Making if-then statements out of not so obvious statements?

lsatextreme
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Making if-then statements out of not so obvious statements?

Postby lsatextreme » Thu Sep 23, 2010 4:15 pm

So after all the conditional practice thus far, I seem to be decently competent at mixing and molding different conditional statements, but I seem to struggle with those statements that aren't as obvious as to how to plug into the if-then statement.

For example:
Wendy appears in every photograph that Selma appears in

Is there an easy way for me to know how to make this into an if-then statement? This was one of the rules in a LG in PT45, and I somehow diagrammed this rule and all the other similar rules, inversely, so apparently I'm not grasping the inherent meaning of an if-then statement yet or something.

I've just always memorized if-then, contrapositives, sufficient -> necessary, etc, but knowing all of these rules doesn't seem to help me on applying them to regular statements like above. Any guidelines?

dakatz
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Re: Making if-then statements out of not so obvious statements?

Postby dakatz » Thu Sep 23, 2010 4:21 pm

When dissecting conditionals, it helps to define the sufficient condition as the one you know more about. What do you know about it? Well, you know a characteristic or trait of EVERY person/thing that falls into the "sufficient" class.

So if Wendy is in EVERY picture that Selma is in, then it is the clear that you know something about EVERY picture Selma is in. What do you know? That Wendy will be in it. Remember what makes something sufficient in the first place. It is sufficient because you know that anyone or anything in that group will have a certain necessary trait. So always say to yourself "Of these two groups of things, which one do I know something about EVERY person/thing in the group?"

lsatextreme
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Re: Making if-then statements out of not so obvious statements?

Postby lsatextreme » Thu Sep 23, 2010 5:07 pm

thank you makes sense :)

lsatextreme
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Re: Making if-then statements out of not so obvious statements?

Postby lsatextreme » Wed Oct 06, 2010 1:28 am

ugh I'm reviving my semi old topic bc this same issue I had inquired about couple weeks ago is creeping up on me again. There's this one stimulus on PT48 that says "Traditional norms in our society prevent sincerity by requiring one to ignore unpleasant realities and tell small lies." Now I think the correct way to diagram this is by saying "If ignore unpleasant realities and tell small lies then Traditional norms in our society prevent sincerity," but I totally thought it was the opposite. I can't seem to wrap my mind around forming these if-then statements...

lsatextreme
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Re: Making if-then statements out of not so obvious statements?

Postby lsatextreme » Wed Oct 06, 2010 7:59 pm

Sorry, I wasn't clear, could anyone happen to help me know how to diagram this into an if-then statement?

"Traditional norms in our society prevent sincerity by requiring one to ignore unpleasant realities and tell small lies."

wjun15
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Re: Making if-then statements out of not so obvious statements?

Postby wjun15 » Wed Oct 06, 2010 8:10 pm

is there any difference between

if and only if
if but only if

?

how about,

Sally will not go to the park only if John goes.

^ does that even make sense? lol...

sally goes -> john goes ... right?

can you guys think of any other confusing ways... that we should be aware of before the test on saturday?

randomwalk
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Re: Making if-then statements out of not so obvious statements?

Postby randomwalk » Wed Oct 06, 2010 8:21 pm

"If and only if" is logically equivalent to "if but only if", so there is no real difference between the two.

"If" sets off the sufficient condition. "Only if" sets off the necessary condition.

The sentence you stated
"Sally will not go to the park only if John goes."
can be translated into

~SP -> JP

because the "only if" sets off the necessary condition.

Just for further clarification, this sentence is logically equivalent to the one you stated:
If Sally will not go to the park, then John goes.

("If" setting off the sufficient condition.)

edit:

Hoping to clarify your sentence more. This might sound redundant. What do you know about Sally? You know it's only possible for her to not go to the park when John goes. So you know that if she doesn't go to the park, John must have went, because it's the only way she could not go. But you don't have enough information for what John does when she does go to the park.
Last edited by randomwalk on Wed Oct 06, 2010 8:26 pm, edited 2 times in total.

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AreJay711
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Re: Making if-then statements out of not so obvious statements?

Postby AreJay711 » Wed Oct 06, 2010 8:22 pm

lsatextreme wrote:Sorry, I wasn't clear, could anyone happen to help me know how to diagram this into an if-then statement?

"Traditional norms in our society prevent sincerity by requiring one to ignore unpleasant realities and tell small lies."


This isn't a conditional statement it is a causal statement. Anyway if you wanted to diagram this like a conditional for whatever reason just look for the sufficient and necessary. traditional norms ---> Ignore unpleasent realities and tell small lies. Contrapositive ~ignore unpleasant realities or ~tell small lies -----> ~Traditional norms. You can tell how this doesn't capture what the original statement said (that the traditional norms CAUSE the other two). Traditional norms is the sufficient (because its existence requires that the other two are there) and ignoring unpleasant realities and telling small lies are the necessary (because if they do not exist then you know there are not traditional norms).

Imagine I said if the house fell down, then there was an earthquake. Certainly the house falling down didn't cause the earthquake. I could also say if there was a thunderstorm, he didn't go for a run. I doubt his not going for a run caused the thunderstorm. Basically, you can't determine a causal relationship from a conditional statement. You can kind of come up with conditional statements from causal statements sometimes however.

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AreJay711
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Re: Making if-then statements out of not so obvious statements?

Postby AreJay711 » Wed Oct 06, 2010 8:28 pm

wjun15 wrote:is there any difference between

if and only if
if but only if

?

how about,

Sally will not go to the park only if John goes.

^ does that even make sense? lol...

sally goes -> john goes ... right?

can you guys think of any other confusing ways... that we should be aware of before the test on saturday?


If and only if means it is both a necessary and sufficient condition.

A if and only if B = If A then B and If B then A. Plus with their contrapositives: IF ~A then ~B, and If ~B then ~A.

Basically just remember that it is saying two things A --> B and B --> A. I don't think I have seen this on very many LSATs though. I know it from a formal logic course I took a few years ago.

EDIT : "Sally will NOT go to the park only if John goes." = J --> ~ S
Last edited by AreJay711 on Wed Oct 06, 2010 8:32 pm, edited 2 times in total.

wjun15
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Re: Making if-then statements out of not so obvious statements?

Postby wjun15 » Wed Oct 06, 2010 8:28 pm

randomwalk wrote:"If and only if" is logically equivalent to "if but only if", so there is no real difference between the two.

"If" sets off the sufficient condition. "Only if" sets off the necessary condition.

The sentence you stated
"Sally will not go to the park only if John goes."
can be translated into

~SP -> JP

because the "only if" sets off the necessary condition.

Just for further clarification, this sentence is logically equivalent to the one you stated:
If Sally will not go to the park, then John goes.

("If" setting off the sufficient condition.)

edit:

Hoping to clarify your sentence more. This might sound redundant. What do you know about Sally? You know it's only possible for her to not go to the park when John goes. So you know that if she doesn't go to the park, John must have went, because it's the only way she could not go. But you don't have enough information for what John does when she does go to the park.


okay thanks. i got confused with Unless and only if...because when i see unless i always negate the sufficient and put as the necessary everything after "unless". i guess you cant do that for this sentence.

wjun15
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Re: Making if-then statements out of not so obvious statements?

Postby wjun15 » Wed Oct 06, 2010 8:31 pm

AreJay711 wrote:
wjun15 wrote:is there any difference between

if and only if
if but only if

?

how about,

Sally will not go to the park only if John goes.

^ does that even make sense? lol...

sally goes -> john goes ... right?

can you guys think of any other confusing ways... that we should be aware of before the test on saturday?


If and only if means it is both a necessary and sufficient condition.

A if and only if B = If A then B and If B then A. Plus with their contrapositives: IF ~A then ~B, and If ~B then ~A.

Basically just remember that it is saying two things A --> B and B --> A. I don't think I have seen this on very many LSATs though. I know it from a formal logic course I took a few years ago.


i saw this on a recent PT and it asked something like "find a situation where the principle occurs" and it was a if and only if statement in the stimulus. So in this case, I would look for an answer that was either A>B OR B>A and they would both be correct right?

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AreJay711
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Re: Making if-then statements out of not so obvious statements?

Postby AreJay711 » Wed Oct 06, 2010 8:37 pm

No you want both if there is one. So if I said there is smoke if and only if there is fire. That would mean that they both always have to be together. smoke --> fire AND fire --> smoke. This isn't really obvious based on how we use English so thats probably why they don't use "if and only if" that much.

wjun15
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Re: Making if-then statements out of not so obvious statements?

Postby wjun15 » Wed Oct 06, 2010 8:39 pm

AreJay711 wrote:No you want both if there is one. So if I said there is smoke if and only if there is fire. That would mean that they both always have to be together. smoke --> fire AND fire --> smoke. This isn't really obvious based on how we use English so thats probably why they don't use "if and only if" that much.


okay thanks. but in a must be true question either one would be good.




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