Necessary Assumptions (LSAT)

The advice is often given during LSAT prep that one way to approach necessary assumption questions is to negate the answer choices. This post is not in any way meant to disagree with that advice; it's quite good. But I wanted to explain the reason behind it. It's important to not just memorize these ideas but to also understand them.

A necessary assumption in an argument is an assumption that is needed for the conclusion to follow from the premises provided. Stated otherwise, without that assumption, the conclusion no longer follows. A necessary assumption question on the LSAT will be worded as follows:

"Which of the following is an assumption on which the argument depends?"

And so why negate the answer choices when attempting to figure out which of the answer choices is an assumption on which the argument depends? Let's say that you negate an answer choice, take the new negated answer choice as true, and in doing so the conclusion of the argument no longer follows from the premises provided. If the argument fell apart such that the conclusion no longer followed simply because you negated the answer choice and took the negated answer choice as true then it follows that the way the answer choice was written (pre-negation) was necessary to that argument. In other words, the pre-negated answer choice was an assumption on which the argument depended. 

To the contrary, if you negate an answer choice, take the negated choice as true, and the conclusion still follows from the premises provided, then the pre-negated answer choice was not necessary to that argument. It couldn't be! If it were then the argument would have fallen apart when that answer choice was negated. 

So, if you negate an answer choice, and the conclusion still follows, you know that that answer choice is not an assumption on which the argument depends and you can confidently cross it out for this question type.

Essentially, a necessary assumption is one in which if the assumption is proven false, then the argument is false. I'll post more about sufficient assumptions at a later time, but the key difference there is that a sufficient assumption is one in which if the assumption is proven true then the argument is true.