Paulsville and Longtown cannot both be included in the candidate's itinerary of campaign stops. The candidate will ma...

Joseph on August 13, 2013


Can this question be explained to me. I do not see how the question is a valid argument. I went with choice A. Thank you so much

4 Replies

Mehran on August 14, 2013

Let's break down the argument...

Principle #1:
"Paulsville and Longtown cannot both be included."

P ==> not L
L ==> not P

Principle #2:
"The candidate will make a stop in Paulsville unless Salisbury is made part of the itinerary."

not P ==> S
not S ==> P

"Unfortunately, a stop in Salisbury is out of the question."

not S

"Clearly, then, a stop in Longtown can be ruled out."

not L

We know from Principle #2 that if "not S" then "P." We also know from Principle #1 that if "P" then "not L." So this is a valid transitive argument.

not S ==> P ==> not L

Let's take a look at the answer choices (A) and (B):

(A) is an invalid argument. The premise "not S" is sufficient to conclude "not R" but "not R" does not lead to anything else.

PR: R ==> not GP
GP ==> not R

PR: R ==> S
not S ==> not R

P: not S
C: not GP

(B) is correct because it exactly parallels the argument.

PR: not M ==> P
not P ==> M

PR: P ==> not C
C ==> not P

P: not M
C: not C

This is a valid transitive argument.

not M ==> P ==> not C

Hope this helps! Please let us know if you have any other questions.

Steven on February 9, 2018

I'm not seeing that all of the logic was written out in answer choice B. Where are you showing: "and Tom will not support both Parker and Chung"? I just want to make sure I am following the argument correctly.

Mehran on February 9, 2018

Hey @erojas, thanks for your post. Great question. "and Tom will not support both Parker and Chung" is diagrammed in our previous post as P - > not C (contrapositive C - > not P). If Tom supports P, he won't support C. If Tom supports C, he won't support P. Hope this helps!

Lexi on March 17 at 07:47PM

Hello, in the question, we use the contrapositive of the second Principal Rule (not S==>P) and the transitive property to come to the conclusion. In the answer choice B, we use the Transitive property but not the contrapositive of one of the Principal Rules. On the LSAT, if I came to B and saw it was valid, should I keep looking at C,D and E to see if one of them used both the transitive property and the contrapositive of a Principal Rule or could I be comfortable selecting B and moving on? Hope that makes sense, thanks!