## Either/Or

LSAT2018
LSAT Master

Posts: 371
Joined: Wed Jan 10, 2018 1:11 am
Points: 371

I was reviewing one of your posts on conditional reasoning and would like to clarify some points:

NOT A → B
(NOT B → A)
This would mean either A or B is selected since there are three possibilities, A, B, or both A and B.

A ↔ NOT B
(B ↔ NOT A)
This would mean either A is selected without B, or else B is selected without A (either A is selected and B is not selected, or else B is selected and A is not selected) since there are two possibilities, A or B.

So I would just like to clarify the differences between this one and the one above. Would this be likened to either A or B is selected, but not both which could also be stated as either A or B is selected, but not otherwise ?
Dave Killoran
PowerScore Staff

Posts: 2780
Joined: Fri Mar 25, 2011 1:18 pm
Points: 2,781

You got it, congrats!

Of the two, the first relationship appears far more on the test than the second, just fyi.

For even more on that first rule, read: The Most Dangerous Conditional Rule on the LSAT. I post that before every LSAT on my Twitter feed, and it was quite timely this June since it appeared in one of the scored games and one of the Experimental games (which means it's coming again at some point, which is no surprise).

Thanks!
Dave Killoran
PowerScore Test Preparation
My LSAT Articles: http://blog.powerscore.com/lsat/author/dave-killoran
LSAT2018
LSAT Master

Posts: 371
Joined: Wed Jan 10, 2018 1:11 am
Points: 371

Thank you for the link on conditionals! However, as a follow-up, I would just like to ask about the either A or B, but not both statement. In the bible you combined these parts:
Not A → B
Not B → A (Contrapositive)

A → Not B
B → Not A (Contrapositive)

So wouldn't the either A or B, but not both statement be a bi-conditional statement?
A ↔ Not B
B ↔ Not A
Dianapoo

Posts: 31
Joined: Sat Sep 15, 2018 10:37 am
Points: 32

LSAT2018 wrote:Thank you for the link on conditionals! However, as a follow-up, I would just like to ask about the either A or B, but not both statement. In the bible you combined these parts:
Not A → B
Not B → A (Contrapositive)

A → Not B
B → Not A (Contrapositive)

So wouldn't the either A or B, but not both statement be a bi-conditional statement?
A ↔ Not B
B ↔ Not A

Yes, the above is correct Dave just didn't write it the way you did at the end there.