Sufficient and necessary assumption questions are asking about two very different types of assumptions, but both question types have something in common: The conclusion almost always introduces new information, and finding the correct answer requires identifying and handling that new information appropriately.
Sufficient Assumptions
Almost all sufficient assumption questions have new information in the conclusion that simply needs to be connected with a premise in order to make the argument valid. Let’s take the following argument:
Michelle is afraid of large dogs. Therefore, Michelle is afraid of Fluffy.
The second sentence, which is the conclusion, introduces a new piece of information: this Fluffy character. We don’t know anything about Fluffy, because he wasn’t mentioned at all in the premises. Since our task is to make the argument 100% valid, we need to link that new information back to a premise — in this case, we need to fill in the important (but currently unstated) premise that Fluffy is a large dog.
Now try this example:
Talking about politics at Thanksgiving dinner guarantees that there will be an argument. When there is an argument, Uncle Mark throws the turkey. Therefore, any conversations about politics at Thanksgiving will end with ordering pizza.
The conclusion introduces a new concept, ordering pizza, that wasn’t discussed in the premises. In order to make this argument perfect, we need to link that new concept (pizza) back to one of the premises; in this case, the sufficient assumption would be that throwing the turkey invariably leads to ordering pizza.
Necessary Assumptions
You should also be on the lookout for new information in the conclusion of necessary assumption questions. Try this one:
Billy is talking to Charlene at a bar. Charlene has had five shots of tequila. Therefore, Billy will get Charlene’s phone number.
The conclusion suddenly introduces the concept of Charlene’s phone number. Since this is a necessary assumption question, rather than making the argument perfect, we need to state that it is possible for the new information to follow from these premises. For instance, it’s necessary that Charlene sometimes gives her phone number to people who talk to her at a bar; if we were to negate that statement, it would say that Charlene never gives her phone number out at bars, which would preclude Billy from getting them digits.
Okay, one more for practice:
Anyone who plays fantasy football will lose money. Therefore, all fantasy football players will have to ask their mothers for a loan.
The conclusion introduces the idea of asking one’s mother for a loan (a truly unappealing prospect); having noticed this new information, we simply need to state that it is possible that people who lose money will have to ask their moms for cash money. In other words, your necessary assumption would be that at least some people who lose money need to get loans from the Bank of Mom and Dad.
The assumption questions tend to be a little tricky for students, but in most cases they aren’t too difficult as long as you’re on the lookout for things in the conclusion that aren’t supported by the rest of the argument. If you’re having a tough time with these question types, keep your eyes peeled for fresh info – you may be surprised by how much it helps.
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