Only experienced salespeople will be able to meet the company's selling quota. Thus, I must not count as an experienc...

warren on August 25, 2015


I can't see the difference between a & b . Please diagram this.

1 Reply

Melody on September 3, 2015

Let's diagram (A):

"Only on Fridays are employees allowed to dress casually."

P1: DC ==> F
not F ==> not DC

"Today is Friday"

P2: F

"but Hector is dressed formally."

P3: not DC

"So he must not be going to work."

C: not GW

So, the argument is mistaking it being Friday for the necessary condition, when it is actually the sufficient condition. We know that it could be Friday and someone doesn't have to dress casually at work. We merely know that if one is dressed casually work, then it is a Friday.

Let's diagram (B):

"Only music lovers take this class."

P1: TC ==> ML
not ML ==> not TC

"Hillary is not taking this class,"

P2: not TC

"she apparently does not love music."

C: not ML

This argument is mistaking not taking the class as the necessary condition, when it is actually the sufficient condition of the contrapositive. The difference between (A) and (B) is that (A) uses the two premises of "F" and "not DC," which are both necessary conditions to conclude that Hector isn't going to work, whereas (B) only uses the necessary condition from its contrapositive to conclude the sufficient condition of its contrapositive.

Hope that clears things up! Please let us know if you have any other questions.