People who have never been asked to do more than they can easily do are people who never do all they can. Alex is som...

djw1117 January 18, 2014

(A) & (D)

Aren't (A) & (D) logically identical? (A) P: HD ==> KTVOC not KTVOC ==> not HD C: KTVOC ==> HD (D) P: Poly ==> CFBSL not CFBSL ==> not Poly C: CFBSL ==> Poly Both erroneously invoke the Necessary Condition to prove the existence of the Sufficient Condition (i.e., they just reverse). If that is true, and there can be only one answer choice, how can (A) or (D) be correct? Thank you, Daniel

Replies

Create a free account to read and take part in forum discussions.

Already have an account? log in

Naz January 21, 2014

Let's diagram the argument:

"People who have never been asked to do more than they can easily do are people who never do all they can."

P1: not AMCED ==> not PDAC
PDAC ==> AMCED

"Alex is someone who has clearly not done all that he is capable of doing."

P2: not PDAC

"So obviously no one has ever pushed him to do more than what comes to him easily."

C: not AMCED

You are correct in identifying that this is a flawed contrapositive. Don't just reverse! Remember to always reverse and negate. The existence of the necessary condition does not prove the sufficient condition.

Now let's look at answer choice (A):

"Anybody who has a dog knows the true value of companionship,"

P1: HD ==> KTVOC
not KTVC ==> not HD

"Alicia has demonstrated that she knows the true value of companionship."

P2: KTVOC

"Thus we can safely conclude that Alicia has a dog."

C: HD

This is the correct answer because, just like the stimulus, we have only reversed, as opposed to reversing and negating.

Answer choice (D) has a different structure.

"A polygon is any closed plan figure bounded by straight lines."

Your issue arose in diagramming the first sentence. You diagrammed it as follows:

P1: Poly ==> CFBSL
not CFBSL ==> not Poly

However, being a polygon is not the sufficient condition. We know that ANY closed plane figure bounded by straight lines is a polygon. Remember that "ANY" like "ALL" and "EACH" introduces a sufficient condition. So it is sufficient that something is a closed plane figure bounded by straight lines, for us to conclude that it is a polygon. We must diagram it like so:

P1: CFBSL ==> Poly
not Poly ==> not CFBSL

"That object pictured on the chalkboard is certainly a closed plane figure bounded by a large number of straight lines."

P2: CFBSL

"So that object pictured on the chalkboard must be a polygon."

C: Poly

This is a valid positive argument invoking the general principle.

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

djw1117 January 27, 2014

Ahh..I see. Thank you for breaking that down.

DJW

yababio May 21, 2015

Theres no video

Naz May 21, 2015

There is no need for a video explanation to this question since it has no major visual components. Please refer to the written explanation above for a breakdown of the problem.

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

sweingar April 21, 2018

@naz I'm having trouble understanding the assigned S and N conditions in P1 of the question, can you explain how you assigned them?