[zach7691]

Hello all, I’m on Lesson 19 in the Trainer, and for some reason I’m completely stumped on the concept of required/necessary assumptions, especially on how to differentiate them from sufficient assumptions. Some of the questions I’m finding have very odd wording, and I can’t quite dissect them to understand what it is they’re saying.

I’ve wrapped my head around the concept of sufficient assumption, but the descriptions for required assumption sound like the description for sufficient assumption, just phrased in another way. I could recognize a required assumption question if I saw it, but I guess I don’t get the difference between finding an answer that would be sufficient to fill in a logical gap, versus what needs to be true for an argument to work. I understand the semantic difference, but in the given book examples, I don’t GET how an argument that “is useful, but not necessary”, or “supports the premise, but isn’t necessary” is wrong.

In the book practice questions, I was able to correctly answer 3/4 of the required assumption questions, but I’m afraid that I was only so lucky because I recognized certain language in the answer choices that I could tell did not relate to the conclusion or support, and not necessarily because I understood that an answer HAD to be true. For what it’s worth, the question I answered incorrectly was the ordinary mountain sickness/cerebral edema question on page 272, and I incorrectly answered it with C.

Does anyone else have this issue? Any advice?

[LSAT Dan]

The mountain sickness question is a tough one that stumps a lot of students. In terms of advice, I’d say to keep working through them, but here’s a simplified example that might help.

My roommate loves maple bars. He went to the donut store this morning. He will buy a maple bar if he can afford one. I happen to know that maple bars only cost 25 cents today, so he will buy a maple bar.

Conclusion: He will buy a maple bar today.

Here’s a possible answer choice:

A) My roommate has three dollars.

Notice that this is great as a sufficient assumption answer – if he has 3 dollars, then we know the conclusion is true: He will buy a maple bar. How do we know? Because they’re only a quarter, so he can afford one with his 3 dollars, and we have a premise that if he can afford one, he will buy one.

But it’s a wrong answer as a necessary assumption question. The argument doesn’t DEPEND on his having three dollars. He’s not REQUIRED to have three dollars in order for the conclusion to follow. Even if he only has two dollars, he might still buy a maple bar.

His having three dollars is sufficient, but not necessary.

Now the reverse:

In order to become an attorney, one must pass the bar exam. Rhonda has been studying hard for the bar exam, so Rhonda will become an attorney.

Conclusion: Rhonda will become an attorney.

Answer choice:

A) Rhonda will pass the bar exam.

This is a good answer for a NECESSARY assumption question – if she doesn’t pass the bar exam, then she won’t be an attorney. So the conclusion that she’ll become an attorney DEPENDS on her passing the bar exam.

But it’s a wrong answer for a sufficient assumption question – passing the bar doesn’t guarantee that the conclusion is true. It doesn’t automatically make you an attorney. You still have to pay bar dues. You have to pass a background check.

Basically, necessary assumptions aren’t sufficient because there might be OTHER necessary conditions. For instance, if beer costs $5, then it’s necessary to have five dollars to buy one. But that’s not sufficient – you still have to be 21 years old or older. There’s more than one necessary condition.

[zach7691]

I think I understand. I know that if you negate a necessary assumption answer and it totally wrecks your conclusion, then it works – I guess the language just bogged me down. Thank you for your help!