## LGB - Flaw Questions - Errors of conditional reasoning

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flash21

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Joined: Fri Apr 19, 2013 8:56 pm

### LGB - Flaw Questions - Errors of conditional reasoning

Hi I was wondering if someone could help explain mistaken reversals and mistaken negations for this type of question, as in what would a type of question look like that demonstrates this flaw?

I am reading through the chapter and this part seems a bit unclear to me.

It states , "The absence of an occurrence as evidence that a necessary condition for that occurrence also did not take place" , (Mistaken negation)

so this appears logically as , A--->B(slashed) (if A then not B)
and mistakenly as: B(slashed) ---> A

but how does this look in a question typically? and What do mistaken reversals look like in a question? Would appreciate the help. Thanks.

ScottRiqui

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Joined: Mon Nov 29, 2010 8:09 pm

### Re: LGB - Flaw Questions - Errors of conditional reasoning

Well, just as an example of "mistaken reversal", let's suppose I told you "If a piece of fruit is red, then it's not a banana" (R --> ~B).

You pick up an orange and say "This piece of fruit is not a banana, so it must be red". That's a mistaken reversal (~B --> R)

A mistaken negation would be if I told you "If the weather's nice tomorrow, I'll pick you up in the convertible" (WN --> C), and you interpreted that to mean if the weather is NOT nice, that I WON'T be picking you up in the convertible (~WN --> ~C). That would be incorrect; there's nothing in my promise that would prevent me from picking you up in the convertible even if the weather is lousy.

RobertGolddust

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### Re: LGB - Flaw Questions - Errors of conditional reasoning

Convertible example is logically valid but weak.

If I was a noob I would ask you why anyone would drive around a convertible in lousy weather.

The banana example is great though.

ScottRiqui

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Joined: Mon Nov 29, 2010 8:09 pm

### Re: LGB - Flaw Questions - Errors of conditional reasoning

RobertGolddust wrote:Convertible example is logically valid but weak.

If I was a noob I would ask you why anyone would drive around a convertible in lousy weather.

The banana example is great though.

I like using the convertible example because it plays on our tendency to mistakenly convert conditional "if" statements into biconditional "if and only if" statements when we're speaking or reading. Try not to read "common sense" into it - you have to look solely at the wording in the statement.

Jeffort

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Joined: Wed Jun 18, 2008 4:43 pm

### Re: LGB - Flaw Questions - Errors of conditional reasoning

ScottRiqui wrote:
RobertGolddust wrote:Convertible example is logically valid but weak.

If I was a noob I would ask you why anyone would drive around a convertible in lousy weather.

The banana example is great though.

I like using the convertible example because it plays on our tendency to mistakenly convert conditional "if" statements into biconditional "if and only if" statements when we're speaking or reading. Try not to read "common sense" into it - you have to look solely at the wording in the statement.

I like your example, especially for use to explain the difference between daily life conversational interpretations of statements vs. proper logical LSAT interpretation for making valid inferences and being aware of possibilities logically left open by statements.

Lots of people score poorly on the LSAT in part due to applying intended/implied conversational meanings of phrases instead of the strict literal and logical interpretation when answering questions. The initial trouble many students have with understanding conditional logic/reasoning and applying it properly in LR questions is because the true logical meaning/impact of many common phrasings of conditional statements is different than what most people would treat the same phrase to mean/imply when used in everyday conversation.

Getting people to see and understand the difference is typically a big breakthrough lightbulb turning on moment in LSAT prep that leads to much of what is going on in LR questions making a heck of a lot more sense, so it's important to bring up the topic and highlight it when teaching or tutoring others.

flash21

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Joined: Fri Apr 19, 2013 8:56 pm

### Re: LGB - Flaw Questions - Errors of conditional reasoning

Thanks guys, appreciate the help

foggynotion

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Joined: Sun Nov 29, 2009 4:19 am

### Re: LGB - Flaw Questions - Errors of conditional reasoning

Jeffort wrote:
ScottRiqui wrote:
RobertGolddust wrote:Convertible example is logically valid but weak.

If I was a noob I would ask you why anyone would drive around a convertible in lousy weather.

The banana example is great though.

I like using the convertible example because it plays on our tendency to mistakenly convert conditional "if" statements into biconditional "if and only if" statements when we're speaking or reading. Try not to read "common sense" into it - you have to look solely at the wording in the statement.

I like your example, especially for use to explain the difference between daily life conversational interpretations of statements vs. proper logical LSAT interpretation for making valid inferences and being aware of possibilities logically left open by statements.

I also like the convertible example, for the reasons Jeffort mentioned. Not only don't we know what the driver will do if the weather is not nice, but it seems somehow to be implied that the convertible's top will be down--yet it never says this! That's just an assumption a reader might make, because that's what someone who talks about the weather in relation to a convertible would probably mean. But it's never specified in the statement. If the driver leaves the top up, then certainly it wouldn't matter very much what the weather was like!

Clearly

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### Re: LGB - Flaw Questions - Errors of conditional reasoning

RobertGolddust wrote:Convertible example is logically valid but weak.

If I was a noob I would ask you why anyone would drive around a convertible in lousy weather.

The banana example is great though.

lol are you serious?