## General Logic Games Question

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jusdel1

Posts: 11
Joined: Wed Jan 12, 2011 3:35 pm

### General Logic Games Question

So far logic games is by far my strongest section. However, there is one rule of logic games that I don't understand. For example, If L is present then A can't be present. This makes a L <--|--> A. Why can't I do the same thing if i have a question such as if L isn't present then A is present?

Also, if I were to have something like this: A --> B, B < - | - > C, I can infer A < - | - > C. Why can't I do A < - | - > C, A --> B, and infer B < - | - > C?

lawgod

Posts: 465
Joined: Mon Dec 28, 2009 3:22 pm

### Re: General Logic Games Question

If L is present then A is not.
If L is not present, what makes you think that A has to be?

TMC116

Posts: 284
Joined: Mon Jun 06, 2011 6:08 pm

### Re: General Logic Games Question

Yeah this is a classic mixup of conditional logic.

You're right about the double-not arrow in the first rule. The two variables cannot occur together.

But in the second rule, ~L --> A, means that you have to have at least one of the variables, A or L, but possibly both. This is a rule of inclusion while the first is a rule of exclusion.

You may be getting tripped up on Powerscore's method of representing the Double Not Arrow. You don't have to draw their diagram if it's not helpful. If it's easier for you to visualize you can just draw each conditional statement and contrapositive.

Jeffort

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

### Re: General Logic Games Question

jusdel1 wrote:So far logic games is by far my strongest section. However, there is one rule of logic games that I don't understand. For example, If L is present then A can't be present. This makes a L <--|--> A. Why can't I do the same thing if i have a question such as if L isn't present then A is present?

Also, if I were to have something like this: A --> B, B < - | - > C, I can infer A < - | - > C. Why can't I do A < - | - > C, A --> B, and infer B < - | - > C?

You are asking about rules that establish a mutually exclusive relationship -AKA both cannot be true together- vs. rules that dictate either/or -AKA at least one of the two has to be true. It's a common area of confusion early into LSAT prep when you first get exposed to these types of rules and try to deal with how they work in the context of LSAT questions.

The bare bones difference between the two types of rules is that one dictates a rule of exclusion (not both, possibly neither) while the other dictates a requirement of inclusion (at least one, possibly both).

With A ---> ~B aka A < - | - > B, the rule prohibits both elements from being true at the same time, or in the context of LG grouping games, being selected/grouped together. In its basic form (absent other related conditions/parameters in the game/LR question that affect how it applies in the full context of the particular LG/LR Q environment), it merely prohibits both from being true/selected together, thus leaving open the possibility of both conditions being false without breaking the rule. Put simply, A < - | - > B does not require either A or B to be true. They both could be false.

With ~A ---> B, the rule dictates that one of the two elements must be true. In its basic form the rule simply prohibits a situation where both are false. Absent additional constraints, it allows both to be true together but does not require it.

Things get more complicated with how to apply such rules in the context of LG's and LR questions depending on the context established by the entire set of LG rules-game parameters/LR question stimulus you have to apply the rule within.

For instance, if you have an in/out selection game and an A < - | - > B rule, both A & B could be put together into the 'out/not selected' group. Whereas in a two group/divide the variables up into two teams/committees/cars/boats/planes/whatever groups (meaning all the elements are technically 'selected/assigned' to a group and nothing is considered out/not selected), it creates both an either/or and not both together situation so that A is in one group and B is in the other.

Fianna13

Posts: 297
Joined: Thu Feb 03, 2011 1:05 am

### Re: General Logic Games Question

Looks like most people explained the first scenario pretty well. For the second one, A is the sufficient condition and B is the necessary condition, which means B does not guarantee A's presence, therefore, it could be that B and C could exist at the same time. Does that help?

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