Assumption-Negation test

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glucose101
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Assumption-Negation test

Postby glucose101 » Mon Aug 01, 2011 5:09 pm

Can someone list various instances on checking assumptions with the negation test, particularly more challenging negations? For example, what if the phrase says "Johnny eats food and doesn't drink juice." Just trying to make sure I understand ALL instances of negation. Thanks in advance!

Manhattan LSAT Noah
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Re: Assumption-Negation test

Postby Manhattan LSAT Noah » Mon Aug 01, 2011 8:17 pm

There's a short exercise here.

Maybe you wanted more LSAT-drawn phrases - that's a bit more back to the basics, but I hope it's helpful.

SanDiegoJake
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Re: Assumption-Negation test

Postby SanDiegoJake » Mon Aug 01, 2011 8:24 pm

Quick tip: Many answer choices are difficult to negate. You're always safe if you negate any answer choice by saying "What if it's not true that (answer choice)" If your answer is, "Then the argument falls apart", you've found your correct answer. If your answer is "Um, I don't know what that would mean", it's likely that you have not yet found your correct answer.

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glucose101
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Re: Assumption-Negation test

Postby glucose101 » Mon Aug 01, 2011 9:40 pm

Thanks Noah!

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Helicio
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Re: Assumption-Negation test

Postby Helicio » Mon Aug 01, 2011 10:32 pm

glucose101 wrote:Can someone list various instances on checking assumptions with the negation test, particularly more challenging negations? For example, what if the phrase says "Johnny eats food and doesn't drink juice." Just trying to make sure I understand ALL instances of negation. Thanks in advance!


"Johnny eats food and doesn't drink juice" would become "Johnny does not eat food and drinks juice."

Though someone please tell me if I'm wrong so I don't mislead you.

Here are some more examples:

Republicans are not willing to compromise/Republicans are willing to compromise

The aquifer is running out of water/the aquifer is not running out of water

The beaver did not have sex, and the deer did have sex/The beaver had sex, and the deer did not have sex


Harvard Law School is better than Yale Law School/ Harvard law school is not better than Yale Law School

Edit: Note that in the previous example, you can't change it to Harvard Law School is WORSE than Yale Law School because it might simply be just as good. So be careful not to use polar opposites and stick with logical negation.

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KevinP
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Re: Assumption-Negation test

Postby KevinP » Tue Aug 02, 2011 1:42 am

Helicio wrote:"Johnny eats food and doesn't drink juice" would become "Johnny does not eat food and drinks juice."

Though someone please tell me if I'm wrong so I don't mislead you.


You're close but not quite right.
The logical negation of "Johnny eats food and doesn't drink juice" would become "Johnny does not eat food OR he drinks juice".
This is due to the fact that in an AND, both operands must be true and hence having at least one operand be false is enough to make the entire proposition false. In isn't necessary to have both operands be false.

Useless fact: The AND/OR negation rules are known as De Morgan's laws and can be generalized to any number of ANDs/ORs.

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Helicio
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Re: Assumption-Negation test

Postby Helicio » Tue Aug 02, 2011 12:58 pm

KevinP wrote:
Helicio wrote:"Johnny eats food and doesn't drink juice" would become "Johnny does not eat food and drinks juice."

Though someone please tell me if I'm wrong so I don't mislead you.


You're close but not quite right.
The logical negation of "Johnny eats food and doesn't drink juice" would become "Johnny does not eat food OR he drinks juice".
This is due to the fact that in an AND, both operands must be true and hence having at least one operand be false is enough to make the entire proposition false. In isn't necessary to have both operands be false.

Useless fact: The AND/OR negation rules are known as De Morgan's laws and can be generalized to any number of ANDs/ORs.


OMG. I made a noob error. I should have remembered how AND/OR works in contrapositives.

Ty




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