PT 69, S4 Q6; negative conditional reasoning? Forum

[drumstickies]

I thought I understood conditional reasoning pretty well until I got this question wrong.

It says something along the lines of "unless he or she in an accountant or his or her membership is unanimously supported, he or she cannot be on the finance committee."

So, when I diagrammed it, it was--

~accountant OR ~unanimous --> ~finance committee

Since OR in the sufficient means that either will trigger the necessary, I thought A was correct. I got it wrong. Does the OR rule change when the sufficient conditions are negated?

[ScottRiqui]

I thought I understood conditional reasoning pretty well until I got this question wrong.

It says something along the lines of "unless he or she in an accountant or his or her membership is unanimously supported, he or she cannot be on the finance committee."

So, when I diagrammed it, it was--

~accountant OR ~unanimous --> ~finance committee

Since OR in the sufficient means that either will trigger the necessary, I thought A was correct. I got it wrong. Does the OR rule change when the sufficient conditions are negated?

Your diagram should have been:

~accountant AND ~unanimous --> ~finance committee

If she is on the finance committee, then either she's an accountant, or her membership was supported unanimously:

finance committee --> accountant OR unanimous

Taking the contrapositive (using DeMorgan's Rule), we get

~accountant AND ~unanimous --> ~finance committee

[drumstickies]

Thanks for the response. You're absolutely right. I do want to push back, though, in order to correct my misunderstanding.

MLSAT says that "unless" and "except perhaps" should be diagrammed as "if not" (page 347). So, using MLSAT's strategy, shouldn't it have been diagrammed ~accountant OR ~unanimous --> ~finance committee?

How do you normally diagram "unless"?

[ScottRiqui]

Thanks for the response. You're absolutely right. I do want to push back, though, in order to correct my misunderstanding.

MLSAT says that "unless" and "except perhaps" should be diagrammed as "if not" (page 347). So, using MLSAT's strategy, shouldn't it have been diagrammed ~accountant OR ~unanimous --> ~finance committee?

How do you normally diagram "unless"?

I generally just "plain English / common sense" it. If a statement says "no one is happy unless they're healthy", then the consequence is that if someone is happy, they could only get that way by being healthy:

happy --> healthy

I guess if you want a formal rule, it would be " The term 'unless' introduces a necessary condition."

[sashafierce]

Thanks for the response. You're absolutely right. I do want to push back, though, in order to correct my misunderstanding.

MLSAT says that "unless" and "except perhaps" should be diagrammed as "if not" (page 347). So, using MLSAT's strategy, shouldn't it have been diagrammed ~accountant OR ~unanimous --> ~finance committee?

How do you normally diagram "unless"?

I generally just "plain English / common sense" it. If a statement says "no one is happy unless they're healthy", then the consequence is that if someone is happy, they could only get that way by being healthy:

happy --> healthy

I guess if you want a formal rule, it would be " The term 'unless' introduces a necessary condition."

And then you make the other part of the statement the sufficient condition and negate it.

Steve from the LSAT blog explains both method really well:

http://lsatblog.blogspot.com/2009/11/wo ... -mean.html