkh above for a another look at the differences between sufficient and necessary and how they apply to this example). Lastly, since not taking account of the contrapositive, or 'ignoring' the contrapositive (assuming this can even be technically accomplished since the contrapositive is implied in the original conditional statement, but putting this aside for sake of argument) does not REMEDY the error in logic that the person making the conclusion has made, it becomes inconsequential to the matter. But, the AC says that the error was made BECAUSE of ignorance, so to fully discount and explain this AC, point #2 above needs examination.

Let's consider #2: If the person making the conclusion DOES take account of the contrapositive, and this somehow results in the incorrect conclusion he/she originally made to be retracted, then the AC "ignores the possibility that if john is not a good boy, then he does not go to school" would have merit. Let's see what happens. The contrapositive once again is implied in the original conditional statement. We have the original, which is "If school, then good boy," and we have the contrapositive, which is "If not good boy, then not school." Notice the 1st clause of each of these statements: "If school" and "If not good boy." These clauses mean that you have definitive information about consequence ONLY regarding school attendance (the first if statement from the original) and not being a good boy (the second if statement from the contrapositive). But the person making the conclusion concludes the following: "John does not go to school, thus he is not a good boy." The 'thus' communicates that the person making the conclusion has derived the part "he is not a good boy" from the information "John does not go to school." BUT, we ONLY have definitive information about consequence on the 2 things stated above, i.e., if John goes to school or if John is not a good boy. We do NOT have information about what happens when "John does NOT go to school," therefore we CANNOT conclude ANYTHING asides from a tautology (i.e., the same thing: that John does NOT go to school) from it. Hence, even when explicitly accounting FOR the contrapositive, the incorrect conclusion made by the person (i.e., 'flawed', as the LSAT wording goes) is NOT CORRECTED, i.e., it is still wrong, meaning that consideration or inconsideration of the contrapositive is not the reason why the reasoning of the argument is flawed. Regardless of whether the person making the conclusion considered only the original conditional statement explicitly or if he considered both the original and contrapositive of that original explicitly, the conclusion would STILL be wrong. Therefore, once again, consideration of the contrapositive had no effect on the flaw in the reasoning.

-MW




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