Formal logic question...

klikme
Posts: 2
Joined: Tue Aug 17, 2010 10:36 am

Formal logic question...

Postby klikme » Tue Aug 17, 2010 10:48 am

I'm preparing lsat with Powerscore. Here I have a question regarding formal logic.

Some A's are B's.
No B's are C's.
All C's are D's.

Diagram : A some B <-/->(double not arrow) C -> D
Inferences : 1. A some not C
2. D some not B (according to the Bible)

Question : Isn't the inference "A some D" possible?
(For in the diagram "A some B <-/-> C" can be replaced with the inference 1 "A some not C", then this would make "A some not C -> D" and again "A some D".)

Thanks.

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Nulli Secundus
Posts: 2625
Joined: Mon Jun 21, 2010 7:19 am

Re: Formal logic question...

Postby Nulli Secundus » Tue Aug 17, 2010 11:56 am

A some not C
C -----> D

Since the second part of first inference is "not C" and first part of second inference is "C" you cannot combine them. HTH.

d34d9823
Posts: 1915
Joined: Wed Apr 14, 2010 2:52 pm

Re: Formal logic question...

Postby d34d9823 » Tue Aug 17, 2010 12:00 pm

nullisecundus wrote:A some not C
C -----> D

Since the second part of first inference is "not C" and first part of second inference is "C" you cannot combine them. HTH.

Yeah, some As being Bs doesn't preclude all As being Bs.

klikme
Posts: 2
Joined: Tue Aug 17, 2010 10:36 am

Re: Formal logic question...

Postby klikme » Wed Aug 18, 2010 7:26 am

:D Thanks guys




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