'if AND only if' VS 'if BUT only if'

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Zeta
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'if AND only if' VS 'if BUT only if'

Postby Zeta » Wed Dec 05, 2012 3:47 am

is there any difference in the diagramming of the two?

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jump_man
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Re: 'if AND only if' VS 'if BUT only if'

Postby jump_man » Wed Dec 05, 2012 4:55 am

Zeta wrote:is there any difference in the diagramming of the two?


Why don't you try it out and see if you get to the same answer (ideally, the correct answer)

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Zeta
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Re: 'if AND only if' VS 'if BUT only if'

Postby Zeta » Wed Dec 05, 2012 5:26 am

I get the same answer.

A <--> B

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TheMostDangerousLG
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Re: 'if AND only if' VS 'if BUT only if'

Postby TheMostDangerousLG » Wed Dec 05, 2012 6:59 am

Zeta wrote:I get the same answer.

A <--> B


Maybe it would be useful if you try thinking of "if and only if" in terms of two distinct statements. So for example, "A if and only if B" can be seen as "A if B" AND "A only if B" (which is functionally equivalent to "but only if"). If you try diagramming them, you should see the difference in the relationship:

"A if and only if B" = "A if B" and "A only if B":
1. "A if B" (which is of course equivalent to "if B, then A"): B --> A
2. "A only if B": ~ B --> ~ A

And then the contrapositive of the second statement being A --> B, you can see how it combines with the first statement to form the statement you posted, A <--> B.

The difference between that and "A but only if B" is that B does not necessarily "trigger" A. You know that if you have A, you must also have B ("but only if"), but you don't necessarily know the opposite to be true. So it would just be written as: A --> B. And from there the contrapositive, ~ B --> ~ A (if not B, then you can't have A). But knowing that you have B does not imply that you have A (as in the first statement above / "A if B").

TL;DR: "A if and only if B" = A <--> B, "A but only if B" = "A --> B".

Gah, that was terribly long-winded, sorry! Middle of an all-nighter = tired rambling.

KaplanLSATInstructor
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Re: 'if AND only if' VS 'if BUT only if'

Postby KaplanLSATInstructor » Wed Dec 05, 2012 10:36 am

OP,

I just want to clarify, since I believe the above poster missed a key word in your original question. (The above poster was discussing X, but only if Y -- leaving out the crucial "if" immediately before the "but.")

If you're told "X if, and only if, Y"... this is EXACTLY the same as being told "X if, but only if, Y."

In both cases, Y is both sufficient to guarantee X (because of the "if") and necessary for X (because of the "only if"). So you get two pieces of Formal Logic for the price of one:

X --> Y
Y --> X

And, of course, both contrapositives are also valid. Some people like to use the double arrow notation (X <--> Y), but that's a personal decision.

Hope this helps.

- Chris

BeenDidThat
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Re: 'if AND only if' VS 'if BUT only if'

Postby BeenDidThat » Wed Dec 05, 2012 10:55 am

KaplanLSATInstructor wrote:OP,

I just want to clarify, since I believe the above poster missed a key word in your original question. (The above poster was discussing X, but only if Y -- leaving out the crucial "if" immediately before the "but.")

If you're told "X if, and only if, Y"... this is EXACTLY the same as being told "X if, but only if, Y."

In both cases, Y is both sufficient to guarantee X (because of the "if") and necessary for X (because of the "only if"). So you get two pieces of Formal Logic for the price of one:

X --> Y
Y --> X

And, of course, both contrapositives are also valid. Some people like to use the double arrow notation (X <--> Y), but that's a personal decision.

Hope this helps.

- Chris



^^^ what he said (want to clarify because the poster two above missed it)

"And" and "But" have the same logical content, but the word "but" serves as a prelude to the second clause of a sentence, indicating that it will qualify the first clause.

It would be very strange to use say "a gun will shoot if but only if it has a bullet in it." It should be "a gun will shoot if and only if it has a bullet in it."

Using "but" here makes little sense because there's no definitive content preceding the "but" that would need qualification, thanks to the first "if."

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scottyc66
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Re: 'if AND only if' VS 'if BUT only if'

Postby scottyc66 » Wed Dec 05, 2012 11:03 am

Zeta wrote:is there any difference in the diagramming of the two?

Tons of responses and text to a very simple question, they're the same

blackandstrong
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Re: 'if AND only if' VS 'if BUT only if'

Postby blackandstrong » Wed Dec 05, 2012 12:42 pm

scottyc66 wrote:
Zeta wrote:is there any difference in the diagramming of the two?

Tons of responses and text to a very simple question, they're the same


LOL. I was going to say that. No difference between the two.

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TheMostDangerousLG
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Re: 'if AND only if' VS 'if BUT only if'

Postby TheMostDangerousLG » Wed Dec 05, 2012 1:54 pm

BeenDidThat wrote:
KaplanLSATInstructor wrote:OP,

I just want to clarify, since I believe the above poster missed a key word in your original question. (The above poster was discussing X, but only if Y -- leaving out the crucial "if" immediately before the "but.")

If you're told "X if, and only if, Y"... this is EXACTLY the same as being told "X if, but only if, Y."

In both cases, Y is both sufficient to guarantee X (because of the "if") and necessary for X (because of the "only if"). So you get two pieces of Formal Logic for the price of one:

X --> Y
Y --> X

And, of course, both contrapositives are also valid. Some people like to use the double arrow notation (X <--> Y), but that's a personal decision.

Hope this helps.

- Chris



^^^ what he said (want to clarify because the poster two above missed it)

"And" and "But" have the same logical content, but the word "but" serves as a prelude to the second clause of a sentence, indicating that it will qualify the first clause.

It would be very strange to use say "a gun will shoot if but only if it has a bullet in it." It should be "a gun will shoot if and only if it has a bullet in it."

Using "but" here makes little sense because there's no definitive content preceding the "but" that would need qualification, thanks to the first "if."


Oh geez, you're right! Feel like an idiot; sorry for the useless post. (Shall hereby refrain from posting in the middle of the night without sufficient caffeine.)

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Zeta
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Re: 'if AND only if' VS 'if BUT only if'

Postby Zeta » Wed Dec 05, 2012 6:28 pm

i appreciate the explanations a lot. they help a lot more than just, 'they're the same.'




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