What is a necessary assumption?

[zainrizvi]

I've heard the definition of a necessary assumption, many many times. But for some reason, it doesn't click in my head 100 percent.

A necessary assumption is something that is REQUIRED for the argument to be true, meaning if this assumption is false, then the argument CANNOT be valid. The conclusion WILL not follow given the evidence.

But does this mean that no matter what premises you add to the current argument, the conclusion will not follow - or just that the GIVEN, limited evidence cannot be used to come to the conclusion? Sometimes I've seen an approach where people look for situations where the conclusion does follow, despite denying the assumption - that's how you can tell it's an incorrect choice. But this does not make intuitive sense to me because I feel like you can always come up with premises where they can make the conclusion follow.


I hope I'm making sense..

[noah]

A necessary assumption is something that is REQUIRED for the argument to be true, meaning if this assumption is false, then the argument CANNOT be valid. The conclusion WILL not follow given the evidence.

This! Don't get involved in adding other premises. Just use the premises you're given - with those, what else do you need (to assume) so that you can definitively draw the conclusion.

I wouldn't stress about this too much - many of our teachers didn't know the difference between necessary and sufficient assumptions when they first took the LSAT (and rocked it). The only things you really need to remember are:

1. Necessary assumptions, when negated, will destroy the argument (meaning, you can't definitively draw the conclusion given the premises).

2. Necessary assumptions can be hard to predict.

3. Look out for sufficient assumption answers when asked for a necessary assumption. That is, an assumption that is broader or stronger than you need.

I hope that helps.

[zainrizvi]

Thanks for your quick response - it definitely does.

I'm just confused about how to utilize the denial test without adding any other premises.

I deny an answer choice, then I try to see if I could up with a situation where the conclusion will still hold with the evidence. Do you suggest I approach it another way?

[noah]

I'm just confused about how to utilize the denial test without adding any other premises.

I deny an answer choice, then I try to see if I could up with a situation where the conclusion will still hold with the evidence. Do you suggest I approach it another way?

You negate the answer choice, and then add it into the argument. If it's truly a necessary assumption, you should find yourself saying "Well, how can we draw that conclusion then?!"

If it's not a necessary assumption, the negated form should be irrelevant to whether we can draw the conclusion or not.

For example:

Tim is sick today. Therefore, he must have become sick because he went out without his jacket last night.


(A) Tim could not be sick because of something he ate.

(B) Tim has been sick before.

The first answer is necessary. If we negate it - Tim might be sick because of something he ate - then why would we conclude he must be because of not having his jacket?

The second answer, when negated - Tim has never been sick before - has no bearing on the argument.

I hope that helps.