Please clarify "unless"

delusional
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Please clarify "unless"

Postby delusional » Tue Nov 30, 2010 11:24 pm

A simple question, I think. Is the following correct?

N cannot be included unless P is included ------------- means that if P is included, N now has the possibility of being included.

J is not included unless K is included ------------- means that if K is included, J must now be included.

ohlawl
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Re: Please clarify "unless"

Postby ohlawl » Tue Nov 30, 2010 11:28 pm

Unless = If not.

If not K then not J. Contrapositive: If J then K.
J may or may not be included if you only know K.

Granted I took the test a few months ago.

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Sh@keNb@ke
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Re: Please clarify "unless"

Postby Sh@keNb@ke » Tue Nov 30, 2010 11:30 pm

delusional wrote:A simple question, I think. Is the following correct?

N cannot be included unless P is included ------------- means that if P is included, N now has the possibility of being included.

J is not included unless K is included ------------- means that if K is included, J must now be included.


The proper diagramming for this is: N ----> P and J ----> K.

If N is included, P is included and if J is included, K is included.

I had this problem with unless when I was first learning the material. The way I think about it is just negate the first part of the statement and take out the unless. Hope this helps!
Last edited by Sh@keNb@ke on Tue Nov 30, 2010 11:31 pm, edited 1 time in total.

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luckyme
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Re: Please clarify "unless"

Postby luckyme » Tue Nov 30, 2010 11:31 pm

ohlawl wrote:Unless = If not.

If not K then not J. Contrapositive: If J then K.
J may or may not be included if you only know K.

Granted I took the test a few months ago.


this is correct

Kaitlyn
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Re: Please clarify "unless"

Postby Kaitlyn » Tue Nov 30, 2010 11:34 pm

I think of it the way it's explained in the Powerscore Bible. That is- anything that comes after unless is the necessary condition. And everything that comes before unless is negated and becomes the sufficient condition. So I would take your first example to mean: If N is included, P must be included. And your second example: If J is included, K must be included.

ETA: Was writing this up before the two posts above were posted.

godofcoffee
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Re: Please clarify "unless"

Postby godofcoffee » Tue Nov 30, 2010 11:37 pm

It might be helpful to know that "unless" is logically equivalent to "or" (technically, "inclusive or" rather than "exclusive or").

"X or Y" is, in turn, logically equivalent to "not X then Y", which has been mentioned already.

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JazzOne
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Re: Please clarify "unless"

Postby JazzOne » Wed Dec 01, 2010 12:55 am

ohlawl wrote:Unless = If not

~P -> ~N
N -> P

~K -> ~J
J -> K

benito
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Re: Please clarify "unless"

Postby benito » Wed Dec 01, 2010 1:27 am

i think its funny how the LSAT twists our minds......in a normal conversation if someone was saying unless this then that we'd have no problem immediately understanding what they mean, in fact 99% of the time you would just infer the meaning automatically without any thought.

delusional
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Re: Please clarify "unless"

Postby delusional » Wed Dec 01, 2010 9:57 am

I think I seem to remember places where it was obvious that "unless" made everything necessary, i.e. "X is not there unless Y is" would have been diagrammed as X <------------> Y. Meaning they each must be there if the other is, and neither can be there alone.

Is that wrong?

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JazzOne
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Re: Please clarify "unless"

Postby JazzOne » Wed Dec 01, 2010 10:24 am

delusional wrote:I think I seem to remember places where it was obvious that "unless" made everything necessary, i.e. "X is not there unless Y is" would have been diagrammed as X <------------> Y. Meaning they each must be there if the other is, and neither can be there alone.

Is that wrong?

That's definitely not right. The correct diagram for "X is not there unless Y is" would be:
~Y -> ~X
X -> Y

The word unless does not "make everything necessary." Unless means "if not."

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Richie Tenenbaum
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Re: Please clarify "unless"

Postby Richie Tenenbaum » Wed Dec 01, 2010 10:32 am

JazzOne wrote:
delusional wrote:I think I seem to remember places where it was obvious that "unless" made everything necessary, i.e. "X is not there unless Y is" would have been diagrammed as X <------------> Y. Meaning they each must be there if the other is, and neither can be there alone.

Is that wrong?

That's definitely not right. The correct diagram for "X is not there unless Y is" would be:
~Y -> ~X
X -> Y

The word unless does not "make everything necessary." Unless means "if not."


Another way to do it that can make things easier if the form is NOT...UNLESS is negate the sufficient clause and make unless=then.

Thus for "No X unless Y", instead of going to negatives first (~Y -> ~ X), you can go X -> Y.

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LSAT Blog
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Re: Please clarify "unless"

Postby LSAT Blog » Wed Dec 01, 2010 12:00 pm

I prefer the way Richie described, but the "if not" method is also correct.

I've found that the "if not" method is more likely to result in statements containing negative variables than the other method is.




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