A Question about Flaw Questions with a Mistaken Reversal or Mistaken Negation (example fr. PT 80 LR)

[CharlesPasselius]

I have a question about how to identify the right answer choice and whether I'm reading things correctly. It seems to me that the correct answer choice for flaw stimulus with either a "Mistaken Reversal" or a "Mistaken Negation" is the same in either case. Is this true or is there ever a distinction? I will give my example below.

I did PT 80 last week. While doing my blind review, I compared two questions, one from each LR section. The first was section 1 question 13. The second was section 4 question 16. They have a similar formal error in reasoning, but not the same one. Yet, the correct answer's language was almost the same.

1-13 is structured something like
P1: purpose -> basis
P2: /purpose
SC: Therefore /basis
C: Therefore, existing laws have legitimacy qua laws.

The portion with the logical fallacy is:
A -> B
/A
Therefore, /B

This is Denying the Antecedent or, in PowerScore terminology, a "Mistaken Negation." The correct answer choice reads, "takes a sufficient condition for a state of affairs to be a necessary condition for it." (aka 'mistakes a sufficient for a necessary condition')


4-18 is structured more simply
Enter -> receive (including t-shirt)
receive
Therefore, entered

OR

A -> B
B
Therefore, A.

This is a case of Affirming the Consequent or, in PowerScore terminology, a "Mistaken Reversal." Yet, the correct answer choice seems to be the same in substance, namely, "Takes a condition that is sufficient for a particular outcome as one that is necessary for that outcome." (aka 'mistakes a sufficient for a necessary condition')

On my actual take, I recognized this and chose "B." But when I went back over the test during my blind review, I realized that I had put the same answer choice for two different formal fallacies (similar though they are). So, I thought that B could not be right on that basis. So, am I right in thinking that the same answer choice is used for both types of formal fallacy? Or is there another way to reconcile this? Thanks in advance.

[CharlesPasselius]

Quick update/edit: I checked the LR Bible to see if it addresses this issue. It does indeed, but the correct answer choices given are divided into two types for two different errors "Confuses a necessary condition for a sufficient condition" and "Confuses a sufficient condition for a necessary condition." However, in the margin it says "Remember, a Mistaken Negation and a Mistaken Reversal are contrapositives of each other, so the error behind both is identical." Could someone demonstrate how this is true? I cannot see it myself.

[Dave Killoran]

Hi Charles,

Thanks for the question! I'm short on time and so I will focus on your second post.

Both a MN and a MR trade on the same basic mistake: confusing something sufficient for necessary. The LSAT tends to look at the specific route of the error (either MR or MN) and then use language that more directly describes that error, which is why you see that distinction in the book. that doesn't change that the underlying error traces to the same idea.

Here how it works:

• Original statement:
  
  
  • A B
  
  And now the MN and MR of that original statement:
  
  
  • MN: A B
    
    MR: B A

If you then isolate just the MN and MR, you can see they are contrapositives of each other, so functionally identical in meaning.

Thanks!

[CharlesPasselius]

Thank you for explaining that , Dave. It is very clear now.