[This is a lesson excerpt from our online course, for which we invite you to enroll.]

Necessary assumptions can wreck an argument.

First, let’s understand what a necessary assumption is. It’s an assumption (definition: unstated premise) that is necessary for the argument. “Necessary” here means the same thing it does when we talk about a “necessary condition” (as opposed to a “sufficient condition”). Let’s put the relationship between argument and necessary assumption in Lawgic and then run the contrapositive.

Argument (valid) –> Necessary Assumption (true)
/Necessary Assumption –> /Argument

In English, this means that when we negate the necessary assumption, the argument falls apart. No necessary assumption, no argument.

Necessary Assumption questions present you with an argument (premises + conclusion) where in order for the conclusion to be valid, there is a necessary (critical) assumption not stated in the argument. Without this particular assumption, the argument falls apart. Your job is to find this sucker, a necessary assumption, in the answer choice.

There are two types of Necessary Assumptions (NA).

NA Question Type I: The Shield

Consider a simple argument:

As trees age, they grow rings. Therefore, counting the number of rings a tree has will tell us how old a tree is.

Before we get into necessary assumptions, let’s establish a point of comparison with sufficient assumptions. What are some sufficient assumptions we can make this argument valid? How about trees grow 1 ring per year? That will make our argument valid. Given this new information, we can say that it must be true that counting the number of rings a tree has will tell us how old a tree is. Because we know that as trees age, they grow 1 ring per year.

But is this sufficient assumption (that “trees grow 1 ring per year”) a necessary assumption? Well, is it necessary? Let’s negate it to see if it wrecks the argument: It’s not the case that trees grow 1 ring per year. Okay, so does this wreck our argument? Well, no. Because so what if trees don’t grow 1 ring per year. Maybe trees grow 3 rings a year, or .5 rings a year. Or whatever. As long as it’s a function of time, we can determine how old a tree is by counting its rings. So, I hope you see that while “trees grow 1 ring per year” is a sufficient assumption, it’s not a necessary assumption. Given the additional premise of trees grow 1 ring per year, our argument becomes valid. But, we can also say trees grow 4 rings a year, which would also be sufficient. For this argument there are plenty of sufficient, but not necessary assumptions.

Let’s consider now a necessary assumption. Trees don’t unpredictably skip its ring growth every once in a while. Is this necessary? Let’s see what happens when we negate it: trees do unpredictably skip its ring growth every once in a while. Well, there goes our argument. How are we supposed to reach our conclusion from our premise if this negated statement is true? We can’t. Our argument is destroyed. It falls apart. If trees unpredictably skip its growth rings every once in a while, there’s no way that we’ll be able to tell how old trees are by counting its rings. The assumption that trees don’t unpredictably skip its ring growth every once in a while is necessary. What else is necessary? That trees don’t grow additional rings during years with lots of rain. Answers to shield type necessary assumption questions protect your argument from being wrecked.

NA Question Type II: The Bridge

Just as the name suggests, these answer choices point out gaps in the logic of the argument. For example, the major premise of the argument might tell you: you edge out your fiercest competitor in a race. The conclusion then claims that you win the race.
The argument has jumped from one idea (beating your fiercest competitor) to another (winning the race). Bridge questions trade on your inclination to conflate two different ideas. They’re assuming that your fiercest competitor was the only competitor that had a shot of beating you. What about all the other guys? Maybe the dark horse will be the once to win the race. The necessary assumption here may say something that edging out your fiercest competitor is in some way relevant to your winning the race.

Think about building a bridge. The premises left you at one side of the river and the conclusion is way over on the other side. It’s your job to find this bridge in the answer choice that takes you from the premise to the conclusion.

Some sample necessary assumption question stems.
1. The argument makes which one of the following assumptions?
2. The argument assumes
3. Which one of the following is an assumption on which the argument relies?
4. Which one of the following is an assumption that the fund-raiser’s argument depends on?
5. Which one of the following is an assumption required by the argument?
6. Which one of the following is an assumption made by the argument?
7. Which one of the following is an assumption necessary for the critic’s conclusion to be properly drawn?

J.Y. has been teaching the LSAT since 2006 and has taught thousands of students. He is the founder of 7Sage and PreProBono, two organizations dedicated to making legal education more accessible. He graduated from Columbia University in 2007 where he studied Economics, Political Science, and Philosophy and holds a J.D. from Harvard Law School.

J.Y. highly encourages all LSAT students to review their logic games using 7Sage's Fool Proof Method and to sign up for 7Sage's free trial LSAT course.

Topic: Lesson Excerpt
• commed

great.i like this

• Alexander

Well, if we’re dealing with the following argument

“As trees age, they grow rings. Therefore, counting the number of rings a tree has will tell us how old a tree is.”,

then is the following assumption

“Trees don’t skip its ring growth every once in a while.”

really necessary?

It seems possible that even though trees skip their growth rings every once in a while, we can nevertheless identify when this has and will happen with perfect accuracy.

• J.Y. Ping

Good catch Alex. Who cares if they skip ring growths as long as the skipping is some predictable function of time, right? I should have said “Trees don’t unpredictably skip its ring growth every once in a while.” I’ve edited the post so it’s better now. Thanks smarty pants.