## What are your Formal Logic "magic" tricks?

Prepare for the LSAT or discuss it with others in this forum.
Journey180

Posts: 45
Joined: Wed Apr 17, 2013 5:22 pm

### What are your Formal Logic "magic" tricks?

During my studies, I have come upon some some tricks in formal logic that weren't mentioned in the books I were studying. Have you discovered any formal logic "magic" tricks that make life in formal logic ville a little easier? Share them, now!

Jeffort

Posts: 1888
Joined: Wed Jun 18, 2008 4:43 pm

### Re: What are your Formal Logic "magic" tricks?

Journey180 wrote:During my studies, I have come upon some some tricks in formal logic that weren't mentioned in the books I were studying. Have you discovered any formal logic "magic" tricks that make life in formal logic ville a little easier? Share them, now!

I'm curious about what 'magic tricks' you think you have found that are not mentioned in the prep books since when students think they have discovered a different/new way to supposedly make things magically easier, they are usually making some sort of logical mistake or just notating something in a slightly different but logically equivalent diagrammatic way.

Journey180

Posts: 45
Joined: Wed Apr 17, 2013 5:22 pm

### Re: What are your Formal Logic "magic" tricks?

Jeffort wrote:
Journey180 wrote:During my studies, I have come upon some some tricks in formal logic that weren't mentioned in the books I were studying. Have you discovered any formal logic "magic" tricks that make life in formal logic ville a little easier? Share them, now!

I'm curious about what 'magic tricks' you think you have found that are not mentioned in the prep books since when students think they have discovered something different to supposedly make things magically easier, they are usually making some sort of logical mistake or just notating something in a slightly different but logically equivalent diagrammatic way.

I am not limiting the question to logical notations, or logical deductions/inferences, but I was hoping to open up discussion on what general tricks people use to make formal logic easier. For example, one trick I know ( and I don't claim I am the inventor of this. I only claim I didn't see it in LGB) is, in certain games, a long logical chain can yield strong inferences if controposited ( is that a word?) lol. Anyway, when you find the contrapositive of a long chain of logic, you get major deductions/inferences.

mojangles

Posts: 197
Joined: Tue Jun 11, 2013 12:39 am

### Re: What are your Formal Logic "magic" tricks?

Journey180 wrote:
Jeffort wrote:
Journey180 wrote:During my studies, I have come upon some some tricks in formal logic that weren't mentioned in the books I were studying. Have you discovered any formal logic "magic" tricks that make life in formal logic ville a little easier? Share them, now!

I'm curious about what 'magic tricks' you think you have found that are not mentioned in the prep books since when students think they have discovered something different to supposedly make things magically easier, they are usually making some sort of logical mistake or just notating something in a slightly different but logically equivalent diagrammatic way.

I am not limiting the question to logical notations, or logical deductions/inferences, but I was hoping to open up discussion on what general tricks people use to make formal logic easier. For example, one trick I know ( and I don't claim I am the inventor of this. I only claim I didn't see it in LGB) is, in pure selection games, a long logical chain can yield strong inferences if controposited ( is that a word?) lol. Anyway, when you find the contrapositive of a long chain of logic, you get major deductions/inferences.

i never used LGB so i can't say that for sure. but that is something i learned from lsatblog, which is a free tool, so i would imagine that it has to be in some of the paid books.

all of my studying was done with:

casebriefs (free but for the most part i found it fairly useless)
lsatblogs
LSAT prep-tests

therefore, i can't really say what is or isn't included in any of the books people may generally use for studying. lsatblogs is tight for trying to work through LG though

magickware

Posts: 359
Joined: Sun Nov 04, 2012 1:27 pm

### Re: What are your Formal Logic "magic" tricks?

Journey180 wrote: a long logical chain can yield strong inferences if controposited ( is that a word?) lol. Anyway, when you find the contrapositive of a long chain of logic, you get major deductions/inferences.

I am fairly certain that is in the PS LGB. Or, at least, the idea that contrapositives in of themselves can be a necessary part of the inference.

Journey180

Posts: 45
Joined: Wed Apr 17, 2013 5:22 pm

### Re: What are your Formal Logic "magic" tricks?

mojangles wrote:
Journey180 wrote:
Jeffort wrote:
Journey180 wrote:During my studies, I have come upon some some tricks in formal logic that weren't mentioned in the books I were studying. Have you discovered any formal logic "magic" tricks that make life in formal logic ville a little easier? Share them, now!

I'm curious about what 'magic tricks' you think you have found that are not mentioned in the prep books since when students think they have discovered something different to supposedly make things magically easier, they are usually making some sort of logical mistake or just notating something in a slightly different but logically equivalent diagrammatic way.

I am not limiting the question to logical notations, or logical deductions/inferences, but I was hoping to open up discussion on what general tricks people use to make formal logic easier. For example, one trick I know ( and I don't claim I am the inventor of this. I only claim I didn't see it in LGB) is, in pure selection games, a long logical chain can yield strong inferences if controposited ( is that a word?) lol. Anyway, when you find the contrapositive of a long chain of logic, you get major deductions/inferences.

i never used LGB so i can't say that for sure. but that is something i learned from lsatblog, which is a free tool, so i would imagine that it has to be in some of the paid books.

all of my studying was done with:

casebriefs (free but for the most part i found it fairly useless)
lsatblogs
LSAT prep-tests

therefore, i can't really say what is or isn't included in any of the books people may generally use for studying. lsatblogs is tight for trying to work through LG though

One more thing is the LGB did explain the seperation principle pretty well; I just added my own spin to it: For example,with 3 boys that can't sit next to each other would look like "B_B_B" right? I noticed that the formula would be where (boys -1= number of needed spaces between each boy. In this example, 3 boys-1= 2 spaces. ) and ( spaces + boys= number of total spaces needed if only boys and girls or any other two variable combination is played). So, for our example...3 boys would need atleast 2 seats without boys and necessitate at least 5 total chairs.

Journey180

Posts: 45
Joined: Wed Apr 17, 2013 5:22 pm

### Re: What are your Formal Logic "magic" tricks?

magickware wrote:
Journey180 wrote: a long logical chain can yield strong inferences if controposited ( is that a word?) lol. Anyway, when you find the contrapositive of a long chain of logic, you get major deductions/inferences.

I am fairly certain that is in the PS LGB. Or, at least, the idea that contrapositives in of themselves can be a necessary part of the inference.

Yea, it discusses contropositives but not to the extent where they can be used for long and even complex chains. Atleast I didn't see it. If you have, please cite page number and year of publication. Thanks.

Clyde Frog

Posts: 8978
Joined: Sun May 26, 2013 2:27 am

### Re: What are your Formal Logic "magic" tricks?

Ha! A magician never reveals their tricks.

TheMostDangerousLG

Posts: 1545
Joined: Thu Nov 22, 2012 4:25 am

### Re: What are your Formal Logic "magic" tricks?

Well, I don't know if counts as a magic trick, but I do sacrifice a young turtle dove before every logic games section as an offering to the lord of conditional reasoning, Ellis Ay-Cee. Let me tell you, the reaction I got on test day was not pleasant!

EzraFitz

Posts: 742
Joined: Mon Jul 01, 2013 10:42 am

### Re: What are your Formal Logic "magic" tricks?

TheMostDangerousLG wrote:Well, I don't know if counts as a magic trick, but I do sacrifice a young turtle dove before every logic games section as an offering to the lord of conditional reasoning, Ellis Ay-Cee. Let me tell you, the reaction I got on test day was not pleasant!

+1 I usually use a small goat.

TheMostDangerousLG

Posts: 1545
Joined: Thu Nov 22, 2012 4:25 am

### Re: What are your Formal Logic "magic" tricks?

jcb5329 wrote:
TheMostDangerousLG wrote:Well, I don't know if counts as a magic trick, but I do sacrifice a young turtle dove before every logic games section as an offering to the lord of conditional reasoning, Ellis Ay-Cee. Let me tell you, the reaction I got on test day was not pleasant!

+1 I usually use a small goat.

Rookie mistake. You have to make sure your offering fits in a gallon-sized ziploc bag (with room left over for your pencils and timer, of course).

sighsigh

Posts: 263
Joined: Wed Oct 20, 2010 8:47 pm

### Re: What are your Formal Logic "magic" tricks?

They might teach this in one of the prep books, but I'm not sure. Say you have A => B. You want to write the contrapositive, but instead of writing ~B => ~A next to it, you can instead just turn A => B into A <=|> ~B

This covers both the original and the contrapositive. The "|" just means 'not.' This saves a lot of time if you have a lot of conditionals and don't want to spend a ton of time to write down all of their contrapositives. In other words, all of:

A => B
~B => ~A

C => D
~D => ~C

E => F
~F => ~E

becomes

A <=|> ~B
C <=|> ~D
E <=|> ~F

I find this extremely useful.
Last edited by sighsigh on Fri Jul 05, 2013 8:16 pm, edited 1 time in total.

EzraFitz

Posts: 742
Joined: Mon Jul 01, 2013 10:42 am

### Re: What are your Formal Logic "magic" tricks?

sighsigh wrote:They might teach this in one of the prep books, but I'm not sure. Say you have A => B. You want to write the contrapositive, but instead of writing ~B => ~A next to it, you can instead just turn A => B into A <=|> ~B

This is exactly what I did for all of them, and always have about 5 minutes to spare per section. It saves tons of time.

And LG, dang, I never thought of that, it's the perfect solution (and much easier than trying to fit a Kid into my ziploc bag)

mojangles

Posts: 197
Joined: Tue Jun 11, 2013 12:39 am

### Re: What are your Formal Logic "magic" tricks?

TheMostDangerousLG wrote:
jcb5329 wrote:
TheMostDangerousLG wrote:Well, I don't know if counts as a magic trick, but I do sacrifice a young turtle dove before every logic games section as an offering to the lord of conditional reasoning, Ellis Ay-Cee. Let me tell you, the reaction I got on test day was not pleasant!

+1 I usually use a small goat.

Rookie mistake. You have to make sure your offering fits in a gallon-sized ziploc bag (with room left over for your pencils and timer, of course).

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