Negating “Assumption Negation”

Nathan Fox

Nathan Fox

Jul 29, 2021

Two weeks ago, I wrote a manifesto declaring war on the “contrapositive” and all other forms of overcomplicated LSAT teacher nerd-speak.

This week, I’ve got “assumption negation” in my crosshairs. (That’s right: I’m about to negate the unnecessary “assumption negation” approach to Necessary Assumption questions—say that five times fast.)

I used to teach Necessary Assumption questions all wrong. I was suckered by LSAT dogma, and I thought that it was my duty to teach Necessary Assumption questions in a confusing, convoluted manner. Eventually, Ben Olson taught me how to think of them in a much simpler way. Before I get into “assumption negation” and all the wrong ways I used to think about it, let me tell you the right way to think about Necessary Assumption questions:

A necessary assumption is one that the author must agree with.

That’s it! The correct answer on a Necessary Assumption question is the only one that the author must agree with. It’s an awful lot like a Must Be True question. Just find the one that the author can’t possibly say no to. That’s all you have to do.

Anyway, back in the old days, my NA spiel went something like this:

Assumption means “missing piece,” so Necessary Assumption questions are about finding a critical missing piece of the argument that, when negated, would disprove the conclusion of the argument. The technique we use is to consider the opposite of each answer choice. If an answer can be false without it destroying the argument, then that’s not the answer. The correct answer, when false, will destroy the author’s argument.

This is flat wrong in part, and even the part that might be technically correct is far more confusing than it needs to be.

The part that’s dead wrong is the whole bit about how an assumption is a “missing piece” of the argument. That’s just not true on Necessary Assumption questions. On a Sufficient Assumption question, yes—you’re looking to build a bridge from the evidence to the conclusion, or from the evidence to another piece of evidence in a way that would prove the argument’s conclusion. You can memorize this:

A sufficient assumption is one that makes the argument win.

But Necessary Assumption questions aren’t about making the argument win at all. The correct answer frequently has nothing to do with building an affirmative case for the conclusion. And it’s not a “missing piece” in any reasonable sense of that term. Consider the following argument:

Nathan wants a dog that will hike the Tahoe Rim Trail with him, his best friends, and their dog Zada. Coco, a rottweiler available nearby for adoption, has lots of energy. So Nathan should adopt Coco to hike the Tahoe Rim Trail.

The correct answer to a Necessary Assumption question very well might be “Coco can walk.”

Nobody in their right mind would ever say that “Coco can walk” is a “missing piece” of the argument. If it’s true that Coco can walk, it does absolutely nothing to prove that Nathan should adopt Coco. If you’re looking for a missing piece, you’re going to miss this question.

But would the author of the argument have to agree that Coco can walk? Of course. Anyone recommending that I adopt Coco to hike 169 miles around the entire damn lake had better agree that Coco can walk. Duh.

Another necessary assumption of the same argument would be “Coco won’t immediately and constantly attempt to murder Zada.” Again, this isn’t a “missing piece” of the argument at all. But if you’re advocating that I adopt Coco to hike the rim trail with my best friends and their dog, then of course you’d have to agree that Coco won’t immediately and constantly try to murder my best friends’ dog—otherwise, what kind of psychopath are you?

The part of my spiel that, while technically correct, was confusing, time consuming, and overcomplicated was the whole bit about “negate each answer choice and look for the one that would disprove the argument.” Ain’t nobody got time for that!

Adding a negative to each answer choice, then seeing if it would negate the argument, is purposely looking for a double negative. And what if the answer choice itself already contains a negative? Now we’re in triple-negative territory. Even if we can handle it, it burns up too much of our time.

Instead of negating every answer, just ask, “Does this seem like something the author would have to agree with?”

The correct answer on a Necessary Assumption question is allowed to be unexpected—frequently, it defends the argument against some unanticipated attack. But the wrong answers are irrelevant or extra in ways that make them obviously incorrect if you’re wearing your Must Be True hat. The correct answer is the only one that would allow you to stand up in court and say, “Yes, Your Honor, the author must agree with this statement because…”

It’s true that sometimes your explanation will start with, “…you see, if the author did not agree with this statement, then their argument would make no sense.” But you really should only bother going into it after eliminating the other four answers.

On your first read, just ask, “Would the author have to agree with this?” The author doesn’t necessarily have to have contemplated the issue in advance. But when confronted with it, the author would be forced to shrug and say, “Well yes, of course Coco can walk” or, “Well yes, of course I agree that Coco isn’t a dog-murderer.”

When an LSAT tip sounds too complicated, it probably is. So let’s resign “assumption negation” to the ever-growing turd pile. Instead, on Necessary Assumption questions, let’s just look for the one answer that the author must agree with.

(Because people will ask—no, I’m not adopting Coco. I had her for four days. As it turned out, she could definitely walk. But she also did try to immediately murder Zada every time they met. So Coco got returned to sender.)

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