The logical function of an "or else" statement

User avatar
sdwarrior403
Posts: 114
Joined: Fri Aug 10, 2012 8:13 pm

The logical function of an "or else" statement

Postby sdwarrior403 » Wed Aug 29, 2012 4:58 pm

I understand that A or B means both could happen.

However, what about the statement A or else B? Does this have a differing function in logic? Does this imply that we have a situation of one of A and B being selected and one of A and B not being selected?

My opinion is that A or else B functions the same as a simple or statement.

I would really appreciate the input of lsat tutors on this forum.

User avatar
sdwarrior403
Posts: 114
Joined: Fri Aug 10, 2012 8:13 pm

Re: The logical function of an "or else" statement

Postby sdwarrior403 » Wed Aug 29, 2012 9:07 pm

Bump

kaiser
Posts: 2940
Joined: Mon May 09, 2011 11:34 pm

Re: The logical function of an "or else" statement

Postby kaiser » Wed Aug 29, 2012 9:10 pm

.
Last edited by kaiser on Wed Aug 29, 2012 9:20 pm, edited 1 time in total.

User avatar
zanda
Posts: 526
Joined: Fri Apr 24, 2009 9:36 am

Re: The logical function of an "or else" statement

Postby zanda » Wed Aug 29, 2012 9:17 pm

If not A, then B.


FWIW, when I was studying for the LSAT I didn't memorize lists of logical connectors for each potential formal expression, but found it was easier to just think about what the words mean. That way if they throw something different at you on the LSAT you're still fine.


EDIT- if not A, then B is equivalent to A or B.

User avatar
sdwarrior403
Posts: 114
Joined: Fri Aug 10, 2012 8:13 pm

Re: The logical function of an "or else" statement

Postby sdwarrior403 » Wed Aug 29, 2012 10:55 pm

Yeah so what you just described above is the exact function of an or statement. So I suppose you are saying that an or else statement is the same as an or statement.

TylerJonesMPLS
Posts: 74
Joined: Wed Jun 20, 2012 11:20 pm

Re: The logical function of an "or else" statement

Postby TylerJonesMPLS » Thu Aug 30, 2012 4:33 am

I googled for 'or else' as logically equivalent to exclusive or, and I was surprised by how little I found. But, in a discussion about the article, "Thinking about Thinking," (in the Princeton Alumni magazine) I found a reference to 'or else' being often understood as exclusive or.

http://www.princeton.edu/paw/web_exclusives/more/more_letters/letters_thinkingabout.html

Here's the quotation:

"The first interpretation, which has been selected by the professor, relies upon the fact that the words "or else" are often used to denote what is referred to in logic as an "exclusive or,"…"

You will find it in the first paragraph of the second letter.

Since the quotation only says 'or else' often is used as exclusive or, your interpretation of 'or else' as an inclusive or is perfectly OK.

However, I do think that the context in PT 21.2.20 indicates that 'or else' means exclusive or in that particular passage. I'll go into why I think so in the 21.2.20 thread.

User avatar
JazzOne
Posts: 2938
Joined: Tue Sep 09, 2008 11:04 am

Re: The logical function of an "or else" statement

Postby JazzOne » Fri Aug 31, 2012 9:42 am

zanda wrote:If not A, then B.


FWIW, when I was studying for the LSAT I didn't memorize lists of logical connectors for each potential formal expression, but found it was easier to just think about what the words mean. That way if they throw something different at you on the LSAT you're still fine.


EDIT- if not A, then B is equivalent to A or B.

Congratulations. That was completely unhelpful.

OP: I can't think of one example of "or else" from a previous LSAT. It's an interesting question though.

User avatar
zanda
Posts: 526
Joined: Fri Apr 24, 2009 9:36 am

Re: The logical function of an "or else" statement

Postby zanda » Sun Sep 02, 2012 1:05 pm

JazzOne wrote:
zanda wrote:If not A, then B.


FWIW, when I was studying for the LSAT I didn't memorize lists of logical connectors for each potential formal expression, but found it was easier to just think about what the words mean. That way if they throw something different at you on the LSAT you're still fine.


EDIT- if not A, then B is equivalent to A or B.

Congratulations. That was completely unhelpful.

OP: I can't think of one example of "or else" from a previous LSAT. It's an interesting question though.

Was my answer incorrect?

My post did 2 things.
1) Answered the Question
2) Attempted to give general advice on how to approach the LSAT that some might find useful.
The 2nd might be unhelpful for some posters, but I fail to see how answering the question asked is "completely unhelpful." Was my answer wrong? If so, perhaps you could offer what you believe is the correct answer. Were you being facetious, by quoting my helpful post, accusing it of being unhelpful, and in doing so making a completely unhelpful post yourself? Am I in for a "that's the joke" meme post? If so, clever stuff on your part.

User avatar
Yardbird
Posts: 1093
Joined: Sun Dec 04, 2011 1:45 pm

Re: The logical function of an "or else" statement

Postby Yardbird » Sun Sep 02, 2012 2:07 pm

zanda wrote:
JazzOne wrote:
zanda wrote:If not A, then B.


FWIW, when I was studying for the LSAT I didn't memorize lists of logical connectors for each potential formal expression, but found it was easier to just think about what the words mean. That way if they throw something different at you on the LSAT you're still fine.


EDIT- if not A, then B is equivalent to A or B.

Congratulations. That was completely unhelpful.

OP: I can't think of one example of "or else" from a previous LSAT. It's an interesting question though.

Was my answer incorrect?

My post did 2 things.
1) Answered the Question
2) Attempted to give general advice on how to approach the LSAT that some might find useful.
The 2nd might be unhelpful for some posters, but I fail to see how answering the question asked is "completely unhelpful." Was my answer wrong? If so, perhaps you could offer what you believe is the correct answer. Were you being facetious, by quoting my helpful post, accusing it of being unhelpful, and in doing so making a completely unhelpful post yourself? Am I in for a "that's the joke" meme post? If so, clever stuff on your part.
Your answer was roundabout and unclear: you say in your unedited post that "If not A, then B" is equivalent to "A or else B" and then at the bottom of your post in an edit you say it's also equivalent to A or B. It would have been simpler and clearer to just say "A or else B" is equivalent to "A or B" (and I'll confirm that this is correct).

For the OP, the only time you have the exclusive OR is if it says "A or B, but not both." Otherwise it's assumed that the OR is inclusive (can be one or the other or both).

User avatar
UVAIce
Posts: 442
Joined: Tue Jul 10, 2012 3:10 pm

Re: The logical function of an "or else" statement

Postby UVAIce » Sun Sep 02, 2012 2:32 pm

In computer programming an else argument is treated as if not A then B. It doesn't seem to postulate that if A there can't be B (exclusive or). So, if there is A there could be a B (can't rule it out), but if there is not an A there definitely is B.

User avatar
Yardbird
Posts: 1093
Joined: Sun Dec 04, 2011 1:45 pm

Re: The logical function of an "or else" statement

Postby Yardbird » Sun Sep 02, 2012 2:46 pm

UVAIce wrote:In computer programming an else argument is treated as if not A then B. It doesn't seem to postulate that if A there can't be B (exclusive or). So, if there is A there could be a B (can't rule it out), but if there is not an A there definitely is B.
This is the LSAT though, not a computer program. The context of the question should make it clear, and if not, the correct answer will be very obvious.

User avatar
TopHatToad
Posts: 85
Joined: Wed Nov 03, 2010 1:09 pm

Re: The logical function of an "or else" statement

Postby TopHatToad » Sun Sep 02, 2012 2:50 pm

OP, I know why you're asking this, because I remember you posting about a relatively confusing LR problem a few days ago.

In terms of strict formal logic, "A or else B" is equivalent to "if not A, then B" so you're right about that. However, the term lends itself to some ambiguity (as evidenced by the replies to this post and the last), which is why its appearance is exceedingly rare on the test. LSAC doesn't want to throw out questions, after all. For your original question, if I remember it right I think it was very clear (in common-sense terms) that the two alternatives were mutually exclusive.

Guy either goes on leave from work and comes back, OR ELSE he quits. You managed some mental acrobatics when you postulated him returning from leave and then quitting. Now technically the rules of formal logic don't preclude the possibility, but I think this is one of those cases where you lose the forest for the trees. Unlike games, LR does occasionally require such commonsense assumptions (I mean, why would the guy return just to quit again immediately?) in order to narrow the number of "correct" answer choices down to 1. Hope that helps a bit!

User avatar
zanda
Posts: 526
Joined: Fri Apr 24, 2009 9:36 am

Re: The logical function of an "or else" statement

Postby zanda » Sun Sep 02, 2012 3:04 pm

shadowofjazz wrote:
zanda wrote:
JazzOne wrote:
zanda wrote:If not A, then B.


FWIW, when I was studying for the LSAT I didn't memorize lists of logical connectors for each potential formal expression, but found it was easier to just think about what the words mean. That way if they throw something different at you on the LSAT you're still fine.


EDIT- if not A, then B is equivalent to A or B.

Congratulations. That was completely unhelpful.

OP: I can't think of one example of "or else" from a previous LSAT. It's an interesting question though.

Was my answer incorrect?

My post did 2 things.
1) Answered the Question
2) Attempted to give general advice on how to approach the LSAT that some might find useful.
The 2nd might be unhelpful for some posters, but I fail to see how answering the question asked is "completely unhelpful." Was my answer wrong? If so, perhaps you could offer what you believe is the correct answer. Were you being facetious, by quoting my helpful post, accusing it of being unhelpful, and in doing so making a completely unhelpful post yourself? Am I in for a "that's the joke" meme post? If so, clever stuff on your part.
Your answer was roundabout and unclear: you say in your unedited post that "If not A, then B" is equivalent to "A or else B" and then at the bottom of your post in an edit you say it's also equivalent to A or B. It would have been simpler and clearer to just say "A or else B" is equivalent to "A or B" (and I'll confirm that this is correct).

For the OP, the only time you have the exclusive OR is if it says "A or B, but not both." Otherwise it's assumed that the OR is inclusive (can be one or the other or both).

I see now that it was a bit unclear. What happened was I tried to figure out what "A or else B" was and came up with "If not A, then B." Only after posting did I re-read OP and see that he asked if it meant "A or B," hence my edit. That was my bad, Still, this bit of a lack of clarity didn't make my post "completely unhelpful," and thus I stand by my calling out the putz for being a putz. I mean hell, the putz didn't even say he had trouble understanding my post. Instead he added nothing at all, like the putz he apparently is. What a putz!

foggynotion
Posts: 46
Joined: Sun Nov 29, 2009 4:19 am

Re: The logical function of an "or else" statement

Postby foggynotion » Tue Sep 04, 2012 4:10 am

"or else" is equivalent to "or", which means:

A or B

is logically the same as

A or else B

which is logically the same as

~A -> B

which is logically the same as

~B -> A

which means that either A is true, or B is true, or both (ie, at least one of them is true).

In the question that was asked about (the Technocomp question), I think the answer to the poster's question is most easily seen by looking at it this way:

The conclusion was essentially, "if she quits, they found out she was offered the fellowship" (Q -> FF). The contrapositive is:

~FF -> ~Q

From the other information in the argument, we know that if Technocomp didn't find out about the fellowship, then they would allow her to take a leave of absence:

~FF -> TALA

The assumption they were looking for was something to effect of "she'll take a leave of absence if Technocomp allows her to take a leave of absence":

TALA -> LA

So if we include the proposed assumption, we would know that if Technocomp didn't find out about the fellowship, they would allow her to take a leave of absence, in which case she would take a leave of absence--and if she's taking a leave of absence, by definition, she hasn't quit (at least at that time). So, if they didn't find out about the fellowship....she didn't quit. So I don't think we're supposed to take "or else" to mean that only one option is possible, it's just that in this particular question, given what they're talking about, if one of the options is true (she takes a leave of absence), the other option can't be true (she quits) (because if you're on a leave, you still work for the company--otherwise, it wouldn't be called a leave).

Hope this helps.




Return to “LSAT Prep and Discussion Forum”

Who is online

Users browsing this forum: blackpi, davey jones, dontsaywhatyoumean, Google [Bot], jagerbom79, Majestic-12 [Bot], paragonloop and 4 guests