Law School Discussion

A if, but only if, B

A if, but only if, B
« on: May 26, 2009, 06:23:03 PM »
the rule  'a if, but only if, b' was recently tested on the games section ... i didn't fully understand the rule, and given the time constraints i didn't have the luxury to stop and think about it, so i took a gamble and assumed that rule implied both 'if a then b' and 'if b then a' ... luckily, i guessed right but i never want to do this on a real LSAT

i know that 'a if, and only if, b' implies 'if a then b' as well as 'if b then a' ... so i understand this relationship ... i just don't get why 'but' in the example above has the same effect as 'and' in this example ... any insights would be helpful

here are my thoughts ... in 'a if, and only if, b' makes sense because it's both 'a if b ... if b then a' and 'a only if b ... if a then b' ... can someone break 'a if, but only if, b' down for me like that? Thanks!

gzl

Re: A if, but only if, B
« Reply #1 on: May 26, 2009, 07:07:24 PM »

here are my thoughts ... in 'a if, and only if, b' makes sense because it's both 'a if b ... if b then a' and 'a only if b ... if a then b' ... can someone break 'a if, but only if, b' down for me like that? Thanks!

Think of "but" as just giving the contrapositive of "and". "if not b then not a"

Re: A if, but only if, B
« Reply #2 on: May 26, 2009, 07:21:10 PM »
thanks gonzolaw, that's a great tip! it clarifies it a lot more ...


here's a link to what i was discussing what other folks but your tip definitely crystallizes the difference in wording ... http://www.top-law-schools.com/forums/viewtopic.php?f=6&t=73348&p=1655601#p1655601

Re: A if, but only if, B
« Reply #3 on: May 26, 2009, 10:27:38 PM »
Understand the difference between the following:

A if B.

This means that if B occurs then A will occur.  But it does not mean that if C occurs then A will not occur, or if D occurs then A will not occur, etc.  In other words, it's possible for A to occur even if B does not occur.

A if, but only if, B.

This means that if B occurs then A will occur. In this situation, though, if C were to occur then A would not occur, and if D were to occur then A would not occur, etc. In other words, it is not possible for A to occur if B does not occur.

.5L

Re: A if, but only if, B
« Reply #4 on: May 27, 2009, 02:12:40 AM »
Understand the difference between the following:

A if B.

This means that if B occurs then A will occur.  But it does not mean that if C occurs then A will not occur, or if D occurs then A will not occur, etc.  In other words, it's possible for A to occur even if B does not occur.

A if, but only if, B.

This means that if B occurs then A will occur. In this situation, though, if C were to occur then A would not occur, and if D were to occur then A would not occur, etc. In other words, it is not possible for A to occur if B does not occur.

I think OP was more confused about why "but" and "and" should be read as equivalent.

EarlCat

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Re: A if, but only if, B
« Reply #5 on: May 29, 2009, 10:19:18 PM »
"But" means "and."

/thread

Julie Fern

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Re: A if, but only if, B
« Reply #6 on: May 30, 2009, 04:13:52 AM »
how "/thread"?  there more boxes down here!

Re: A if, but only if, B
« Reply #7 on: June 04, 2009, 05:59:22 AM »
i know that 'a if, and only if, b' implies 'if a then b' as well as 'if b then a' ... so i understand this relationship ... i just don't get why 'but' in the example above has the same effect as 'and' in this example ... any insights would be helpful  here are my thoughts ... in 'a if, and only if, b' makes sense because it's both 'a if b ... if b then a' and 'a only if b ... if a then b' ... can someone break 'a if, but only if, b' down for me like that? Thanks!

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Julie Fern

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Re: A if, but only if, B
« Reply #8 on: June 04, 2009, 06:02:57 AM »
told ya.

Julie Fern

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Re: A if, but only if, B
« Reply #9 on: June 04, 2009, 09:00:39 AM »
rhetorically speaking, "but" usually suggest unexpected, much like julie.  here, it just thrown in mess with you.