Sufficient and Necessary Assumptions Made Easy | Demon Daily, Ep. 1424

Hinzugefügt: 2026-05-12

[00:00:02]Hey there, welcome to LSAT Demon Daily.

[00:00:03]I'm Nathan Fox, one of the founders of lsatdemon.com and the weekly podcast Thinking LSAT. With me is Josh Mansfield, a teacher with the Demon and a rising 2L.

[00:00:13]Josh, we have a an email here from a This is actually from a Demon student via the ask button who is asking about sufficient versus necessary assumptions.

[00:00:25]Okay. Hi Demon team. I've heard it said a number of times that there are many questions you can get right without fully understanding the difference between necessary and sufficient assumptions.

[00:00:35]But some that you will get wrong if you don't understand the difference.

[00:00:38]Is this an effective way to test your understanding of the two question types?

[00:00:42]Thanks for all you guys do. So I'm assuming that that's probably a reference in there to the question. Uh but I think the base under here is like the the need of understanding sufficient versus necessary assumptions.

[00:00:55]>> is there an effective way to >> There you understanding of these two question types. That might be it. I'm guessing that was a typo. Um look on easier questions, this is actually a pretty good test. If you understand this, if this really clicks for you, then I think you understand the difference between sufficient necessary assumption questions.

[00:01:16]Okay, toy argument. The toy argument is uh Josh wants to go hit a bucket of balls at his local driving range.

[00:01:27]>> [sighs] >> The bucket of balls costs $10.

[00:01:32]So Josh is going to go this afternoon hit a bucket of ball at the driving range.

[00:01:39]Two different questions I could ask you.

[00:01:42]Which one of the following is an assumption required for Josh to hit balls at the driving range?

[00:01:50]That's the necessary assumption question. I could also ask you which one of the following, if assumed would justify the conclusion that Josh is going to hit balls or can hit balls at the driving range? Mhm.

[00:02:03]And uh that's the sufficient assumption question.

[00:02:09]Necessary assumption questions are about figuring out which one must be true or else the logic fails.

[00:02:17]Sufficient assumption questions are about which one, if true will make the argument win.

[00:02:24]So the example here is uh and and you've got to be able to We can walk uh through the tests um individually. Mhm.

[00:02:35]But he he said uh there are some you can get right without fully understanding the difference between necessary and sufficient assumptions. And that's absolutely true because regardless of whether I had said which one of the following is an assumption required by the argument above or which one of the following, if assumed would allow the conclusion to be properly drawn, the sufficient assumption question regardless, if one of the answer choices was Josh has at least $10 in his pocket remember, I said that the driving range costs $10. Yep.

[00:03:10]>> If I said Josh has at least $10 in his pocket or Josh has the ability to pay $10, let's say so people don't argue with me about Apple Pay or whatever, but Josh has at least 10 bucks Yep. that he can pay.

[00:03:24]Uh that is necessary.

[00:03:28]That That must be true. If he doesn't have the $10, then he can't hit balls.

[00:03:32]Nope. It's also sufficient. I said it costs $10. If he has $10, then he can hit balls.

[00:03:39]So if there's an answer that says Josh has at least $10 ability to pay because it's both necessary and sufficient you don't have to know the different analysis. You could apply the wrong analysis and still get it right. You could go which one of these has to be true? Oh, that's the answer. Oh, which one of these, if true proves? Oh, that one. That's the answer.

[00:04:01]Yeah, it is the answer. One in the same.

[00:04:03]And that's how uh assumption questions tend to be just painting with a really broad brush, but that's how assumption questions tend to be in the first 10 or first 15 questions in a section. The LSAT mostly is easy.

[00:04:17]The LSAT mostly isn't testing the distinction between those two.

[00:04:23]But when you get to harder assumption questions and this is where listeners can really like take a minute, think about it.

[00:04:32]But if there was an answer there that says Josh has $12 in his pocket it's two separate analyses to figure out whether that's necessary and whether it's sufficient.

[00:04:46]So if the question had said which one of the following, if assumed, would allow the conclusion to be properly drawn well, $12 that does it. I can still hit a bucket of balls. I can buy a Coke.

[00:05:00]>> [laughter] >> Exactly. Yeah, bucket was 10.

[00:05:03]If you got $12 or $100 or a million dollars then sure, that's enough. That's sufficient. If that is assumed, it allows the conclusion to be properly drawn.

[00:05:15]But if the question had said which one of the following is an assumption required by the argument above the analysis changes. You You have to go instead of asking which one makes the argument work, you ask which one has to be true or else the argument can't work.

[00:05:35]And $12 would not be a correct answer there because you don't have to have $12.

[00:05:42]You don't have to have $11. You don't have to have $10 and one penny.

[00:05:47]You have to have $10 because that's the price.

[00:05:50]And on harder questions they'll ask you the sufficient assumption question and there will be wrong answers Or sorry, they'll ask you the necessary assumption question. Yep.

[00:06:05]And there will be answers like $12 that aren't necessary.

[00:06:12]They are sufficient, so it's a trap, you know, it feels like, oh yeah, that does it. That's perfect. That's what they left out.

[00:06:18]But it's too much. It's more than what they actually claimed. It's It does not have to be true. It's not necessary.

[00:06:24]Mhm. And then flip side of that is consider $8.

[00:06:30]If Josh has $8 in his pocket, you know, if the question said which one of the following is an assumption required by the argument in order to have 10, he does have to have eight.

[00:06:41]So he having at least $8 is required.

[00:06:46]It can be a little harder to see there because you're like, whoa, but no, you said it was $10. I know, but he has to have at least eight or else he doesn't have 10.

[00:06:53]And I have to have at least seven or whatever. I can say a dollar.

[00:06:57]Yeah. He has to have at some point in his life possessed money.

[00:07:02]Yeah.

[00:07:04]Right? Yep. Um So sure. So any number 10 and below is necessary. Mhm.

[00:07:15]But if they said which one of the following, if assumed, allows the conclusion to be properly drawn and it said any number between zero and 999 no, it's 10 bucks. So yeah, you >> I can't go golfing. I have 999.

[00:07:28]>> Yep.

[00:07:30]But if that's all you got, it's not enough. You have to have 10 or more.

[00:07:35]So again vast majority of LSAT questions, not vast majority. Majority though, I think, of LSAT questions aren't that hard where they're not going to ask you the necessary assumption question and have a sufficient assumption wrong answer.

[00:07:48]They're also not going to ask you the sufficient assumption question and have a necessary assumption wrong answer on the majority of LSAT questions. Yep.

[00:07:55]Most of the time it's going to just be like, yeah, they ask you one or the other of those questions and the answer is 10 bucks.

[00:08:01]And it's just like, yeah, okay, easy, too easy. Too easy.

[00:08:04]But when we start like differentiating the 160s from the 165s and the 165s from the 170s then they're going to ask you questions where you will fall into a trap if you don't understand the difference between those two questions.

[00:08:19]And it is a sure uh a sure tell if your LSAT tutor talks to you about assumption questions.

[00:08:29]As like a category. That's That is what I was going to say. Uh I think your explanation is is phenomenal. Thank you. Um >> [laughter] >> I've only done it like 10,000 times and >> Oh, is that all?

[00:08:42]Yeah.

[00:08:42]I I the the $8 one is the one I think people don't think about so frequently.

[00:08:46]And I also I see students who dismiss arguments because they're too strong for sufficient assumption questions. Yeah.

[00:08:52]So I think that highlights I think that highlights perfectly. The thing I wanted to add quit thinking about these two in connec- connection with each other. Yeah. You don't need to. That's because they both Yes, it is. Everybody does it. Like even, you know, LSAC or Khan Academy or all the big prep companies, they always are like, okay, here's our section on assumption questions. Stop it. Yeah, you you do have to stop it because I mean, at the Demon, we actually have them in entirely different fundamental families where we've got closed record questions and open record questions. Closed record questions are about which one must be true. That's a necessary assumption question. Mhm. Open record questions are which one, if true, would change the game.

[00:09:33]And that's a sufficient assumption question, right? I mean, if there was an answer there that said Josh has a trillion dollars it's like, well, okay, that does it. I mean, he just certainly does not have to have a trillion dollars, but if he did, he could go get that bucket of balls. I would have my own driving range if I had a trillion dollars. Let's just be clear.

[00:09:50]Yeah, a trillion dollars. [laughter] You'd have a lot more than a driving range. That's my my own golf course, yeah. I'm I'm not I'm not going to the the public driving range, the muni.

[00:09:58]Yeah.

[00:09:58]>> [laughter] [gasps] >> Uh yes, but that's that that is why one of the things I love about the way the demon lessons function is that separation of the two into open and closed. Think about it that way. Oh, this this is a necessary assumption, so this is a closed question type like >> Yeah, they're not the same. Separate those. Do not put those two things together in your head, and you're going to have a lot better time that way.

[00:10:20]Yeah.

[00:10:21]Cool. I hope that was helpful.

[00:10:24]Uh if you have any other questions about question types, want to send that in, you can email [email protected], and you can share some LSAT or law school admissions news there as well.

[00:10:32]Thanks for listening.

[00:10:37]>> [music]