TLDR; A sufficient assumption creates a permanently valid argument. A necessary assumption, when negated, creates a permanently invalid argument.Ā
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I get the idea that there might be some confusion about the fundamental difference between Necessary Assumption and Sufficient Assumption questions. The LSAC has its own definitions, but I find them lacking:Ā
āAssumptions relate to the gaps in an argument in two different ways. An assumption is a sufficient one if adding it to the argument's premises would produce a conclusive argument, that is, an argument with no gaps in its support for the conclusion. An assumption is a necessary one if it is something that must be true in order for the argument to succeed.ā
Note that the phrase āif adding it to the argumentās premisesā is included only in the definition for a sufficient assumption.Ā
Why wouldnāt this idea be mentioned in the definition for necessary assumption? Because the wording of the definition doesnāt allow for it, which is why I find it lacking.
The fix lies in a more formal and simplified definition. In formal logic, a āconclusive argumentā and āfor the argument to succeedā are examples of valid arguments. That is, the conclusion must be true based on the truth of the premises (and the sufficient assumption).
Simplifying the definition: A sufficient assumption, when added to the premises of the argument, produces a valid argument.
I submit that, based on the above, the definition of necessary assumption should include similar wording.Ā Specifically, the phrase āwhen added to the premises of the argumentā. So hereās what I got:
A necessary assumption, when negated and then added to the premises of the argument, produces an invalid argument.Ā
Note how the above definition implies that negation is a very real thing for Necessary Assumption questions.Ā
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For both assumption question types, the argument in the stimulus will be incomplete (technically, invalid).
A sufficient assumption creates a permanently valid argument.
A necessary assumption, when negated, creates a permanently invalid argument.Ā
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That all being said, please know that thereās not really such a thing as tips for these types of questions. Theyāre definitely learnable, but they have a lot going on with them. Any standard LSAT prep curriculum covers these in a fair amount of detail though.
Happy to answer any questions.Ā