Please clarify "unless" Forum

[delusional]

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]

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.

[Sh@keNb@ke]

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!

[luckyme]

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]

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]

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.

[JazzOne]

Unless = If not

~P -> ~N
N -> P

~K -> ~J
J -> K

[benito]

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]

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?

[JazzOne]

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."

[Richie Tenenbaum]

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.

[LSAT Blog]

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.