Formal logic diagramming help Forum
- risktaker
- Posts: 687
- Joined: Tue Sep 22, 2009 3:10 pm
Formal logic diagramming help
"Unless public transportation becomes much more popular, traffic will become more congested and pollution from cars will get worse."
TMCnot and PWnot----->PTMP
PTMPnot------>TMC or PW
This is what i thought the disgram should be like. The answer shows the contra positive of this as:
PTMPnot----->TMCnot and PWnot
Why does "and" not switch to "or" when the contrapositive is taken?
Any help greatly appreciated.
TMCnot and PWnot----->PTMP
PTMPnot------>TMC or PW
This is what i thought the disgram should be like. The answer shows the contra positive of this as:
PTMPnot----->TMCnot and PWnot
Why does "and" not switch to "or" when the contrapositive is taken?
Any help greatly appreciated.
- theZeigs
- Posts: 150
- Joined: Fri Nov 28, 2008 3:26 pm
Re: Formal logic diagramming help
Weird, the post that is currently right below yours has a similar thing...I just wrote a long thought-process there:
http://top-law-schools.com/forums/viewt ... 6&t=103815
To summarize, "unless" means "if not" so your FIRST if/then should be
PTMPnot --> TMC and PCW
and the contrapositive
TMCnot OR PCWnot --> PTMP.
In English: If public transportation is NOT more popular, then traffic will be more congested AND pollution form cars will get worse. The only way to prevent traffic from being congested is for public transportation to be more popular. The only way to prevent pollution from cars from being worse is for public transportation to be more popular. Thus, if traffic is not more congested, then public transportation must have become more popular. Similarly, if pollution from cars is not worse, then public transportation is more popular.
http://top-law-schools.com/forums/viewt ... 6&t=103815
To summarize, "unless" means "if not" so your FIRST if/then should be
PTMPnot --> TMC and PCW
and the contrapositive
TMCnot OR PCWnot --> PTMP.
In English: If public transportation is NOT more popular, then traffic will be more congested AND pollution form cars will get worse. The only way to prevent traffic from being congested is for public transportation to be more popular. The only way to prevent pollution from cars from being worse is for public transportation to be more popular. Thus, if traffic is not more congested, then public transportation must have become more popular. Similarly, if pollution from cars is not worse, then public transportation is more popular.
- theZeigs
- Posts: 150
- Joined: Fri Nov 28, 2008 3:26 pm
Re: Formal logic diagramming help
*MODS* - Am I in violation of any trademark/copyright issue here because of the LRB? Thanks.
Ahhh...so I just got tripped up on a similar type of question:
"Unless they find an eyewitness and put the defendant on the stand, they will lose the case."
Here, if you use my (uncorrected) method in the previous post, you diagram
-FEW & -DOS --> LC
therefore
-LC --> FEW or DOS
BUT
this is not the correct way of doing it! The reason is because the contrapositive is now not correct. (here: if I didn't lose the case, then either I found an eyewitness or I put the defendant on the stand. But really, I had to do BOTH according to the language of the prompt in order to not lose the case!) The key here is if you do it my way, negation (i.e. "if no") causes any "AND" that's part of the "IF NOT" clause to become "OR" and any "OR" must become "AND".
This is why the LRB suggests doing something called the "Unless Equation" (TM). To paraphrase, basically, anything modified by "unless" (or anything that means "if no...") becomes the necessary condition.The rest is then negated and is the sufficient. Unfortunately, in the LRB, they don't mention that when you negate something, like in my method, ANDs become ORs. They only tell you to negate "the rest" and make the "unless" the sufficient.
For my example: "Unless they find an eyewitness and put the defendant on the stand, they will lose the case."
-LC --> FEW and DOS
therefore
-FEW or -DOS --> LC
For your example: "Unless public transportation becomes much more popular, traffic will become more congested and pollution from cars will get worse."
-TMC or -PCW --> PTMP
therefore
-PTMP --> TMC AND PCW
So you can use either method you like, but just remember that if you say that "unless" is "if no" you are effectively negating whatever that clauses is, and must then reverse any AND/OR that are present.
This has been helpful for me to think through this, I can't believe the LRB people didn't put this in the book. Although I haven't finished it yet, so we'll see
Ahhh...so I just got tripped up on a similar type of question:
"Unless they find an eyewitness and put the defendant on the stand, they will lose the case."
Here, if you use my (uncorrected) method in the previous post, you diagram
-FEW & -DOS --> LC
therefore
-LC --> FEW or DOS
BUT
this is not the correct way of doing it! The reason is because the contrapositive is now not correct. (here: if I didn't lose the case, then either I found an eyewitness or I put the defendant on the stand. But really, I had to do BOTH according to the language of the prompt in order to not lose the case!) The key here is if you do it my way, negation (i.e. "if no") causes any "AND" that's part of the "IF NOT" clause to become "OR" and any "OR" must become "AND".
This is why the LRB suggests doing something called the "Unless Equation" (TM). To paraphrase, basically, anything modified by "unless" (or anything that means "if no...") becomes the necessary condition.The rest is then negated and is the sufficient. Unfortunately, in the LRB, they don't mention that when you negate something, like in my method, ANDs become ORs. They only tell you to negate "the rest" and make the "unless" the sufficient.
For my example: "Unless they find an eyewitness and put the defendant on the stand, they will lose the case."
-LC --> FEW and DOS
therefore
-FEW or -DOS --> LC
For your example: "Unless public transportation becomes much more popular, traffic will become more congested and pollution from cars will get worse."
-TMC or -PCW --> PTMP
therefore
-PTMP --> TMC AND PCW
So you can use either method you like, but just remember that if you say that "unless" is "if no" you are effectively negating whatever that clauses is, and must then reverse any AND/OR that are present.
This has been helpful for me to think through this, I can't believe the LRB people didn't put this in the book. Although I haven't finished it yet, so we'll see
- blhblahblah
- Posts: 132
- Joined: Mon Aug 27, 2007 10:54 pm
Re: Formal logic diagramming help
You both are confused. For the sake of posterity, the credited diagram is:"Unless public transportation becomes much more popular, traffic will become more congested and pollution from cars will get worse."
~Congested AND ~Pollution worse --> Public transportation popular
contrapositive:
~Public transporation popular --> Congested OR Pollution worse
/end thread.
- blhblahblah
- Posts: 132
- Joined: Mon Aug 27, 2007 10:54 pm
Re: Formal logic diagramming help
Also, this isn't formal logic. It's conditional logic.
Formal logic deals strictly with categories and quantities. Conditional logic deals with sufficiency and necessity.
Formal logic deals strictly with categories and quantities. Conditional logic deals with sufficiency and necessity.
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- theZeigs
- Posts: 150
- Joined: Fri Nov 28, 2008 3:26 pm
Re: Formal logic diagramming help
I have to admit, this is not my strength.
I am quoting directly from the LRB when I discuss the "Unless equation." (page 126 in my book, which I got in 2008)
Put the "Unless" as necesssary ( ... ---> PTMP) and negate the rest and put as sufficient (-TMC and -PCW --> PTMP) but I think when you negate you SHOULD make the AND into OR.
Obviously, I think that the LRB didn't go far enough in explaining that when a clause has an AND/OR in it and must be negated, the AND becomes OR and the OR becomes AND. If I am correct and LRB is correct otherwise, then we should get the same answer as risktaker's book, but....
PTMPnot ----> TMC and PW
Risktaker, can you advise?
I am quoting directly from the LRB when I discuss the "Unless equation." (page 126 in my book, which I got in 2008)
So: "Unless public transportation becomes much more popular, traffic will become more congested and pollution from cars will get worse."theZeigs wrote: This is why the LRB suggests doing something called the "Unless Equation" (TM). To paraphrase, basically, anything modified by "unless" (or anything that means "if no...") becomes the necessary condition.The rest is then negated and is the sufficient.
Put the "Unless" as necesssary ( ... ---> PTMP) and negate the rest and put as sufficient (-TMC and -PCW --> PTMP) but I think when you negate you SHOULD make the AND into OR.
Obviously, I think that the LRB didn't go far enough in explaining that when a clause has an AND/OR in it and must be negated, the AND becomes OR and the OR becomes AND. If I am correct and LRB is correct otherwise, then we should get the same answer as risktaker's book, but....
I think that he must have typed the wrong answer here. I think he meant:risktaker wrote:The answer shows the contra positive of this as:
PTMPnot----->TMCnot and PWnot
PTMPnot ----> TMC and PW
Risktaker, can you advise?
- bluejayk
- Posts: 178
- Joined: Tue Jun 17, 2008 3:06 pm
Re: Formal logic diagramming help
No, different systems of formal logic vary, but they often include conditional logic. People on this board are typically referring to sentential logic, one type of formal logic. That's just translating natural language into logical sentences using the operators "and", "or", "not", "if/then".blhblahblah wrote:Also, this isn't formal logic. It's conditional logic.
Formal logic deals strictly with categories and quantities. Conditional logic deals with sufficiency and necessity.
- risktaker
- Posts: 687
- Joined: Tue Sep 22, 2009 3:10 pm
Re: Formal logic diagramming help
Okay. This is quoting from LSAT 180 from Kaplan TM. This was their way of diagramming it, which is different from the other poster.
"If public transport isn't popular, then we'll have more traffic and pollution." TM
I have a feeling that with unless statements when there's "and" involved, the contrapositive retains the "and". The same also goes for "or" i think.
So it would be PTP~------>More traffic and more pollution
Contrapositive: More traffic~ and more pollution~------->PTP
Any thoughts on this would be appreciated.
"If public transport isn't popular, then we'll have more traffic and pollution." TM
I have a feeling that with unless statements when there's "and" involved, the contrapositive retains the "and". The same also goes for "or" i think.
So it would be PTP~------>More traffic and more pollution
Contrapositive: More traffic~ and more pollution~------->PTP
Any thoughts on this would be appreciated.
-
- Posts: 1710
- Joined: Wed Sep 16, 2009 12:01 am
Re: Formal logic diagramming help
My thought process is this:risktaker wrote:"Unless public transportation becomes much more popular, traffic will become more congested and pollution from cars will get worse."
"Unless" means "if not." So it becomes "If public transportation doesn't become much more popular, traffic will become more congested and pollution from cars will get worse."
Now I represent that (~ is negation):
~public transport more popular -> traffic more congested AND pollution worse
~traffic more congested OR ~pollution worse -> public transit more popular
As a double-check, you could turn this back into words. If traffic doesn't become more congested or pollution doesn't get worse, then we know that public transit got more popular, because unless public transit gets more popular, traffic will become more congested and pollution will get worse.
As for why "and" becomes "or" and vice-versa when negating, I think of it this way. I could say, "If I get good grades and a good score on the LSAT, I'll get into law school." Then if I come to you and say, "Man, life sucks. I didn't get into law school," you couldn't conclude that I BOTH got terrible grades AND got a bad score on the LSAT; all you could say was that I screwed up at least one of those two things, either by failing to get good grades or by failing to get a good score (or both). So the symbols look like this:
good grades AND good score -> get in
~get in -> ~good grades OR ~good score
What was originally creating the conditional (an "if," an "unless") is irrelevant. The rules are the same regardless.
- risktaker
- Posts: 687
- Joined: Tue Sep 22, 2009 3:10 pm
Re: Formal logic diagramming help
Yeah, that makes sense. Thank you.
- chewdak
- Posts: 106
- Joined: Fri Apr 03, 2009 5:54 pm
Re: Formal logic diagramming help
To get this concept, it helps me to break complex statements into simple ones.
A --> B and C is equivalent to two statements: A --> B and A --> C
contrapositive of either will suffice to negate A:
~B --> ~A or ~C --> ~A, and can be shortened to ~B or ~C --> ~A
A --> B and C is equivalent to two statements: A --> B and A --> C
contrapositive of either will suffice to negate A:
~B --> ~A or ~C --> ~A, and can be shortened to ~B or ~C --> ~A
- theZeigs
- Posts: 150
- Joined: Fri Nov 28, 2008 3:26 pm
Re: Formal logic diagramming help
This is a nice, concise summary. +1chewdak wrote:To get this concept, it helps me to break complex statements into simple ones.
A --> B and C is equivalent to two statements: A --> B and A --> C
contrapositive of either will suffice to negate A:
~B --> ~A or ~C --> ~A, and can be shortened to ~B or ~C --> ~A
And I was right! You had mis-entered what the book said! Man, I lost sleep last night because of that. I even was talking about it with my lawyer friend, who replied "I care about real problems." HAHAH
- risktaker
- Posts: 687
- Joined: Tue Sep 22, 2009 3:10 pm
Re: Formal logic diagramming help
Oh man. Yeah, sorry about that.
- theZeigs
- Posts: 150
- Joined: Fri Nov 28, 2008 3:26 pm
Re: Formal logic diagramming help
No worries.risktaker wrote:Oh man. Yeah, sorry about that.
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