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Justify vs. Assumption, Necessary vs. Sufficient

ctrah38
LSAT Novice
 
Posts: 1
Joined: Mon Jun 25, 2018 1:02 pm
Points: 1

I'm having trouble understanding the difference between justify and assumption questions I think because I don't fully understand the difference between necessary and sufficient. Justify questions involve a statement that is sufficient to prove the conclusion, and assumption questions involve a statement that is necessary to prove the conclusion, so the conclusion depends on the assumption, correct? Even in conditional reasoning, I know the format of if (sufficient) :arrow: then (necessary), and if not (necessary) :arrow: then not (sufficient), but I don't really know what that means. I can only identify which term is which by finding the stated or implied "if." I think this is why I'm struggling on justify vs. assumption questions.
James Finch
PowerScore Staff
PowerScore Staff
 
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Joined: Wed Sep 06, 2017 5:06 pm
Points: 389

Hi Ctrah,

The distinction between sufficient assumptions (Justify questions) and necessary assumptions (Assumption questions) is where their stimuluses' conclusions fall in the conditional chain. For sufficient assumptions/justifications, whenever the justification is true, then the stimulus's conclusion must be true as well:

Sufficient Assumption :arrow: Conclusion

For Assumption questions, the opposite relationship exists: when the conclusion is true, then the assumption must be true as well:

Conclusion :arrow: Necessary Assumption

which in turn is why we have to use the Assumption Negation technique (the contrapositive of the above relationship) to test our answer choices against the conclusion:

Assumptionfalse :arrow: Conclusionfalse

The question stem will tell you whether you're dealing with an Assumption or Justify question. Assumption questions will ask for an answer choice that, if not true, would make the conclusion false (ie "the above does not follow unless which of the following is assumed," "the argument depends upon which of the following," etc). The correct answer choice does not, however, necessarily make the conclusion true; just know that if the correct answer choice is false, the conclusion will be too.

Justify questions are more straightforward. Their question stems deal with certainty ("properly drawing/inferring" the conclusion) and the correct answer choice being true will make the conclusion true 100% of the time. This means that no negation is required, simply test the answer choices and see which makes the conclusion true.

In practice, Justify questions and Supporter Assumption questions can generally be answered using a similar approach: a missing element is needed, like a hole to be filled, either to link the conclusion to a certain premise or to link two premises to complete the logical chain. These both often yield Prephrases that make it simple to identify the correct answer choice.

Defender Assumption questions are some of the most difficult for students, because they often deal with some of the most basic, everyday assumptions that would otherwise make the whole argument moot/wrong. This means there are a near infinite number that could potentially answer the question, and Prephrasing is near-impossible. Instead, the best way to approach these types (ie any Assumption question where no obvious logical hole is found in the stimulus) is to quickly identify and eliminate the irrelevant Loser and answer choices, then use the Assumption Negation technique to choose between Contenders. Otherwise, it's very easy to bog down on these questions.

Hope this helps!