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Two of the most difficult question types in the Logical Reasoning section of the LSAT are necessary and sufficient assumption questions. Both of these questions ask you to analyze what an argument leaves out, or the gap between its premise and conclusion. But each of them requires a very different response from the other, and the LSAT will try to trick you into confusing the two. To understand the crucial difference between necessary and sufficient assumptions, let’s imagine them in the context of an experience even more challenging than the LSAT: assembling IKEA furniture.
You’ve proudly arrived home with a giant cardboard box labeled with an unpronounceable name, otherwise known as your new coffee table. Of course you can tackle this. You begin to lay out your supplies. You will need boards A-E. You will need a hammer. You will need six nails. You will need your wit. You will need persistence. You will need a stiff drink. Each of these parts, individually, is necessary to building your IKEA coffee table. But none of them alone are sufficient. You can’t build your coffee table with just a hammer, and sadly, you definitely can’t build it with just a stiff drink. You will need all of the parts together to have sufficient supplies to begin
So what does this have to do with the LSAT? Well, the LSAT could ask you two questions about this situation. On the one hand, they could ask what’s necessary for building your coffee table. That question might look something like this:
Building your coffee table depends on which of the following supplies?
Building your coffee table requires which of the following supplies?
Either way, your answer might be:
- A) Nails
It could also be:
- E) Persistence
Moral of the story? For necessary assumption questions, the LSAT wants you to choose an answer that describes one truth that the argument needs in order to function. But it doesn’t want you to go overboard. You wouldn’t select an answer like:
- B) Nails, boards A-E, a stiff drink, persistence, wit, a hammer, safety goggles, a good sense of humor, a can of paint, your best friend, your oldest beanie baby, and some Nutella
Because, theoretically, that’s just a little bit more than you actually need.
What would it look like if the LSAT asked you a sufficient assumption question about your IKEA experience? Something like this:
Which of the following would logically allow you to complete your coffee table?
And your answer might be:
- A) Nails, a hammer, boards A-E, a stiff drink, persistence, and wit
It could even be:
- A) Nails, Boards A-E, a stiff drink, persistence, wit, a hammer, safety goggles, a good sense of humor, a can of paint, your best friend, your oldest beanie baby, and some Nutella
After all, that would be more than sufficient to build that table. But it would not be:
- B) Nails
Of course, LSAT material is usually even less fun than building an IKEA table (I know, right?). In order to find out what’s necessary and sufficient on an LSAT argument, you have to start by identifying the premise and conclusion of the argument. (Need help with that? Check out our Logical Reasoning Strategy Guide). Got that? Here’s how you can go about finding necessary and sufficient assumptions:
- Sufficient assumptions will make the argument completely true. You can often figure them out by connecting together the premise and conclusion: If the premise is true, then the conclusion is true—that’s the basic formula for a sufficient assumption. In the context of my IKEA example, if I have nails, a hammer, boards A-E, a stiff drink, persistence, and wit, then I can build my coffee table.
- Necessary assumptions will address a partial, but important, assumption that the argument makes. You can see if an assumption is truly necessary by negating it. If the conclusion doesn’t work without that assumption, then you know it’s necessary. In the context of my IKEA example, let’s say that I don’t have nails. Can I still build my IKEA table? Nope, not a chance. My coffee table falls apart without nails, so I know they are necessary!
Now, my IKEA example isn’t technically an argument—it’s more of a procedure. But the conceptual basis behind necessary and sufficient assumptions is tricky, so if this example makes sense to you, use it to help you as you work your way through tougher material. Wait, did I just admit that the LSAT is actually tougher than building an IKEA coffee table? Uh-oh, you better get to work! 📝
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Allison Bell is a Manhattan Prep Instructor who lives in the Washington, DC metro area. Allison first encountered the LSAT while getting her Bachelor of Arts in English and History at Duke University. In college, she scored a 178 and very nearly applied to law school. In the end, she followed her true passion, teaching. Allison currently has the pleasure of being an eighth grade English teacher in Northern Virginia. As an LSAT teacher, she has the opportunity to blend her love for teaching with her passion for logical argument. Check out Allison’s upcoming LSAT Complete Courses here.